THE MACHINERY OF THOUGHT
By: Richard j.Kosciejew
The Brain is a shared out portion of the central nervous system contained within the cranial walls of the skull. The brain is the control centre for movement, sleep, hunger, thirst, and virtually every other vital activity necessary to survival. All human emotions - including love, hate, fear, anger, elation, and sadness - are controlled by the brain. It also receives and interprets the countless signals sent to it from other parts of the body and from the external environment. The brain makes us conscious, emotional, and intelligent.
Adult human brain is a 1.3-kg (3-lb) mass of pinkish-gray jellylike tissue made up of approximately 100 billion nerve cells, or neurons; neuroglia (supporting-tissue) cells; and vascular (blood-carrying) and other tissues.
Between the brain and the protective shell of vertebrate, the cranium, is explained as the part of the skull that directly covers the brain, and posits to some protective or meninges. Progressing, to more or less of a regulatory fact, such that the outermost cell membrane (biology), is that - the dura mater - of which is the thickest of membranes, containing a thin, pliable layer of tissue covering surfaces or connecting non-localities to regions sustaining the cell’s membrane.
Below, the dura mater of a mind separating, or connecting to region placements are attributed to the membrane, as this is called the arachnoid layer.
Again the innermost membrane, the pia mater, consists mainly of small blood vessels and follows the contours of the surface of the brain.
A clear liquid, the cerebrospinal fluid, bathes the entire brain and fills a series of four cavities, called ventricles, near the centre of the brain. The cerebrospinal fluid protects the internal portion of the brain from varying pressures and transports chemical substances within the nervous system.
From the outside, the brain appears as three distinctively, but connected parts: the cerebrum (the Latin word for brain) - large, almost symmetrical hemispheres; the cerebellum (‘little brain’) - two smaller hemispheres at the back of the cerebrum; and the brain stem - a central core that gradually becomes the spinal cord, exiting the skull through an opening at its base called the foramen magnum. Two other major parts of the brain, the thalamus and the hypothalamus, lie in the midline above the brain stem underneath the cerebellum.
The brain and the spinal cord together make up the central nervous system, which contributively distributes a communication with the rest of the body through the peripheral nervous system. The peripheral nervous system consists of 12 pairs of cranial nerves extending from the cerebrum and brain stem: A system of other nerves branching throughout the body from the spinal cord. The autonomic nervous system, which regulates vital functions that are unformidably under conscious control, such as the activity of the heart muscle, smooth muscle (involuntary muscle found in the skin, blood vessels, and internal organs), and glands.
Most high-level brain functions take place in the cerebrum. Its two large hemispheres make up approximately 85 percent of the brain's weight. The exterior surface of the cerebrum, the cerebral cortex, is a convoluted, or folded, grayish layer of cell bodies known as the gray matter. The brain’s colouration is a gray sustaining matter and covers an underlying mass of fibres called the white matter. The convolutions are made up of a ridge like bulges, known as gyri, separated by small grooves called sulci and larger grooves called fissures. Approximately two-thirds of the cortical surface is hidden in the folds of the sulci. The extensive convolutions enable a very large surface area, where brain cortices in the adult are about 1.5 m2 (16 ft2), - if not only to fit within the cranium. The pattern or conduct of these convolutions is similar, although not identical within all living humans.
The split of half-and-half a cerebral helical hemisphere is partially separated from each other by a deep fold known as the longitudinal fissure. Communication between the two hemispheres is through several concentrated bundles of axons, called commissures, the largest being the corpus callosum.
Several major sulci divides the cortex into distinguishable regions. The central sulcus, or Rolandic fissure, runs from the middle of the top of each hemisphere downward, forwards, and toward another major sulcus, the lateral (‘side’), or Sylvian, sulcus. These and other sulci and gyri divide the cerebrum into five lobes: the frontal, parietal, temporal, and occipital lobes and the insula.
The frontal lobe is the largest of the five and consists of all the cortices in front of the central sulcus. Broca's area, a part of the cortex related to speech is situated or having positioned in location within the frontal lobe. The parietal lobe subsists in containment with the cortex, back of the central sulcus to some sulcus near the back of the cerebrum, which we know as the parieto-occipital sulcus. The parieto-occipital sulcus, in turn, form the front border of the occipital lobe, which is the rearmost part of the cerebrum. The temporal lobe is to the side of and below the lateral sulcus. Wernicke's area, a part of the cortex related to the understanding of language, is found in the temporal lobe, as the insula lies deep within the folds of the lateral sulcus.
The cerebrum receives information from all the sense organs and sends motor commands (signals that results in activity in the muscles or glands) to other parts of the brain and the rest of the body. Motor commands are transmitted by the motor cortex, a strip of cerebral cortex extending from side to side across the top of the cerebrum just in front of the central sulcus. The sensory cortex, a parallel strips of cerebral cortex just behind the central sulcus, receives information from the sense organs.
Many other areas of the cerebral cortex have also been mapped according to their specific functions, such as vision, hearing, speech, emotions, language, and other aspects of perceiving, thinking, and remembering. Cortical regions known as associative cortices are responsible for integrating multiple stimulants, of which are the participators in processing the information, and carrying out complex responses.
The cerebellum coordinates body movements become visible and emerge as situated or given to the location at the lower back of the brain beneath the occipital lobes. As, too, the cerebellum is divided into two lateral (side-by-side) lobes connected by a finger like a bundle of white fibres called the vermis. The outer layer, or cortex, of the cerebellum consists of fine folds called folia. As in the cerebrum, the outer layer of cortical gray matter surrounds a deeper layer of white matter and nuclei (groups of nerve cells). Three fibre bundles called cerebellar peduncles connect the cerebellum to the three parts of the brain stem - the midbrain, the pons, and the medulla oblongata.
The cerebellum coordinates voluntary movements by fine-tuning commands from the motor cortex in the cerebrum. The cerebellum also maintains posture and balance by controlling muscle tone and sensing the position of the limbs. All motor activity, from hitting a baseball to fingering a violin, depends on the cerebellum.
The thalamus and the hypothalamus lie underneath the cerebrum and connect it to the brain stem. The thalamus consist of two rounded masses of gray tissue lying within the middle of the brain, between the two cerebral hemispheres. The thalamus are the main relay station for incoming sensory signals to the cerebral cortex and for outgoing motor signals from it. All sensory information to the brain, except that of the sense of smell, connects to individual nuclei of the thalamus.
The hypothalamus lies beneath the thalamus on the midline at the base of the brain. It regulates or is involved directly in the control of many body's vital drives and activities, such as eating, drinking, temperature regulation, sleep, emotional behaviour, and sexual activity. It also controls the function of internal body organs by means of the autonomic nervous system, interacts closely with the pituitary gland, and helps coordinate activities of the brain stem.
The brain stem is evolutionarily the most primitive part of the brain and is responsible for sustaining the basic functions of life, such as breathing and blood pressure. It includes three main structures lying between and below the two cerebral hemispheric zones are — the midbrain, pons, and medulla oblongata.
The topmost anatomical structure of the brain stem is the midbrain and contains the actions on passing the neuron transmitting processes. Travelling excitably and comprehensively given purposes, therein it is adaptive adjustable as directly aimed for the giving to signals, and is applied to the cerebral cortex. As, many reflex centre—pathways carrying sensory information and motor (output signals) are participatorially given to interactional centres applicably lend too auditory (hearing) abilities. That being, to work or in some effective operations, as this dynamic function is found in the top portion of the midbrain. A pair of nuclei called the superior colliculus control reflex actions of the eye, such as blinking, opening and closing the pupil, and focussing from on the crystalline periphery of lensing. A second pair of nuclei, called the inferior colliculus, controls auditory reflexes, such as adjusting the ear to effects for which the Doppler sensations affecting receptors with sound waves. Where to be giving and taking toward adjustments, the naked ear is to become adaptive to the pitch of increasing or the decreasing of volume. At the bottom of the midbrain are reflex and relay centres relating to pain, temperature, and touch, and several regions associated with the control of movement, such as the red nucleus and the substantia nigra.
Continuous with and below the midbrain and directly in front of the cerebellum is a prominent bulge in the brain stem called the pons. The pons consists of large bundles of nerve fibres that connect the two halves of the cerebellum and connect each side of the cerebellum with the opposite-side cerebral hemisphere. The pons serves mainly as a relay station linking the cerebral cortex and the medulla oblongata.
The long, stalklike lowermost portion of the brain stem is called the medulla oblongata. At the top, it is continuous with the pons and the midbrain; at the bottom, it makes a gradual transition into the spinal cord at the foramen magnum. Sensory and motor nerve fibres connecting the brain and the rest of the body cross over to the opposite side as they pass through the medulla. Thus, the left half the brain contributively distributes a communication with the right half the body, and the right half the brain with the left half the body.
Running up the brain stem from the medulla oblongata through the pons and the midbrain is a netlike formation of nuclei known as the reticular formation. The reticular formation controls respiration, cardiovascular function, digestion, levels of alertness, and patterns of sleep. It also determines which parts of the constant flow of sensory information into the body are received by the cerebrum.
Yet, toward that place are two main types of brain cells: Neurons and neuroglia. Neurons are responsible for the transmission and analysis of all electrochemical communication within the brain and other parts of the nervous system. Each neuron is composed of a cell body called a soma, if to recognize of a major fibre as called an axon, and a system of branches called dendrites, the difference between is the dexterity as understood by whomever. Axons, also called nerve fibres, convey electrical signals away from the soma and can be up to 1 m (3.3 ft) in length. Most axons are covered with a protective sheath of myelin, a substance made of fats and protein, which insulates the axon. Myelinated axons conduct neuronal signals faster than do unmyelinated axons. Dendrites convey electrical signals toward the soma, are shorter than axons, and are usually multiple and branching.
Neuroglial cells are twice as many as neurons and account for half the brain's weight. Neuroglia (from glia, Greek for ‘glue’) provides structural support to the neurons. Neuroglial cells also form myelin, guide developing neurons, take up chemicals involved in cell-to-cell communication, and contribute to the maintenance of the environment around neurons.
Twelve pairs of cranial nerves arise symmetrically from the base of the brain and are numbered, from front to back, in the order in which they arise. They connect mainly with structures of the head and neck, such as the eyes, ears, nose, mouth, tongue, and throat. Some are motor nerves, controlling muscle movement; some are sensory nerves, conveying information from the sense organs; and others contain fibres for both sensory and motor impulses. The first and second pairs of cranial nerves - the olfactories (smell) nerve and the optic (vision) nerve - carry sensory information from the nose and eyes, respectively, to the undersurface of the cerebral hemispheres. The other ten pairs of cranial nerves originate in or end in the brain stem.
The brain functions by complex neuronal, or nerve cell, circuits. Communication between neurons is both electrical and chemical and always travels from the dendrites of a neuron, through its soma, and out its axon to the dendrites of another neuron.
Dendrites of one neuron receive signals from the axons of other neurons through chemicals known as neurotransmitters. The neurotransmitters set off electrical charges in the dendrites, which then carry the signals electrochemically to the soma. The soma integrates the information, which is then transmitted electrochemically down the axon to its tip.
At the tip of the axon, small, bubble like structures called vesicles’ release neurotransmitters that carries the signal across the synapse, or gap, between two neurons. There are many types of neurotransmitters, including norepinephrine, dopamine, and serotonin. Neurotransmitters can be excitatory (that is, they excite an electrochemical response in the dendrite receptors) or inhibitory (they block the response of the dendrite receptors).
One neuron may obtain the interconnectivity with thousands of other neurons, and many thousands of neurons are involved with even the simplest behaviour. It is believed that these connections and their efficiency can be modified, or altered, by experience.
Scientists have used two primary approaching works. One approach is to study brain function after parts of the brain have been damaged. Functions that disappear or that is no longer normal after injury to specific regions of the brain can often be associated with the damaged areas. The second approach is to study the response of the brain to direct stimulation or to stimulation of various sense organs.
Neurons are grouped by function into collections of cells called nuclei. These nuclei are connected to form sensory, motor, and other systems. Scientists can study the function of somatosensory (pain and touch), motor, olfactory, visual, auditory, language, and other systems by measuring the physiological (physical and chemical) change that occur in the brain when these senses are excited. For example, electroencephalography (EEG) measures the electrical activity of specific groups of neurons through electrodes attached to the surface of the skull. Electrodes introduced by them inserting directly into the brain can give readings of individual neurons. Changes in blood flow, glucose (sugar), or oxygen consumption in groups of active cells can also be mapped.
Although the brain appears symmetrical, how it functions is not. Each hemisphere is specific and dominates the other in certain functions. Research has shown that hemispheric dominance is related to whether a person is predominantly right-handed or left-handed. In most right-handed people, the left hemisphere processes arithmetic, language, and speech. The right hemisphere interprets music, complex imagery, and spatial relationships and recognizes and expresses emotion. In left-handed people, the pattern of brain organization is more variable.
Hemispheric specialization has traditionally been studied in people who have sustained damage to the connections between both the right and left hemispheres. It has unduly occurrence as might be occasioned upon the action set of some concurring evidence. This can result of a stroke, an abrupt change of blood flow to an area of the brain that causes the death of nerve cells in that area. The division of functions between the two hemispheres has also been studied in people who have had to have the connection drawn between the left and the right spheres, however, some having to be surgically cut to control severe epilepsy: It is a neurological disease outlined by convulsions and loss of consciousness.
The visual system inherent to humans is one of the most advanced sensory systems in the body, it is more directive to the information that exemplifies a furthering structure of the naked eye. Several cortical regions are collectively called a primary visual and visual associative cortex - and, the midbrain is involved in the visual system. Conscious processing of visual information occurs in the primary visual cortex, but reflexive - that is, its immediate and conscious—responses that occur at the superior colliculus in the brain. Associatory, cortical regions of specialized localities are often associated, or simply to facilitate its integration with multiple receptions. Are that, the parietal and frontal lobes along with parts of the temporal lobe are involved in the processing of visual information and the establishment of visual remembering or being visually retentive? Language involves specialized cortical regions in a complex interaction that allows the brain to comprehend and communicatively expresses in the accompaniment with abstract ideas. The motor cortex initiates impulses that travel through the brain stem to produce audible sounds. Neighbouring regions of motor cortices, called the supplemental motor cortex, are involved in sequencing and coordinating sounds. Broca's area of the frontal lobe is responsible for the sequencing of language elements for output. The comprehension of language is dependent upon Wernicke's area of the temporal lobe. Other cortical circuits connect these areas.
Memory is usually considered a diffusely stored associative process - that is, it puts together information from many different sources. Although research has failed to identify specific sites in the brain as locations of individual memories, certain brain areas are critical for memory to function. Immediate recall - the ability to repeat short series of words or numbers immediately after hearing them - is thought to be found in the auditory associative cortex. Short-term memory - the ability to retain a limited amount of information for up to an hour - is found in the deep temporal lobe. Long-term memory probably involves exchanges between the medial temporal lobe, various cortical regions, and the midbrain.
The autonomic nervous system regulates the life support systems of the body reflexively - that is, without conscious direction. It automatically controls the muscles of the heart, digestive system, and lungs; certain glands; and homeostasis - that is, the equilibrium of the internal environment of the body. The autonomic nervous system itself is controlled by nerve centres in the spinal cord and brain stem and is fine-tuned by regions higher in the brain, such as the midbrain and cortex. Reaction such as blushing suggests briefly that cognitive, or thinking, centres of the brain are also involved in autonomic responses.
The brain is guarded by several highly developed protective mechanisms. The bony cranium, the surrounding meninges, and the cerebrospinal fluid all contribute to the mechanical protection of the brain. In addition, a filtration system called the blood-brain barrier protects the brain from exposure to potentially harmful substances carried in the bloodstream.
Brain disorders have a wide range of causes, including head injury, stroke, bacterial diseases, complex chemical imbalances, and changes associated with aging.
Head injury can initiate a cascade of damaging events. After a blow to the head, a person may be stunned or may become unconscious, through this action. The act or resultant infliction on or upon that which causes injury, especially the loss of conveying visualizations rather than any other means. In spite of the distressing pain, in that the damage is called (in the Latin, concussiõ or concussion from concussis, to a participle of concutere, meaning to strike together, as with ‘concuss’), so the concussion is usually harmful to the brain, and produced by a violent blow and followed by a temporary or prolonged loss of function, leaving no more or, perhaps permanent damage. If the blow is more severe and haemorrhage (excessive bleeding) and swelling occurs. However, severe headache, dizziness, paralysis, a convulsion, or temporary blindness may result, depending on the area of the brain affected. Damage to the cerebrum can also result in profound personality changes.
Truly, damage to Broca's area in the frontal lobe causes difficulty in speaking and writing, a problem known as Broca's aphasia. Injury to Wernicke's area as located and dependable of the left temporal lobe, is that it results in an inability to comprehend spoken language, called Wernicke's aphasia.
An injury or disturbance to a part of the hypothalamus may cause a variety of symptoms, such as loss of appetite with an extreme drop in body weight; increase in appetite leading to obesity; extraordinary thirst with excessive urination (diabetes insipidus); failure in body-temperature control, resulting in either low temperature (hypothermia) or high temperature (fever); excessive emotionality; and uncontrolled anger or aggression. If the relationship between the hypothalamus and the pituitary gland is damaged, other vital bodily functions may be disturbed, such as sexual function, metabolism, and cardiovascular activity.
Injury to the brain stem is even more serious because it houses the nerve centres that control breathing and heart action. Damage to the medulla oblongata usually results in immediate death.
A stroke is damage to the brain due to an interruption in blood flow. The interruption may be caused by a blood clot, constriction of a blood vessel, or rupture of a vessel accompanied by bleeding. A pouch like expansion of the wall of a blood vessel, called an aneurysm, may weaken and burst, for example, because of high blood pressure.
Sufficient quantities of glucose and oxygen, transported through the bloodstream, are needed to keep nerve cells alive. When the blood supply to a small part of the brain is interrupted, the cells in that area die and the function of the area is lost. A massive stroke can cause a one-sided paralysis (hemiplegia) and sensory loss on the side of the body opposite the hemisphere damaged by the stroke.
Epilepsy is a broad term for a variety of brain disorders characterized by seizures, or convulsions. Epilepsy can result from a direct injury to the brain at birth or from a metabolic disturbance in the brain anytime later in life.
Some brain diseases, such as multiple sclerosis and Parkinson disease, are progressive, becoming worse over time. Multiple sclerosis damages the myelin sheath around axons in the brain and spinal cord. As a result, the affected axons cannot transmit nerve impulses properly. Parkinson disease destroys the cells of the substantia nigra in the midbrain, resulting in a deficiency in the neurotransmitter dopamine that affects motor functions.
Cerebral palsy is a broad term for brain damage sustained close to birth that permanently affects motor function. The damage may take place either in the developing fetus, during birth, or just after birth and is the result of the faulty development or breaking down of motor pathways. Cerebral palsy is nonprogressive - that is, it does not worsen with time.
A bacterial infection in the cerebrum or in the coverings of the brain that causes a swelling or an abnormal growth of healthy brain tissue, also, it can cause an increase in intracranial pressure and result in serious damage to the brain.
Scientists are finding that certain brain chemical imbalances are associated with mental disorders such as schizophrenia and depression. Such findings have changed scientific understanding of mental health and have resulted in new treatments that chemically correct these imbalances.
During childhood development, the brain is particularly susceptible to damage because of the rapid growth and reorganization of nerve connections. Problems that originate in the immature brain can appear as epilepsy or other brain-function problems in adulthood.
Several neurological problems are common in aging. Alzheimer's disease damages many areas of the brain, including the frontal, temporal, and parietal lobes. The brain tissue of people with Alzheimer's disease shows characteristic patterns of damaged neurons, known as plaques and tangles. Alzheimer's disease produces the progressive dementia as characterized by symptoms such as failing attention and memory, loss of mathematical ability, irritability, and poor orientation in space and time.
Several commonly used diagnostic methods give images of the brain without invading the skull. Some portray anatomy that is, the structure of the brain - whereas others measure brain function. Two or more methods may be used to complement each other, together providing a complete picture than would be possible by one method alone.
Magnetic resonance imaging (MRI), introduced in the early 1980s, beams high-frequency radio waves into the brain in a highly magnetized field that causes the protons that form the nuclei of hydrogen atoms in the brain to reemit the radio waves. The reemitted radio waves are analysed by computer to create thin cross-sectional images of the brain. MRI provides the most detailed images of the brain and is safer than imaging methods that use X rays. However, MRI is a lengthy process and cannot be used with people who have pacemakers or metal implants, both of which are adversely affected by the magnetic field. Computed tomography (CT), also known as CT scans, developed in the early 1970s. This imaging method provides the proper X-rays for which the brain is examined from its many lobular constructions, thus, feeding the information into a computer that produces a series of cross-sectional images. CT is particularly useful for diagnosing blood clots and brain tumours. It is a much quicker process than magnetic resonance imaging and is therefore advantageous in certain situations - for example, with people who are extremely ill.
Changes in brain function due to brain disorders can be visualized in several ways. Magnetic resonance spectroscopy measures the concentration of specific chemical compounds in the ban rain that may change during specific behaviours. Functional magnetic resonance imaging (fMRI) maps changes in oxygen concentration that correspond to nerve cell activity.
Positron emission tomography (PET), developed in the mid-1970s, uses computed tomography to visualize radioactive tracers, radioactive substances are introduced into the brain intravenously or by inhalation. PET can measure such brain functions as cerebral metabolism, blood flow and volume, oxygen use, and the formation of neurotransmitters. Single photon emission computed tomography (SPECT), developed in the 1950s and 1960s, used radioactive tracers to visualize the circulation and volume of blood in the brain.
Brain-imaging studies have provided new insights into sensory, motor, language, and memory processes, and Dissociative brain disorders, such as an ataxia, epilepsy; cerebrovascular disease; Alzheimer's, Parkinson, and Huntington's diseases, and various mental disorderedness, such as those in confusion or disarray. What is more, that the severity of a group of psychotic disorders usually characterised by withdrawal from reality, illogical patterns of thinking, delusions, and hallucinations. Too, be included of one’s accompanying within the varying degrees of dementia, and they are in themselves lost in the differing degrees to other emotional behaviours, or intellectual disturbances. Schizophrenia is associated with dopamine imbalances in the brain and defects of the frontal lobe and is caused by genetic and other corresponding results, perhaps, from the coexistence of disparate or antagonistic qualities.
Although all vertebrate brains share the same basic three-part structure, the development of their constituent parts varies across the evolutionary scale. In fish, the cerebrum is dwarfed by the rest of the brain and serves mostly to process information from the senses. In reptiles and amphibians, the cerebrum is proportionably larger and begins to connect and form conclusions about this input signal. Birds have well-developed optic lobes, making the cerebrum even larger. Among mammals, the cerebrum dominates the brain. It is mostly revealed in the perturbations within the frontale lobe as amid the accorded primates, in whom cognitive ability is the highest.
In lower vertebrates, such as fish and reptiles, the brain is often tubular and bears a striking resemblance to the early embryonic stages of the brains of more highly evolved animals. The total envisaging all vertebrates, the brain is divided into three regions: the forebrain (prosencephalon), the midbrain (mesencephalon), and the hindbrain (rhombencephalon). These three regions further the subdivided segmentation where differences to the structures, systems, nuclei, and layers. The more highly evolved the animal, the more complex is the brain structure. Human beings have the most complex brains of all animals. Evolutionary forces have also resulted in a progressive increase in the size of the brain. In vertebrates lower than mammals, the brain is small. Meat-eating animals, particularly primates, the brain increases dramatically in size.
The cerebrum and cerebellum of higher mammals are highly convoluted to fit the most gray matter surface within the confines of the cranium. Such highly convoluted brains are called gyrencephalic. Many lower mammals have a smooth, or lissencephalic (‘smooth head’), cortical surfaces.
There is also evidence of evolutionary adaption of the brain. For example, many birds depend on an advanced visual system to identify food at great distances while in flight. Consequently, their optic lobes and cerebellum are well developed, giving them keen sight and outstanding motor coordination in flight. Rodents, on the other hand, as nocturnal animals, do not have a well-developed visual system. Instead, they rely more heavily on other sensory systems, such as a highly-developed sense of smell and facial whiskers.
A Cell (biology), is the basic unit of life. Cells are the smallest structures that are ably to possess of or marked by a high level of efficiency and ability, for being capable or competent of basic life processes, such as taking in nutrients, expelling waste, or reproducing. All living things are composed of cells. Some microscopic organisms, such as bacteria and protozoa, are unicellular, meaning they consist of a single cell. Plants, animals, and fungi are multicellular; that is, they are composed of many cells working in concert. Yet whether it makes up an entire bacterium or is justifiably more or less than the trillions in a human body, the cell is a marvel of design and efficiency. Cells carry out thousands of biochemical reactions each minute and reproduce new cells that perpetuate life.
Cells vary considerably in size. The smallest of cells, is a type of bacterium known as a mycoplasma, and measures 0.0001 mm (0.000004 in) in diameter and 10,000 mycoplasma’s, and that it is implicitly given by verifiable measures for being as wide as the diameter of a single strand of human hair. However, among the largest of cells are the nerve cells that run down a giraffe’s neck: These cells can exceed 3 m’s (9.7 ft) in length. Human cells also display a variety of sizes, from small red blood cells that measure 0.00076 mm (0.00003 in) and liver cells measuring to an approximate expression as a quantity for being ten times its own size. About 10,000 average-sized human cells can fit on the head of a pin.
Along with their differences in size, cells present an array of shapes. Some, such as the bacterium Escherichia coli, resemble rods. The paramecium, a type of protozoan, is a slipper shaped. The amoeba, another protozoan, has an irregular form that changes shape as it moves around. Plant cells typically resemble boxes or cubes. In humans, the outermost layers of skin cells are flat, while muscle cells are long and thin. Some nerve cells, with their elongated, tentacle-like extensions, suggest an octopus.
In multicellular organisms, shape is typically tailored to the cell’s job. For example, flat skin cells pack tightly into a layer that protects the underlying tissues from an invasion by bacteria. Long, thin muscle cells’ contract readily to move bones. The many extensions from a nerve cell enable it to connect to several other nerve cells to send and receive messages rapidly and efficiently.
By itself, each cell is a model of independence and self-containment. Like some miniature, walled city in perpetual rush hour, the cell constantly bustles with traffic, shuttling essential molecules from place to place to carry out the business of living. Despite their individuality, however, cells also display a remarkable ability to join, express, and coordinate with other cells. The human body, for example, consists of an estimated 20 to 30 trillion cells. Dozens of different kinds of cells are organized into specialized groups called tissues. Tendons and bones, for example, are composed of connective tissue, whereas skin and mucous membranes are built from epithelial tissue. Different tissue types are assembled into organs, which are structures specialized to do particular functions. Examples of organs include the heart, stomach, and brain. Organs, in turn, are organized into systems such as the circulatory, digestive, or nervous systems. Together, these assembled organ systems form the human body.
The components of cells are molecules, nonliving structures formed by the union of atoms. Small molecules serve as building blocks for larger molecules. Proteins, nucleic acids, carbohydrates, and lipids, which include fats and oils, are the four major molecules that underlie cell structure and participate in cell functions. For example, a tightly organized arrangement of lipids, proteins, and protein-sugar compounds forms the plasma membrane, or outer boundary, of certain cells. The organelles, membrane-bound compartments in cells, are built largely from proteins. Biochemical reactions in cells are guided by enzymes, specialized proteins that speed up chemical reactions. The nucleic acid deoxyribonucleic acid (DNA) contains the hereditary information for cells, and another nucleic acid, ribonucleic acid(RNA), works with DNA to build the thousands of proteins the cell needs.
Cells fall into one of two categories: prokaryotic or eukaryotic. In a prokaryotic cell, found only in bacteria and archaebacteria, all the components, including the DNA, mingle freely in the cell’s interior, a single compartment. Eukaryotic cells, which make up plants, animals, fungi, and all other life forms, contain many compartments, or organelles, within each cell. The DNA in eukaryotic cells is enclosed in a special organelle called the nucleus, which serves as the cell’s command centre and information library. The term prokaryote comes from Greek words that mean ‘before the nucleus’ or ‘prenucleus,’ while eukaryote means ‘a true nucleus.’
Prokaryotic cells are among the tiniest of all cells, ranging in size from 0.0001 to 0.003 mm (0.000004 to 0.0001 in) in diameter. About a hundred typical prokaryotic cells lined up in a row would match the thickness of a book page. These cells, which can be rodlike, spherical, or spiral in shape, are surrounded by a protective cell wall. Like most cells, prokaryotic cells live in a watery environment, whether it is soil moisture, a pond, or the fluid surrounding cells in the human body. Tiny pores in the cell wall enable water and the substances dissolved in it, such as oxygen, to flow into the cell; these pores also allow wastes to flow out.
Pushed up against the inner surface of the prokaryotic cell wall is a thin membrane called the plasma membrane. The plasma membrane, composed of two layers of flexible lipid molecules and interspersed with durable proteins, is both supple and strong. Unlike the cell wall, whose open pores allow the unregulated traffic of materials in and out of the cell, the plasma membrane is selectively permeable, meaning it allows only certain substances to pass through. Thus, the plasma membrane actively separates the cell’s contents from its surrounding fluids.
While small molecules such as water, oxygen, and carbon dioxide diffuse freely across the plasma membrane, the passage of many larger molecules, including amino acids (the building mechanisms of proteins) and sugars, is carefully regulated. Specialized transport proteins accomplish this task. The transport proteins span the plasma membrane, forming an intricate system of pumps and channels through which traffic is conducted. Some substances swirling in the fluid around the cell can enter it only if they bind to and are escorted in by specific transport proteins. In this way, the cell fine-tunes its internal environment.
The plasma membrane encloses the cytoplasm, the semifluid that fill the cell. Composed of about 65 percent liquids, as the watering maintains the cytoplasm is packed with up to a billion molecules per cell, a rich storehouse that includes enzymes and dissolved nutrients, such as sugars and amino acids. This water oasis provides a favourable environment for the thousands of biochemical reactions that take place in the cell.
Within the cytoplasm of all prokaryotes is deoxyribonucleic acid (DNA), a complex of molecules forms a double helix, a shape similar that of a spiral staircase. The DNA is about 1,000 times the length of the cell, and to fit inside, it repeatedly twists and folds to form a compact structure called a chromosome. The chromosome in prokaryotes is circular, and is found in a region of the cell called the nucleoid. Often, smaller chromosomes called plasmids are found in the cytoplasm. The DNA is divided into units called genes, just like a long train is divided into separate cars. Depending on the species, the DNA contains several hundred or even thousands of genes. Typically, one gene contains coded instructions for building all or part of a single protein. Enzymes, which are specialized proteins, determine most of the biochemical reactions that support and sustain the cell.
Also, immersed in the cytoplasm are the only organelles in prokaryotic cells—tiny beadlike structures called ribosomes. These are the cell’s protein factories. Following the instructions encoded in the DNA, ribosomes churn out proteins by the hundreds every minute, providing needed enzymes, the replacements for worn-out transport proteins, or other proteins required by the cell.
While simple in construction, prokaryotic cells display extremely complex activities. They have a greater range of biochemical reactions than those found in their larger relatives, the eukaryotic cells. The extraordinary biochemical diversity of prokaryotic cells is manifested in the wide-ranging lifestyles of the archaebacteria and the bacteria, whose habitants include polar ice, deserts, and hydrothermal vents — such deep regions found of the oceans, that under great pressure where hot water geysers erupt from cracks in the ocean floor.
Eukaryotic cells are typically about ten times larger than prokaryotic cells. In animal cells, the plasma membrane, rather than a cell wall, forms the cell’s outer boundary. With a design similar to the plasma membrane of prokaryotic cells, it separates the cell from its surroundings and regulates the traffic across the membrane.
The eukaryotic cell cytoplasm is similar to that of the prokaryote cell except one major difference: Eukaryotic cells house a nucleus and many other membrane-enclosed organelles. Like separate rooms of a house, these organelles enable specialized functions to be carried out efficiently. The building of proteins and lipids, for example, takes place in separate organelles where specialized enzymes geared for each job are found.
The nucleus is the largest organelle in an animal cell. It contains many strands of DNA, and the length of each strand is often equal to the diameter of the cell. Unlike the circular prokaryotic DNA, long sections called eukaryotic DNA pack into the nucleus by wrapping around proteins. As a cell begins to divide, each DNA strand folds over onto itself several times, forming a rod-shaped chromosome.
The nucleus is surrounded by a double-layered membrane that protects the DNA from potentially damaging chemical reactions that occur in the cytoplasm. Messages pass between the cytoplasm and the nucleus through nuclear pores, which are holes in the membrane of the nucleus. In each nuclear pore, molecular signals flash back and forth as often as ten times per second. For example, a signal to begin a specific gene comes into the nucleus and instructions for production of the necessary protein go out to the cytoplasm.
Attached to the nuclear membrane is an elongated membranous sac called the endoplasmic reticulum. This organelle tunnels through the cytoplasm, folding back and forth on itself to form a series of membranous stacks. An endoplasmic reticulum takes two forms: Rough and smooth. A rough endoplasmic reticulum (RER) is so called because it appears bumpy under a microscope. The bumps are factually numbered thousands of ribosomes attached to the membrane’s surface. The ribosomes in eukaryotic cells have the same function as those in prokaryotic cells - protein synthesis - but they differ slightly in structure. Eukaryote ribosomes bound to the endoplasmic reticulum help assemble proteins that typically are exported from the cell. The ribosomes work with other molecules to link amino acids to partially completed proteins. These incomplete proteins then travel to the inner chamber of the endoplasmic reticulum, where chemical modifications, such as the addition of a sugar, are carried out. Chemical modifications of lipids are also carried out in the endoplasmic reticulum.
The endoplasmic reticulum and its bound ribosomes are particularly dense in cells that produce many proteins for export, such as the white blood cells of the immune system, which produce and secrete antibodies. Some ribosomes that manufacture proteins are not attached to the endoplasmic reticulum. These so-called free ribosomes are dispersed in the cytoplasm and typically make proteins - many of them enzymes - that remain in the cell.
The second form of an endoplasmic reticulum, remains connected by endoplasmic reticulum (SER.), but, lacks’ ribosomes and an even surface, that within the curves of threaded channels of smooth endoplasmic reticulums are the enzymes needed for the construction of molecules, such as carbohydrates and lipids. The smooth endoplasmic reticulum is prominent in liver cells, where it also serves to detoxify substances such as alcohol, drugs, and other poisons.
Proteins are transported from free and bound ribosomes to the Golgi apparatus, an organelle that resembles a stack of deflated balloons. It is packed with enzymes that complete the processing of proteins. These enzymes add sulfur or phosphorus atoms to certain regions of needed proteins, for example, or chop off tiny pieces from the ends of the proteins. The completed protein then leaves the Golgi apparatus for its final destination interiorly or exterior to the cell. Its assembly on the ribosome, each protein has generatively found a group of 4 to 100 amino acids called a signal. The signal works as a molecular shipping label to direct the protein to its proper location.
Lysosomes are small, often spherical organelles that function as the cell’s recycling centre and garbage disposal. Powerful digestive enzymes concentrated in the Lysosomes break down worn-out organelles and ship their building blocks to the cytoplasm where they are used to construct new organelles. Lysosomes also dismantle and recycle proteins, lipids, and other molecules.
The mitochondria is the powerhouse of the cell. Within these long, slender organelles, which can appear oval or bean shaped under the electron microscope, enzymes convert the sugar glucose and other nutrients into adenosine triphosphate (ATP). This molecule, in turn, serves as an energy battery for countless cellular processes, including the shuttling of substances across the plasma membrane, the
building and transport of proteins and lipids, are then recycled onto molecules and organelles, and the evincing division of cells. Muscle and of liver cells are particularly active and require dozens and sometimes more of up to a hundred mitochondria per cell to meet their energy needs. Mitochondria is unusual in that they contain their own DNA in a prokaryote-like circular chromosome; have their own ribosomes, which resemble prokaryotic ribosomes; and divide independently of the cell.
Unlike the tiny prokaryotic cell, the larger eukaryotic cells require structural support. The cytoskeleton, a dynamic network of protein tubes, filaments, and fibres, crisscrosses the cytoplasm, anchoring the organelles in place and providing shape and structure to the cell. Many components of the cytoskeleton are assembled and disassembled by the cell as needed. During cell division, for example, a special structure called a spindle is built to move chromosomes around. After cell division, the spindle, no longer needed, is dismantled. Some components of the cytoskeleton serve as microscopic tracks along which proteins and other molecules travel like miniature trains. Recent research suggests that the cytoskeleton also may be a mechanical communication structure that converses with the nucleus to help organize events in the cell.
Plant cells have all the components of animal cells and boast several added features, including chloroplasts, a central vacuole, and a cell wall. Chloroplasts convert light energy - typically from the Sun - into the sugar glucose, a form of chemical energy, in a process known as photosynthesis. Chloroplasts, like mitochondria, possess a circular chromosome and prokaryote-like ribosomes, which manufacture the proteins that the chloroplasts typically need.
The central vacuole of a mature plant cell typically takes up most of the room in the cell, for which the vacuole, a membranous bag, crowds the cytoplasm and organelles to the edges of the cell. The central vacuole stores water, salts, sugars, proteins, and other nutrients. In addition, it stores the blue, red, and purple pigments that give certain flowers their colours. The central vacuole also contains plant wastes that taste bitter to certain insects, thus discouraging the insects from feasting on the plant.
In plant cells, a sturdy cell wall surrounds and sheds of a protection of the plasma membrane. Its pores enable materials to pass freely into and out of the cell. The strength of the wall also enables a cell to absorb water into the central vacuole and swell without bursting. The resulting pressure in the cells gives plants rigidity and support for stems, leaves, and flowers. Without sufficient water pressure, the cells collapse and the plant wilts.
To stay alive, cells can probably carry out a variety of functions. Some cells can probably move, and most of the cells could probably divide or populate. All cells must maintain the right concentration of chemicals in their cytoplasm, ingest food and use it for energy, recycle molecules, expel wastes, and construct proteins. Cells must also be able to respond to changes in their environment, that is, the adaptive adjustments that prevail by some environments, whether hostile or friendly acquainting.
Many unicellular organisms swim, glide, thrash, or crawl to search for food and escape enemies. Swimming organisms often move by means of a flagellum, a long tail-like structure made of protein. Many bacteria, for example, have one, two, or many flagella along which rotate like propellers to drive the organism. Some single-celled eukaryotic organisms, such as the euglena, also have a flagellum, but it is longer and thicker than the prokaryotic flagellum. The eukaryotic flagellums work by waving up and down like a whip. In higher animals, the sperm cell uses a flagellum to swim toward the female egg for fertilization.
Movement in eukaryotes is also accomplished with cilia, short, hairlike proteins built by centrioles, which are barrel-shaped structures in the cytoplasm that assemble and break down protein filaments. Typically, thousands of cilia extend through the plasma membrane and cover the surface of the cell, giving it a dense, hairy appearance. By overcoming its apparent celiac movement as if they were wavering, an organism such as the paramecium propels itself through its watery environment, but in cells that do not move, cilia are used for other purposes? In the respiratory tract of humans, for example, millions of ciliated cells prevent inhaled dust, smog, and microorganisms from entering the lungs by sweeping motions of cilia, they currently them up on a current of mucus into the throat, where they are swallowed. Eukaryotic flagella and cilia are formed from basal bodies, and small protein structures found just inside the plasma membrane. Basal bodies also help to anchor flagella and cilia.
Still other eukaryotic cells, such as amoebas and white blood cells, move by amoeboid motion, or crawling. They extrude their cytoplasm to form temporary pseudopodia, or false feet, which effectively placed in front of the cell, quite like an extended arm. They then drag the trailing end of their cytoplasm up to the pseudopodia. A cell using amoeboid motion would lose a race to a euglena or paramecium. However, while it is slow, amoeboid motion is strong enough to move cells against a current, enabling water-dwelling organisms to pursue and devour prey, for example, or white blood cells roaming the blood stream to stalk and engulf a bacterium or virus.
All cells require nutrients for energy, and they display a variety of methods for ingesting them. Simple nutrients dissolved in pond water, for example, can be carried through the plasma membrane of pond-dwelling organisms via a series of molecular pumps. In humans, the cavity of the small intestine contains the nutrients from digested food, and cells that form the walls of the intestine use similar pumps to pull amino acids and other nutrients from the cavity into the bloodstream. Certain unicellular organisms, such as amoebas, can reach out and grabbing food. They used a process known as endocytosis, in which the plasma membrane surrounds and engulfed the food particle, enclosing it in a sac, called a vesicle, that is within the amoeba’s interior.
Cells require energy for a variety of functions, including moving, building up and breaking down molecules, and transporting substances across the plasma membrane. Nutrients contain energy, but cells must convert the energy locked in nutrients to another form - specifically, the ATP molecule, the cell’s energy battery - before it is useful. In single-celled eukaryotic organisms, such as the paramecium, and in multicellular eukaryotic organisms, such as plants, animals, and fungi, mitochondria is responsible for this task. The interior of each mitochondrion consists of an inner membrane folded into a mazelike arrangement of separate compartments called cristae. Within the cristae, enzymes form an assembly line where the energy in glucose and other energy-rich nutrients is harnessed to build ATP; thousands of ATP molecules are constructed each second in a typical cell. In most eukaryotic cells, this process requires oxygen and is known as aerobic respiration.
Some prokaryotic organisms also carry out aerobic respiration. They lack mitochondria, however, and carry out aerobic respiration in the cytoplasm with the help of enzymes sequestered there. Many prokaryote species live in environments where there are no or little amounts of oxygen. Environments such as mud, stagnant ponds, or within the intestines of animals, that some of these organisms produce ATP without oxygen in a process known as anaerobic respiration, where sulfur or other substances replace oxygen. Still other prokaryotes, and yeast, a single-celled eukaryote, build ATP without oxygen in a process known as fermentation.
Most organisms rely on the sugar glucose to produce ATP. Glucose is made by the process of photosynthesis, in which light energy is transformed to the chemical energy of glucose. Animals and fungi cannot carry out photosynthesis and depend on plants and other photosynthetic organisms for this task. In plants, as we have seen, photosynthesis takes place in organelles called chloroplasts. Chloroplasts contain many internal compartments called thylakoids where enzymes aid in the energy conversion process. A single leaf cell contains 40 to 50 chloroplasts. With sufficient sunlight, one large tree is capable of producing upwards of two tons of sugar in a single day. Photosynthesis in prokaryotic organisms - typically aquatic bacteria - is carried out with enzymes clustered in plasma membrane folds called chromatophores. Aquatic bacteria produce the food consumed by tiny organisms living in ponds, rivers, lakes, and seas.
A typical cell must have at least a containment of about 30,000 proteins at anyone particular time. Many of these proteins are enzymes needed to construct the major molecules used by cells - carbohydrates, lipids, proteins, and nucleic acids - or to aid in the breakdown of such molecules after they have worn out. Other proteins are part of the cell’s structure - the plasma membrane and ribosomes, for example. In animals, proteins also function as hormones and antibodies, and they function like delivery trucks to transport other molecules around the body. Hemoglobin, for example, is a protein that transports oxygen in red blood cells. The cell’s demand for proteins never ceases.
Before a protein can be made, however, the molecular directions to build, it must be extracted from one or more genes. In humans, for example, one gene holds the information for the protein insulin, the hormone that cells need to import glucose from the bloodstream, while at least two genes hold the information for collagen, the protein that imparts strength to skin, tendons, and ligaments. The process of building proteins begins when enzymes, in response to a signal from the cell, bind to the gene that carries the code for the required protein, or part of the protein. The enzymes transfer the code to a new molecule called messenger RNA, which carries the code from the nucleus to the cytoplasm. This enables the original genetic code to remain safe in the nucleus, with messenger RNA delivering small bits and pieces of information from the DNA to the cytoplasm as needed. Depending on the cell type, hundreds or even thousands of molecules of messenger RNA are produced each minute.
Once in the cytoplasm, the messenger RNA molecule links with a ribosome. The ribosome moves along the messenger RNA like a monorail car along a track, stimulating another form of RNA - transfer RNA - to gather and link the necessary amino acids, pooled in the cytoplasm, to form the specific protein, or section of protein. The protein is modified as necessary by the endoplasmic reticulum and Golgi apparatus before embarking on its mission. Cells teem with activity as they forge the many diverse proteins that are indispensable for life.
Most cells divide at some time during their life cycle, and some divide dozens of times before they die. Organisms rely on cell division for reproduction, growth, and repair and replacement of damaged or worn out cells. Three types of cell division occur: binary fission, mitosis, and meiosis. Binary fissions, the method used by prokaryotes, produce two identical cells from one cell. The more complex process of mitosis, which also produces two genetically identical cells from a single cell, is used by many unicellular eukaryotic organisms for reproduction. Multicellular organisms use mitosis for growth, cell repair, and cell replacement. In the human body, for example, an estimated 25 million mitotic cell’s divisions occur every second to replace cells that have completed their normal life cycles. Cells of the liver, intestine, and skin may be replaced every few days. Recent research suggested that even brain cells, once thought to be incapable of mitosis, undergo cell division in the part of the brain, as, associated with memory.
The type of cell division required for sexual reproduction is meiosis. Sexually reproducing organisms include seaweeds, fungi, plants, and animals—including, of course, human beings. Meiosis differs from mitosis in that cell division begins with a cell that has a full complement of chromosomes and ends with gamete cells, such as sperm and eggs, that have only half the complement of chromosomes. When a sperm and egg unite during fertilization, the cell resulting from the union, called a zygote, contains the full number of chromosomes.
The story of how cells evolved remains an open and actively investigated question in science. The combined expertise of physicists, geologists, chemists, and evolutionary biologists has been required to shed light on the evolution of cells from the nonliving matter of early Earth. The earth was formed about 4.5 billion years ago, and for millions of years, violent volcanic eruptions blasted substances such as carbon dioxide, nitrogen, water, and other small molecules into the air. These small molecules, bombarded by ultraviolet radiation and lightning from intense storms, collided to form the stable chemical bonds of larger molecules, such as amino acids and nucleotides - the constructing apparatuses in proteins and nucleic acids. Experiments show that these larger molecules form spontaneously under laboratory conditions that simulate the probable early environment of Earth.
Scientists speculate that rain may have carried these molecules into lakes to create a primordial soup - a breeding ground for the assembly of proteins, the nucleic acid RNA, and lipids. Some scientists postulate that these more complex molecules formed in hydrothermal vents rather than in lakes. Other scientists propose that these key substances may have reached Earth on meteorites from outer space. No matter the origin or environment, may, however, that scientists do agree that proteins, nucleic acids, and lipids provided the raw materials for the first cells. In the laboratory, scientists have observed lipid molecules joining to form spheres that resemble a cell’s plasma membrane. From these observations, scientists postulate that millions of years of molecular collisions resulted in lipid spheres enclosing RNA, the simplest molecule can initiate self-replication. These primitive aggregations would have been the ancestors of the first prokaryotic cells.
Fossil studies hint that cyanobacteria, bacteria capable of photosynthesis, and were among the earliest bacteria to evolve, an estimated 3.4 billion to 3.5 billion years ago. In the environment of the early Earth, there were no apparent oxygen, and cyanobacteria probably used fermentation to produce ATP. Over the eons, cyanobacteria did have photosynthesis, which produces oxygen as a byproduct; the result was the gradual accumulation of oxygen in the atmosphere. The presence of oxygen set the stage for the evolution of bacteria that used oxygen in aerobic respiration, a more efficient ATP-producing process than fermentation. Some molecular studies of the evolution of genes in archaebacteria suggest that these organisms may have evolved in the hot waters of hydrothermal vents or hot springs earlier than cyanobacteria, around 3.5 billion years ago. Like cyanobacteria, archaebacteria probably relied on fermentation to synthesize ATP.
Eukaryotic cells may have evolved from primitive prokaryotes in and around 2 billion years ago. One hypothesis suggests that some prokaryotic cells lost their cell walls, permitting the cell’s plasma membrane to expand and fold. These folds, ultimately, may have caused separate compartments within the cell—the forerunners of the nucleus and other organelles now found in eukaryotic cells. Another key hypothesis is known as endosymbiosis. Molecular studies of the bacteria-like DNA and ribosomes in mitochondria and chloroplasts show that mitochondrion and chloroplast ancestors were once free-living bacteria. Scientists propose that these free-living bacteria were engulfed and maintained by other prokaryotic cells for their ability to produce ATP efficiently and to provide a steady supply of glucose. Over generations, eukaryotic cells complemented with mitochondria - the ancestors of animals—or with both mitochondria and chloroplasts—the ancestors of plants - evolved.
The first observations of cells were made in 1665 by English scientist Robert Hooke, who used a crude microscope of his own invention to examine a variety of objects, including a thin piece of cork. Noting the rows of tiny boxes that made up the dead wood’s tissue, Hooke coined the term cell because the boxes reminded him of the small cells occupied by monks in a monastery. While Hooke was the first to observe and describe cells, he did not comprehend their significance. While, the Dutch maker of microscopes Antoni van Leeuwenhoek pioneered the invention of one of the best microscopes of the time, Using his invention, as Leeuwenhoek was the first to observe, draw, and describe a variety of living organisms, including bacteria gliding in saliva. One-celled organisms cavorting in ponds of water, and sperm swimming in semen, as two centuries passed before scientists grasped to the thought of the true importance of cells.
Modern ideas about cells appeared in the 1800s, when improved light microscopes enabled scientists to observe more details of cells. Working together, German botanist Matthias Jakob Schleiden and German zoologist Theodor Schwann recognized the fundamental similarities between plant and animal cells. In 1839 they proposed the revolutionary idea that all living things are made up of cells. Their theory led to modern biology: A new way of seeing and investigating the natural world, as we are inherent to the perceptions of the world.
By the late 1800s, as light microscopes improved still further, scientists could observe chromosomes within the cell. Their research was aided by new techniques for staining parts of the cell, which made possible the first detailed observations of cell division, including observations of the differences between mitosis and meiosis in the 1880s. In the first few decades of the 20th century, many scientists focussed on the behaviour of chromosomes during cell division. Then, it was generally held that mitochondria transmitted the hereditary information. By 1920, however, scientists determined that chromosomes carry genes and that genes transmit hereditary information from generation to generation.
During the same period, scientists began to understand some chemical processes in cells. In the 1920s, the ultracentrifuge was developed. The ultracentrifuge is an instrument that spins cells or other substances in test tubes at high speeds, which causes the heavier parts of the substance to fall to the bottom of the test tube. This instrument enabled scientists to separate the abundant and heavy mitochondria from the rest of the cell and study their chemical interactions. By the late 1940s, scientists were able to explain the role of mitochondria in the cell. Using refined techniques with the ultracentrifuge, scientists subsequently isolated the smaller organelles and gained an understanding of their functions.
While some scientists were studying the functions of cells, others were examining details of their structure. They were aided by a crucial technological development in the 1940s: the invention of the electron microscope, which uses high-energy electrons instead of light waves to view specimens. New generations of electron microscopes have provided resolution, or the differentiation of separate objects, thousands of times more powerful than that available in light microscopes. This powerful resolution revealed organelles such as the endoplasmic reticulum, Lysosomes, the Golgi apparatus, and the cytoskeleton. The scientific fields of cell structure and function continue to complement each other as scientists explore the enormous complexity of cells.
The discovery of the structure of DNA in 1953 by American biochemist James D. Watson and British biophysicist Francis Crick ushered in the era of molecular biology. Today, - investigations inside the world of cells - in that, genes and proteins, upon the molecular level, made of the largest and fastest moving areas in all of science. One particularly active field in recent years has been the investigation of cell-signalling, the process by which molecular messages find their way into the cell via a series of complex protein pathways in the cell.
Another busy area in cell biology concerns programmed cell death, or apoptosis. Millions of times per second in the human body, cells commit suicide as an essential part of the normal cycle of cellular replacement. This also might be a check against disease: When mutations build up within a cell, the cell will usually self-destruct. If this fails to occur, the cell may divide and lead to mutated daughter cells, which continue to divide and spread, gradually forming a growth called a tumour. This unregulated growth by rogue cells can be benign, or harmless, or cancerous, which may threaten healthy tissue. The study of apoptosis is one avenue that scientists explore in understanding how cells become cancerous.
Scientists are also discovering exciting aspects of the physical forces within cells. Cells employ a form of architecture called tensegrity, which enables them to withstand battering by a variety of mechanical stresses, such as the pressure of blood flowing around cells or the movement of organelles within the cell. Tensegrity stabilizes cells by evenly distributing mechanical stresses to the cytoskeleton and other cell components. Tensegrity also may explain how a change in the cytoskeleton, where certain enzymes are anchored, initiates biochemical reactions within the cell, and can even influence the action of genes. The mechanical rules of tensegrity may also account for the assembly of molecules into the first cells. Such new - insights made some 300 years after the tiny universe of cells was first glimpsed - show that cells continue to yield fascinating new worlds of discovery.
A neuron is a long cell that has a thick central area containing the nucleus; it also has one long process called an axon and one or more short, bushy processes called dendrites. Dendrites receive impulses from other neurons. (The exceptions are sensory neurons, such as those that transmit information about temperature or touch, in which the signal is generated by specialized receptors in the skin.) These impulses are propagated electrically along the cell membrane to the end of the axon. At the tip of the axon the signal is chemically transmitted to an adjacent neuron or muscle cell.
Like all other cells, neurons contain charged ions: potassium and sodium (positively charged) and chlorine (negatively charged). Neurons differ from other cells in that they can produce a nerve impulse. A neuron is polarized - that is, it has an overall negative charge inside the cell membrane because of the high concentration of chlorine ions and low concentration of potassium and sodium ions. The concentration of these same ions is exactly reversed outside the cell. This charge differential represents stored electrical energy, sometimes called membrane potential or resting potential. The negative charge inside the cell is maintained by two features. The first is the selective permeability of the cell membrane, which is more permeable to potassium than sodium. The second feature is sodium pumps within the cell membrane that actively pump sodium out of the cell. When depolarization occurs, this charge differential across the membrane is reversed, and a nerve impulse is produced.
Depolarization is a rapid change in the permeability of the cell membrane. When sensory information or any other kind of stimulating current is received by the neuron, the membrane permeability is changed, allowing a sudden influx of sodium ions into the cell. The high concentration of sodium, or action potential, changes the overall charges within the cell from negativity to positivity. The local changes in ion concentration triggers similar reactions along the membrane, propagating the nerve impulse. After a brief period called the refractory period, during which the ionic concentration returned to resting potential, the neuron can repeat this process.
Nerve impulses travel at different speeds, depending on the cellular composition of a neuron. Where speed of impulse is important, as in the nervous system, axons are insulated with a membranous substance called myelin. The insulation provided by myelin maintains the ionic charge over long distances. Nerve impulses are propagated at specific points along the myelin sheath; these points are called the nodes of Ranvier. Examples of myelinated axons are those in sensory nerve fibres and nerves connected to skeletal muscles. In non-myelinated cells, the nerve impulse is propagated more diffusely.
When the electrical signal reaches the tip of an axon, it stimulates small presynaptic vesicles in the cell. These vesicles contain chemicals called neurotransmitters, which are released into the microscopic space between neurons (the synaptic cleft). The neurotransmitters attach to specific receptors on the surface of the adjacent neuron. This stimulus causes the adjacent cell to depolarize and propagate an action potential of its own. The duration of a stimulus from a neurotransmitter is limited by the breakdown of the chemicals in the synaptic cleft and the reuptake by the neuron that produced them. Formerly, each neuron was thought to make only one transmitter, but recent studies have shown that such cells construct of two or more.
Mental Illness mental disorders as characterized by disturbances in a person’s thoughts, emotions, or behaviour. The term mental illness can refer to a variety of disorders, ranging from those that cause mild distress to those that severely impair a person’s ability to function. Mental health professionals sometimes use the terms’ psychiatric disorder or psychopathology to refer to mental illness.
Severe mental illness usually alters a person’s life dramatically. People with severe mental illness sets experience conflicting or problematic symptoms that can make it difficult for one to hold a job, or of going to school, relate to others, or cope with ordinary life demands. Some individuals require hospitalization because they become unable to care for themselves or because they are at risk of committing suicide.
The symptoms of mental illness can be very distressing. People who develop schizophrenia may hear voices inside their head that say nasty things about them or command them to act in strange or unpredictable ways, as, perhaps, they may be paralysed by paranoia—the deep conviction that everyone, including their closest family members, wants to injure or destroy them. People with major depression may feel that nothing brings pleasure and that life is so dreary and unhappy that being dead is better. People with panic disorder may experience heart palpitations, rapid breathing, and anxiety so extreme that they probably could not leave home. People whom experience episodes of mania may engage in reckless sexual behaviour or may spend money indiscriminately, acts that later cause them to feel guilt, shame, and desperation.
Other mental illnesses, while not always debilitating, create certain problems in living. People with personality disorders may experience loneliness and isolation because their personality style interferes with social relations. People with an eating disorder may become so preoccupied with their weight and appearance that they force themselves to vomit or refuse to eat. Individuals who develop post-traumatic stress disorder may become angry easily, experience disturbing memories, and have trouble concentrating.
Experiences of mental illness often differentiated by which of depending on one’s culture or social group, sometimes greatly so. For example, in most of the non-Western world, people with depression complain principally of physical ailments, such as lack of energy, poor sleep, loss of appetite, and various kinds of physical pain. Even so, in northerly depressed people and mental health professionals who treat them have a tendency to emphasize psychological problems, such as feelings of sadness, worthlessness, and despair. The experience of schizophrenia also differs by cultures. In India, for example, in, at least, one-third of all new cases of schizophrenia involve catatonia - a behavioural condition in which a person maintains a bizarre statuelike pose for hours or days. This condition is rare in Europe and North America.
With appropriate treatment, most people can recover from mental illness and return to normal life. Even those with persistent, long-term mental illnesses can usually learn to manage their symptoms and live productive lives.
In most societies mental illness carries a substantial stigma, or mark of shame. The mental ill’s are often accountable for bringing on or upon their own illnesses, and others may see them as victims of bad fate, or religious and moral transgression, or witchcraft. Such stigmas may keep families from acknowledging that a family member is ill. Some families may hide or overprotect a member with mental illness - keeping the person from receiving potentially effective care - or they may reject the person from the family. When magnified from individuals to a whole society, such attitudes lead to under funding of mental health services and terribly inadequate care. In much of the world, even today, the mentally ill are chained, caged, or hospitalized in filthy, brutal institutions. Yet attitudes toward mental illness have improved in many areas, especially owing to health education and advocacy for the mentally ill.
Mental illness creates enormous social and economic costs. Depression, for example, affects some 500 million people in the world and results in more time lost to disability than such chronic diseases as diabetes mellitus and arthritis. Estimating the economic cost of mental illness is complex because there are direct costs (actual medical expenditures), indirect costs (the cost to individuals and society due to reduced or lost productivity, for example), and support costs (time lost to care of family members with mental illnesses). One study estimated that in 1985 the economic costs of mental illness in the United States totalled $103.7 billion. Of this, treatment and support costs totalled $42.5 billion, which represented 11.5 percent of the total cost of care for all illnesses.
Another method of estimating the cost of mental illness to society measures the impact of premature deaths and disablements. Research by the World Health Organization and the World Bank estimated that in 1990, among the world’s population aged 15 to 44 years, depression accounted for more than 10 percent of the total burden attributable to all diseases. Two other illnesses, bipolar disorder and schizophrenia, accounted for another 6 percent of the burden. This research has helped governments recognize that mental illnesses make up a far greater challenge to public health systems than previously realized.
No universally accepted definition of mental illness exists. Usually, the definition of mental illness depends on a society’s norms, or rules of behaviour. Behaviours that violate these norms are considered signs of deviance or, in some cases, of mental illness.
Because norms vary between cultures, Behaviours considered signs of mental illness in one culture may be considered normal in other cultures. For example, in the United States, a person who experiences trance and possession states (altered states of consciousness) is usually diagnosed as suffering from a mental illness. Yet, in many non-Western countries, people consider such states an essential part of human experience. In Native American culture, hearing the voices of recently deceased loved ones are common for people grieving. In contrast, most mental health professionals in Western cultures would consider such behaviour a possible symptom of schizophrenia or psychosis.
The variation in behavioural norms does not mean, however, that definitions of mental illness are necessarily incompatible across cultures. Many Behaviours are recognized throughout the world for being indicative of mental illness. These include extreme social withdrawal, violence to oneself, hallucinations (false sensory perceptions), and delusions (fixed, false ideas).
Another way of defining mental illness is based on whether a person’s behaviours are maladaptive - that is, whether they cause a person to experience problems in coping with common life demands. For example, people with a social phobia may avoid interacting with other people and experience problems at work as a result. Critics note that under this definition, political dissidents could be considered mentally ill for refusing to accept the dictates of their government.
In the United States, researchers estimate that about 24 percent of people 18 or older, or about 44 million adults, experience a mental illness or substance-related disorder during the course of any given year. The most common of these disorders is depression, alcohol dependence, and various phobias (irrational fears of things or situations). An estimated 2.6 percent of adults in the United States, or about 4.8 million people, suffer from a severe and persistent mental illness—such as schizophrenia, bipolar disorder, or a severe form of depression or panic disorder - in any given year. An additional 2.8 percent of adults, or about 5.2 million people, experience a mental illness that seriously interferes with one or more aspects of their daily life, such as their ability to work or relate to other people. All of these figures exclude people who are homeless and those living in prisons, nursing homes, or other institutions - populations that have high rates of mental illness.
International surveys have explained that from 30 to 40 percent of people in a given population experience a mental illness during their lives. These surveys also reveal that anxiety disorders are usually even more common than depression.
Young people can suffer from mental illnesses and psychological problems just as adults can. Prevalence estimates in industrialized countries show that from 14 to 20 percent of individuals under age 18 suffer from a diagnosable mental disorder. In the United States, an estimated 9 to 13 percent of children between the ages of 9 and 17 suffer from a serious emotional disturbance - that is, a disorder that severely disrupts a child's daily functioning in the family, school, or community.
Anxiety disorders are the most common childhood mental disorders, affecting an estimated 8 to 10 percent of children and adolescents in the United States. Children with these disorders experience persistent, unrealistic worry or uneasiness that interferes with their ability to function normally. About 4 percent of children and young adolescents experience severe separation anxiety and worry excessively about becoming separated from their parents. Depression is another common childhood mental disorder, affecting up to 2.5 percent of children (under age 13) and up to 8.3 percent of adolescents in the United States. Depression in children can lead to failure in school, poor self-image, troubled social relations, and even suicide.
A number of mental disorders are usually first diagnosed in infancy, childhood, or adolescence. Autism is a rare disorder that appears before the age of three and severely impairs a child's ability to interact socially and to express communicatively. With others. Attention-deficit hyperactivity disorder begins before the age of seven. Its symptoms include an inability to sit still, focus attention, or control impulses. Eating disorders, such as anorexia nervosa and bulimia nervosa, most often affect adolescent women.
With a greater percentage of people living substantially beyond the age of 65, as both in the industrialized nations of the West, in is, in the forming of developing countries of Asia, Africa, and Latin America - the problem of mental illness among the elderly has grown significantly. Researchers estimate that from 15 to 25 percent of elderly people in the United States suffer from significant symptoms of mental illness. Dementia, characterized by confusion, memory loss, and disorientation, occurs mostly among the elderly. A study of residents of Boston, Massachusetts, revealed that about 10 percent of people over the age of 65 suffer from Alzheimer’s disease, the most common form of dementia, and research on residents of Shanghai, China found that 4.6 percent of people more than 65 suffer from this condition.
Major depression, the most severe form of depression, affects from 1 to 2 percent of people aged 65 or older who are living in the community (rather than in nursing homes or other institutions). The prevalence of depression and other mental illnesses is much higher among elderly residents of nursing homes. Although most older people with depression respond to treatment, many cases of depression among the elderly go undetected or untreated. Research suggests that depression be a major risk factor for suicide among the elderly in the United States. People over age 65 in the United States have the highest suicide rate of any age group.
Like physical diseases, the highest rates of mental illness occur among people in the lower socioeconomic classes, especially those living in severe poverty. Rates of almost all mental illnesses decline as levels of income and education increase. A national survey published in 1994 said that people who earned $19,000 or less annually in the United States were twice as likely to have experienced an anxiety disorder as people who earned $70,000 or more. The hardships associated with poverty seem to contribute to the development of some mental illnesses, particularly anxiety disorders and depression. In addition, debilitating mental illnesses, such as schizophrenia, may cause individuals to drift to lower socioeconomic classes.
Generally, the overall prevalence rates of mental illnesses between men and women are similar. However, men have much higher rates of antisocial personality disorder and substance abuse. In the United States, women suffer from depression and anxiety disorders at about twice the rate of men. The gender gap is even wider in some countries. For example, in China, women suffer from depression at nine times the rate of men.
Mental illness is becoming an increasing problem for two reasons. First, increases in life expectancy have brought increased numbers of certain chronic mental illnesses. For example, because more people are living into old age, more people are suffering from dementia. Second, a number of studies provide evidence that rates of depression are rising throughout the world. The reasons may be related to such factors as economic change, political and social violence, and cultural disruptions. While some have questioned these findings, dramatic increases in the numbers of refugees and people dislocated from their homes by economic forces or civil strife are associated with great increases in a variety of mental illnesses for those populations. According to the United Nations High Commissioner for Refugees, the number of refugees worldwide increased from 2.5 million in 1971 to 13.2 million in 1996, peaking at 17 million in 1991.
A number of mental illnesses - such as depression, anxiety disorders, schizophrenia, and bipolar disorder - occur worldwide. Others seem to occur only in particular cultures. For example, eating disorders, such as anorexia nervosa (compulsive diets associated with unrealistic fears of fatness), occur mostly between young women and women in Europe, North America, and Westernized areas of Asia, whose cultures view thinness as an essential component of female beauty. In Latin America, people who live through an overwhelming fright after a dangerous or traumatic events are said to have [fright], an illness in which their soul has been frightened away. In some societies of West Africa and elsewhere, brain lag describes individuals (usually students) who experience difficulties in concentrating and thinking, as well as physical symptoms of pain and fatigue.
Most mental health professionals in the United States use the Diagnostic and Statistical Manual of Mental Disorders(DSM), a reference book published by the American Psychiatric Association, as a guide to the different kinds of mental illnesses. The fourth edition, known as DSM-IV, describes more than 300 mental disorders, behavioural disorders, addictive disorders, and other psychological problems and groups them into broad categories. This article describes some of the major categories, including anxiety disorders, mood disorders, schizophrenia and other psychotic disorders, personality disorders, cognitive disorders, Dissociative disorders, Somatoform disorders, factitious disorders, substance-related disorders, eating disorders, and impulse-control disorders. Mental health professionals in many other parts of the world use a different classification system, the International Classification of Diseases (ICD), published by the World Health Organization.
The DSM and ICD are both categorical systems of classification, in which each mental illness is defined by its own unique set of symptoms and characteristics. In theory, each disorder should possess diagnostic criteria that are independent of one another, just as tuberculosis and lung cancer are discrete diseases. Yet symptoms of many mental disorders overlap, and many people - such as those who experience both depression and severe anxiety - show symptoms of more than one disorder at the same time. For these reasons, some mental health professionals advocate a dimensional system of classification. In contrast to the categorical approach, which sees mental disorders as qualitatively distinct from normal behaviour, a dimensional system views behaviour as falling along a continuum of normality, with some Behaviours considered more abnormally than others. In a dimensional system, diagnoses do not describe discrete diseases but portray the relative importance of an array of symptoms.
Definitions and classifications of mental illnesses change as research improves understanding of them. For example, DSM-IV allows a diagnosis of schizophrenia only when characteristic symptoms have lasted at least one month, whereas the previous edition of DSM required a duration of only one week.
Anxiety disorders involve excessive apprehension, worry, and fear. People with generalized anxiety disorder experience constant anxiety about routine events in their lives. Phobias are fears of specific objects, situations, or activities. Panic disorder is an anxiety disorder in which people experience sudden, intense terror and such physical symptoms as rapid heartbeat and shortness of breath. People with obsessive-compulsive disorder experience intrusive thoughts or images (obsessions) or feel compelled to do certain behaviours (compulsions). People with post-traumatic stress disorder relive traumatic events from their past and feel extreme anxiety and distress about the event.
Mood disorders, also called affective disorders, create disturbances in a person’s emotional life. Depression, mania, and bipolar disorder are examples of mood disorders. Symptoms of depression may include feelings of sadness, hopelessness, and worthlessness, as well as complaints of physical pain and changes in appetite, sleep patterns, and energy level. In mania, on the other hand, an individual experiences an abnormally elevated mood, often marked by exaggerated self-importance, irritability, agitation, and a decreased need for sleep. In bipolar disorder, also called manic-depressive illness, a person’s mood alternates between extremes of mania and depression.
People with schizophrenia and other psychotic disorders lose contact with reality. Symptoms may include delusions and hallucinations, disorganized thinking and speech, bizarre behaviour, a diminished range of emotional responsiveness, and social withdrawal. In addition, people who suffer from these illnesses experience and association with the inability to function in one or more important areas of life, such as social inter-relations, work, or school.
Personality disorders are mental illnesses in which one’s personality results in personal distress or a significant impairment in social or work functioning. Generally, people with personality disorders have poor perceptions of themselves or others, as, perhaps, they might have a low self-esteem or overwhelming narcissism, poor impulse control, troubled social relationships, and inappropriate emotional responses. Considerable controversy exists toward the overflowing emptiness, as whether to formulate the distinction between a normal personality and a personality disorder.
Cognitive disorders, such as delirium and dementia, involve a significant loss of mental functioning. Dementia, for example, is characterized by impaired memory and difficulties in such functions as speaking, abstract thinking, and the ability to identify familiar objects. The conditions in this category usually result from a medical condition, substance abuse, or adverse reactions to medication or poisonous substances.
Dissociative disorders involve disturbances in a person’s consciousness, memories, identity, and perception of the environment. Dissociative disorders include amnesia that has no physical cause; Dissociative identity disorder, in of which, a person, as having more than one distinct characterization or a seeming amount of yet, the personalities and alternative measures to take upon the efforts to control one’s particular, and, yet, peculiarized behaviour.
Depersonalization disorder is as much characterized by a chronic feeling of being detached from one’s body or mental processes: And Dissociative Fugue, the episodical departure from home or one’s work with an accompanying loss in the suffering departure or vanquishing of memory. In some parts of the world people experience Dissociative states as rooted on the ‘possessions’, through which a god or ghost instead of some separately disposed personality. In many societies, trance and possession states are normal parts of cultural and religious practices and are not considered Dissociative disorders.
Somatoform disorders are characterized by the presence of physical symptoms that cannot be explained by a medical condition or another mental illness. Thus, physicians often judge that such symptoms result from psychological conflicts or distress. For example, in conversion disorder, also called hysteria, a person may experience blindness, deafness, or seizures, but a physician cannot find anything wrong with the person. People with another Somatoform disorder, hypochondriasis, constantly fear that they will develop a serious disease and misinterpret minor physical symptoms as evidence of illness. The term Somatoform comes from the Greek word soma, meaning ‘body.’
In contrast to people with Somatoform disorders, people with factitious disorders intentionally produce or fake physical or psychological symptoms in order to receive medical attention and care. For example, an individual might falsely report shortness of breath to gain admittance to a hospital, report thoughts of suicide to solicit attention, or fabricate blood in the urine or the symptoms of rash so as to appear ill. Munchausen syndrome represents the most extreme and chronic variant of the factitious disorders.
Substance-related disorders result from the abuse of drugs, side effects of medications, or exposure to toxic substances. Many mental health professionals regard these disorders as behavioural or addictive disorders rather than as mental illnesses, although substance-related disorders commonly occur in people with mental illnesses. Common substance-related disorders include alcoholism and other forms of drug dependence. In addition, drug use can contribute to symptoms of other mental disorders, such as depression, anxiety, and psychosis. Drugs associated with substance-related disorders include alcohol, caffeine, nicotine, cocaine, heroin amphetamines, hallucinogens, and sedatives.
Eating disorders are conditions in which an individual experiences severe disturbances in eating Behaviours. People with anorexia nervosa have an intense fear about gaining weight and refuse to eat adequately or maintain a normal body weight. People with bulimia nervosa repeatedly engage in episodes of binge eating, usually followed by self-induced vomiting or the use of laxatives, diuretics, or other medications to prevent weight gain. Eating disorders occur mostly among young women in Western societies and certain parts of Asia.
People with impulse-control disorders cannot control an impulse to engage in harmful Behaviours, such as explosive anger, stealing (kleptomania), setting fires (pyromania), gambling, or pulling out their own hair (trichotillomania). Some mental illnesses - such as mania, schizophrenia, and antisocial personality disorder - may include symptoms of impulsive behaviour.
People have tried to understand the causes of mental illness for thousands of years. The modern era of psychiatry, which began in the late 19th and early 20th centuries, has witnessed a sharp debate between biological and psychological perspectives of mental illness. The biological perspective views mental illness in terms of bodily processes, whereas psychological perspectives emphasize the roles of a person’s upbringing and environment.
These two perspectives are exemplified in the work of German psychiatrist Emil Kraepelin and Austrian psychoanalyst Sigmund Freud. Kraepelin, influenced by the work in the mid-1800s of German psychiatrist Wilhelm Griesinger, believed that psychiatric disorders were disease entities that could be classified like physical illnesses. That is, Kraepelin believed that the fundamental causes of mental illness lay in the physiology and biochemistry of the human brain. His classification system of mental disorders, first published in 1883, formed the basis for later diagnostic systems. Freud, on the other hand, argued that the source of mental illness lay in unconscious conflicts originating in early childhood experiences. Freud found evidence for this idea through the analysis of dreams, free association, and slips of speech.
This debate has continued into the late 20th century. Beginning in the 1960s, the biological perspective became dominant, supported by many breakthroughs in psychopharmacology, genetics, neurophysiology, and brain research, for example, scientists discovered many medications that helped to relieve symptoms of certain mental illnesses and verbally give a demonstration of an explanation to that of some people can come to the conclusion of a vulnerable interaction within the standards as accorded to mental illnesses. Psychological perspectives also remain influential, including the psychodynamic perspective, the humanistic and existential perspectives, the behavioural perspective, the cognitive perspective, and the sociocultural perspective.
Many mental health professionals today favour a combination of perspectives, acknowledging that both its biology and a personalized individuality become an environment that exhibits some really important roles in the imbalanced fields of mental illness. This approach recognizes that people are not only products of the genes inherited from their parents, but products of the families and social worlds into which they are born. In this view, environments shape how biological factors will be manifested. For example, an infant may inherit genes that could enable her to become a tall adult, but if she is malnourished as a child, she will never achieve that potential. Likewise, an individual who does not possess a biological vulnerability for depression may nevertheless become severely depressed following the death of a loved one or after experiencing an act of torture.
Scientists have identified a number of neurotransmitters, or chemical substances that enable brain cells to negotiate with each other correspondingly, nonetheless, their appearances are important to the regulatory manners of a person’s emotions and behaviour. These include dopamine, serotonin, norepinephrine, gamma-amino butyric acid (GABA), and acetylcholine. Excesses and deficiencies in levels of these neurotransmitters have been associated with depression, anxiety, and schizophrenia, but scientists have yet to figure out the exact mechanisms involved.
Advances in brain imaging techniques, such as magnetic resonance imaging (MRI) and positron emission tomography (PET), have enabled scientists to study the role of brain structure in mental illness. Some studies have revealed structural brain abnormalities in certain mental illnesses. For example, some people with schizophrenia have enlarged brain ventricles (cavities in the brain that learn with certainty of the cerebrospinal fluid). Nonetheless, this may be a result of dementia praecox, but as an alternative over that which a cause and effect, are not, but all people with schizophrenia show this abnormality.
A variety of medical conditions can cause mental illness. Brain damage and strokes can cause loss of memory, impaired concentration and speech, and unusual changes in behaviour. In addition, brain tumours, if left to grow, can cause psychosis and personality changes. Other possible biological factors in mental illness include an imbalance of hormones, deficiencies in diet, and infections from viruses.
The psychodynamic perspective views mental illness as caused by unconscious and unresolved conflicts in the mind. As stated by Freud, these conflicts arise in early childhood and may cause mental illness by impeding the balanced development of the three systems that make up the human psyche: the id, which comprises innate sexual and aggressive drives: the ego, the conscious portion of the mind that mediates between the unconscious and reality; and the superego, which controls the primitive impulses of the id and represents moral ideals. In this view, generalized anxiety disorder stems from a signal of unconscious danger whose source can only be identified through a thorough analysis of the person’s personality and life experiences. Modern psychodynamic theorists tend to emphasize sexuality less than Freud did and focus more on problems in the individual’s relationships with others.
Both the humanistic and existential perspectives view abnormal behaviour as resulting from a person’s failure to find meaning in life and fulfill his or her potential. The humanistic school of psychology, as represented in the work of American psychologist Carl Rogers, views mental health and personal growth as the natural conditions of human life. In Rogers’s view, every person possesses a drive toward self-actualization, the fulfilment of one’s greatest potential. Mental illness inclines when certain events or the constituent compositions that factor into the circumstances of one’s immediate and surrounding environment, however, when this successive sequence is blocked from this drive, the existential perspectives are seen to be emotional disturbances. As to a resultant amounts for which a person’s failure to properly act authentically, is to say, to behave in accordance with one’s own goals and values, than the goals and values of others.
The pioneers of behaviourism, American psychologists’ John B. Watson and B. F. Skinner, maintained that psychology should confine itself to the study of observable behaviour, rather than explore a person’s unconscious feelings. The behavioural perspective explains mental illness, as well as all of human behaviour, as a learned response to stimuli. In this view, rewards and punishments in a person’s environment shape that person’s behaviour. For example, a person involved in a serious car accident may develop a phobia of cars or generalize the fear to all forms of transportation.
The cognitive perspective holds that mental illness results from problems in cognition - -that is, problems in how a person reasons, perceives events, and solves problems. American psychiatrist Aaron Beck proposed that some mental illnesses - such as depression, anxiety disorders, and personality disorders—result from a way of thinking learned in childhood that is not consistent with reality. For example, people with depression tend to see themselves in a negative light, exaggerate the importance of minor flaws or failures, and misinterpret the behaviour of others in negative ways. It remains unclear, however, whether these kinds of cognitive problems cause mental illness or merely represent symptoms of the illnesses themselves.
The sociocultural perspective regards mental illness as the result of social, economic, and cultural factors. Evidence for this view comes from research that has shown an increased risk of mental illness among people living in poverty. In addition, the incidence of mental illness rises in times of high unemployment. The shift in the world population from rural areas to cities - with their crowding, noise, pollution, decay, and social isolation—has also been implicated in causing proportionally high rates of mental illness. Furthermore, rapid social change, which has particularly affected indigenous peoples throughout the world, brings about high rates of suicide and alcoholism. Refugees and victims of social disasters—warfare, displacement, genocide, violence - have a higher risk of mental illness, especially depression, anxiety, and post-traumatic stress disorder.
Social scientists emphasize that the link between social ills and mental illness is correlational rather than causal. For example, although societies undergoing rapid social change often have high rates of suicide the specific causes have not been identified. Social and cultural factors may create relative risks for a population or class of people, but it is unclear how such factors raise the risk of mental illness for an individual.
There are no blood tests, imaging techniques, or other laboratory procedures that can reliably diagnose a mental illness. Thus, the diagnosis of mental illness is always a judgment or an interpretation by an observer based on the speech, ideas, Behaviours, and experiences of the patient.
For the most part, mental health professionals determine the presence of mental illness in an individual by conducting an interview intended to reveal symptoms of abnormal behaviour. That is, the professional asks the patient questions about his or her mental state: ‘Do you hear voices of people who are not with you?’ ‘Have you felt depressed or lost interest in most activities?’ ‘Have you experienced a marked increase or decrease in your appetite?’ ‘Have you been sleeping less than normal?’ ‘Are you easily distracted?’ The answers to these questions will suggest other questions. Eventually, the clinician will feel that he or she has enough information to learn whether the patient is suffering from a mental illness and, if so, to make a diagnosis.
The process of diagnosis is not as simple as it might seem. Patients often have difficulty remembering symptoms or feel reluctant to talk about their fantasies, sex life, or use of drugs and alcohol. Many patients have experienced the endurable tolerance that chimes in the suffering from more than one disorder at a time — for example, depression and anxiety, or schizophrenia and depression — and determining which symptoms make up the primary problem is complicated and complex. In addition, symptoms may not be specific to mental illnesses. For example, brain tumours, malaria, and infections of the central nervous system can produce symptoms that mimic those of the Psychotic disorders.
Another problem in diagnosis is that mental health professionals may interpret symptoms differently based on their personal or cultural biases. One study examined this effect by showing 300 American and British psychiatrists videotaped interviews of eight patients with mental illnesses. Although the psychiatrists’ diagnoses substantially agreed for patients with ‘textbook’ cases of schizophrenia, their diagnoses varied widely for patients who had symptoms of both schizophrenia and other disorders, depending on whether the psychiatrist was American or British. The risk of misdiagnosis is even greater when the mental health professional and the patient come from different cultural groups.
Mental health professionals use a number of methods to treat people with mental illnesses. The two most common treatments by far are drug therapy and psychotherapy. In drug therapy, a person takes regular doses of a prescription medication intended to reduce symptoms of mental illness. Psychotherapy is the treatment of mental illness through verbal and nonverbal communication between the patient and a trained professional. A person can receive psychotherapy individually or in a group setting.
The type of treatment administered depends on the type and severity of the disorder. For example, doctors usually treat schizophrenia primarily with drugs, but specialized forms of psychotherapy may more effectively relieve phobias. For some mental illnesses, such as depression, the most effective treatment seems to be a combination of drug therapy and psychotherapy. Although some people with severe mental illnesses may never fully recover, most people with mental illnesses improve with treatment and can resume normal lives. Despite the availability of effective treatments, only about 40 percent of people with mental illnesses ever seek professional help.
A variety of mental health professionals offer treatment for mental illness. These include psychiatrists, psychologists, psychotherapists, psychiatric social workers, and psychiatric nurses.
Drugs introduced in the mid-1950s enabled many people who otherwise would have spent years in mental institutions to return to the community and live productive lives. Since then, advances in psychopharmacology have led to the development of drugs of even greater effectiveness. These drugs often relieve symptoms of schizophrenia, depression, anxiety, and other disorders. However, they may produce undesirable and sometimes serious side effects. In addition, relapses may occur when they have ceased or have ceased for distinctly long-term usages, that this may be required. Drugs that control symptoms of mental illness are called psychotherapeutic medications. The major categories of psychotherapeutic drugs include antipsychotic drugs, antianxiety drugs, antidepressant drugs, and antimanic drugs.
Antipsychotic drugs, also called neuroleptics and major tranquillizers, control symptoms of a psychosis, such as hallucinations and delusions, which characterize schizophrenia and related disorders. They can also prevent such symptoms from returning. Antipsychotic drugs may produce side effects ranging from dry mouth and blurred vision to a tardive dyskinesia, a permanent condition that produces involuntary movements of the lips, mouth, and tongue.
Antianxiety drugs, also called minor tranquillizers, reduce high levels of anxiety. They may help people with generalized anxiety disorder, panic disorder, and other anxiety disorders. Benzodiazepines, a class of drugs that includes diazepam (Valium), are the most widely prescribed antianxiety drugs. Benzodiazepines can be addictive and may cause drowsiness and impaired coordination during the day.
Antidepressant drugs help relieve symptoms of depression. Some antidepressant drugs can relieve symptoms of other disorders as well, such as panic disorder and obsessive-compulsive disorder. Antidepressant drugs comprise three major classes: tricyclics, monoamine oxidase inhibitors (MAO inhibitors), and selective serotonin reuptake inhibitors (SSRIs). Side effects of tricyclics may include dizziness upon standing, blurred vision, dry mouth, difficulty urinating, constipation, and drowsiness. People who take MAO inhibitors may experience some of the same side effects, and must follow a special diet that excludes certain foods. SSRIs generally produce fewer side effects, although these may include anxiety, drowsiness, and sexual dysfunction. One type of SSRI, fluoxetine (Prozac), is the most widely prescribed antidepressant drug.
Antimanic drugs help control the mania that occurs as part of bipolar disorder. One of the most effective antimanic drugs is lithium carbonates, a natural mineral salt. Common side effects include nausea, stomach upset, vertigo, and increased thirst and urination. In addition, long-term use of lithium can damage the kidneys.
Psychotherapy can be an effective treatment for many mental illnesses. Unlike drug therapy, psychotherapy produces no physical side effects, although it can cause psychological damage when improperly administered. On the other hand, psychotherapy may take longer than drugs to produce benefits. In addition, sessions may be expensive and time-consuming. In response to this complaint and demands from insurance companies to reduce the costs of mental health treatment, many therapists have started providing therapy of shorter duration.
Psychotherapy encompasses a wide range of techniques and practices. Some forms of psychotherapy, such as psychodynamic therapy and humanistic therapy, focus on helping people understand the internal motivations for their problematic behaviour. Other forms of therapy, such as behavioural therapy and cognitive therapy, focus on the behaviour itself and teach people skills to correct it. The majority of therapists today incorporate treatment techniques from a number of theoretical perspectives. For example, cognitive-behavioural therapy combines aspects of cognitive therapy and behavioural therapy.
Psychodynamic therapy is one of the most common forms of psychotherapy. The therapist focuses on a person’s experiences as a source of internal, unconscious conflicts and tries to help the person resolve those conflicts. Some therapists may use hypnosis to uncover repressed memories. Psychoanalysis, a technique developed by Freud, is one kind of psychodynamic therapy. In psychoanalysis, the person lies on a couch and says whatever comes to mind, a process called free association. The therapist interprets these thoughts along with the person’s dreams and memories. Classical psychoanalysis, which requires years of intensive treatment, is not as widely practised today as in previous years.
Both humanistic therapy and existential therapy treat mental illnesses by helping people achieve personal growth and attain meaning in life. The best-known humanistic therapy is client-entered therapy, developed by Carl Rogers in the 1950s. In this technique, the therapist provides no advice but restates the observations and insights of the client (the person in treatment) in nonjudgmental terms. In addition, the therapist offers the person unconditional empathy and acceptance. Existential therapists help people confront basic questions about the meaning of their lives and guide them toward discovery of their own uniqueness.
Psychotherapists whom practice behavioural therapy do not focus on a person’s experiences or inner life. Instead, they help the person to change patterns of abnormal behaviour by applying established principles of conditioning and learning. Behavioural therapy has proven effective in the treatment of phobias, obsessive-compulsive disorder, and other disorders.
The goal of cognitive therapy is to identify patterns of irrational thinking that cause a person to behave abnormally. The therapist teaches skills that enable the person to recognize the irrationality of the thoughts. The person eventually learns to perceive people, situations, and himself or herself in a more realistic way and develops improved problem-solving and coping skills. Psychotherapists use cognitive therapy to treat depression, panic disorder, and some personality disorders.
Rehabilitation programs help people with severe mental illnesses in learning independent living skills and in obtaining community services. Counsellors may teach them personal hygiene skills, home cleaning and maintenance, meal preparation, social skills, and employment skills. In addition, case managers or social workers may help people with mental illnesses obtain employment, medical care, housing, education, and social services. Some intensive rehabilitation programs strive to provide active follow-up and social support to prevent hospitalization.
Therapists often use play therapy to treat young children with depression, anxiety disorders, and problems stemming from child abuse and neglect. The therapist spends time with the child in a playroom filled with dolls, puppets, and drawing materials, which the child may use to act out personal and family conflicts. The therapist helps the child recognize and confront his or her feelings.
In group therapy, a number of people gather to discuss problems under the guidance of a therapist. By sharing their feelings and experiences with others, group members learn their problems are not unique, receive emotional support, and learn ways to cope with their problems. Psychodrama is a type of group therapy in which participants act out emotional conflicts, often on a stage, with the goals of increasing their understanding of their Behaviours and resolving conflicts. Group therapy generally costs less per person than individual psychotherapy.
Family intervention programs help families learn to cope with and manage a family member’s chronic mental illness, such as schizophrenia. Family members learn to monitor the illness, help with daily life problems, ensure adherence to medication, and cope with stigma.
Electroconvulsive therapy (ECT) is a treatment for severe depression in which an electrical current is passed through the patient’s brain for one or two seconds to induce a controlled seizure. The treatments are repeated a period of several weeks. For unknown reasons, ECT often relieves severe depression even when drug therapy and psychotherapy have failed. The treatment has created controversy because its side effects may include confusion and memory loss. Both of these effects, however, are usually temporary.
Seeking a treatment for extreme cases of mental illness, Portuguese neurologist António Egas Moniz invented the lobotomy, a surgical technique that destroys tissue in the frontal lobe of the brain. The procedure, widely performed in the 1940s and 1950s, often left people in a vegetative state or caused drastic changes in personality and behaviour.
Even more controversial than ECT is psychosurgery, the surgical removal or destruction of sections of the brain in order to reduce severe and chronic psychiatric symptoms. The best-known example of psychosurgery is the lobotomy, a procedure developed by Portuguese neurologist António Egas Moniz that was widely acted in the 1940s and early 1950s. Psychosurgery is now rarely done because no research has proven it effective and because it can produce drastic changes in personality and behaviour.
A significant portion of the homeless population in the United States suffers from a chronic mental illness, such as schizophrenia. The shortage of mental health treatment centres in many cities may partly account for the large number of mentally ill people who are homeless or in jail.
Treatment for mental illness takes places in a number of settings. Mental hospitals or psychiatric wards in general hospitals are used to treat patients in acute phases of their illnesses and when the severity of their symptoms requires constant supervision. Most individuals who suffer from severe mental illness, however, do not require such close attention, and they can usually receive treatment in community settings.
Often, patients who have just completed a period of hospitalization go to group homes or halfway houses before returning to independent living. These facilities offer patients the opportunity to take part in group activities and to receive training in social and job skills. In supportive housing, mentally ill individuals can live independently in an environment that offers an array of mental health and social services. Some people with chronic and severe mental illnesses require care in long-term facilities, such as nursing homes, where they can receive close supervision.
Unfortunately, many areas have a shortage of treatment centres, especially community mental health centres and supportive housing environments. This shortage may partly account for the large number of mentally ill people who are homeless or in jail.
Most non-Western countries still lack adequate treatment facilities and services for the mentally ill. In China, with its 1.2 billion people, there are 4.5 million patients with schizophrenia, but only about 100,000 beds for the mentally ill and fewer than 10,000 psychiatrists. On the other hand, there are hundreds of thousands of traditional healers, many of whom treat mentally ill patients. Other people with mental illnesses receive treatment from general physicians. In most countries of sub-Saharan Africa, psychiatric services are so limited that most people with mental illnesses receive little if any professional care. Some developing countries, however, have begun substantial reform and expansion of mental health services.
Evidence for trepanning, the surgical procedure of cutting a hole in the skull, dates back 4,000 to 5,000 years. Some anthropologists speculate that Stone Age societies performed trepanning on people with mental illnesses to release evil spirits or demons from their heads. Without written records, however, knowing why the operation was performed is impossible.
The literature of ancient Greece and Rome contains evidence of the belief that spirits or demons cause mental illness. In the 5th century Bc the Greek historian Herodotus wrote an account of a king who was driven mad by evil spirits. The legend of Hercules describes how, driven insane by a curse, he killed his own children. The Roman poets Virgil and Ovid repeated these themes in their works. The early Babylonian, Chinese, and Egyptian civilizations also viewed mental illness as possession, and used exorcism - which sometimes involved beatings, restraint, and starvation - to drive the evil spirits from their victim.
Not all ancient scholars agreed with this theory of mental illness. The Greek physician Hippocrates believed that all illnesses, including mental illnesses, had natural origins. For example, he rejected the prevailing notion that epilepsy had its origins in the divine or sacred, viewing it as a disease of the brain. Hippocrates classified mental illnesses into categories that included mania, melancholia (depression), and phrenitis (brain fever), and he advocated humane treatment that included rest, bathing, exercise, and dieting. The Greek philosopher Plato, although adhering to a supernatural view of mental illness, believed that childhood experiences shaped adult Behaviours, anticipating modern psychodynamic theories by more than 2000 years.
The Middle Ages in Europe, from the fall of the Roman empire in the 5th century ad, up til about the 15th century, was a period in which religious beliefs, specifically Christianity, dominated notions of mental illness. Much of the society believed that mentally ill people were possessed by the devil or demons, or accused them of being witches and infecting others with madness. Thus, instead of receiving care from physicians, the mentally ill became objects of religious inquisition and barbaric treatment. On the other hand, some historians of medicine cite evidence that even in the Middle Ages, many people believed mental illness to have its basis in physical and psychological disturbances, such as imbalances in the four bodily humours (blood, black bile, yellow bile, and phlegm), poor diet, and grief.
The Islamic world of North Africa, Spain, and the Middle East generally held far more humane attitudes toward people with mental illnesses. Following the belief that God loved insane people, communities began establishing asylums beginning in the 8th century ad, first in Baghdad and later in Cairo, Damascus, and Fez. The asylums offered patients special diets, baths, drugs, music, and pleasant surroundings.
The Renaissance, which began in Italy in the 14th century and spread throughout Europe in the 16th and 17th centuries, brought both deterioration and progress in perceptions of mental illness. On the one hand, witch-hunts and executions escalated throughout Europe, and the mentally ill were among those persecuted. The infamous Malleus Maleficarum, which serve as a handbook for inquisitors, claimed that witches could be identified by delusions, hallucinations, or other peculiar behaviour. To make matters worse, many of the most eminent physicians of the time fervently advocated these beliefs.
On the other hand, some scholars vigorously protested these supernatural views and called renewed attention to more rational explanations of behaviour. In the early 16th century, for example, the Swiss physician Paracelsus returned to the views of Hippocrates, asserting that mental illnesses were due to natural causes. Later in the century, German physician Johann Weyer argued that witches were mentally disturbed people in need of humane medical treatment.
Physicians in the 18th and 19th centuries used crude devices to treat mental illness, none of which offered any real relief. The circulating swing, top left, was used to spin depressed patients at high speed. American physician Benjamin Rush devised the tranquillizing chair, top right, to calm people with mania. The crib, bottom, was widely used to restrain violent patients.
During the Age of Enlightenment, in the 18th and early 19th centuries, people with mental illnesses continued to suffer from poor treatment. For the most part, they were left to wander the countryside or committed to institutions. In either case, conditions were generally wretched. One mental hospital, the Hospital of Saint Mary of Bethlehem in London, England, became notorious for its noisy, chaotic conditions and cruel treatment of patients.
Yet as the public’s awareness of such conditions grew, improvements in care and treatment began to appear. In 1789 Vincenzo Chiarugi, superintendent of a mental hospital in Florence, Italy, introduced hospital regulations that provided patients with high standards of hygiene, recreation and work opportunities, and least restraint. At nearly the same time, Jean-Baptiste Pussin, superintendent of a ward for ‘incurable’ mental patients at La Bicêtre hospital in Paris, France, forbade staff to beat patients and released patients from shackles. Philippe Pinel continued these reforms upon becoming chief physician of La Bicêtre’s ward for the mentally ill in 1793. Pinel began to keep case histories of patients and developed the notion of ‘moral treatment,’ which involved treating patients with kindness and sensitivity, and without cruelty or violence. In 1796 a Quaker named William Tuke established the York Retreat in rural England, which became a model of compassionate care. The retreat enabled people with mental illnesses to rest peacefully, talk about their problems, and work. Eventually these humane techniques became widespread in Europe.
People living in the colonies of North America in the 17th and 18th centuries generally explained bizarre or deviant behaviour as God’s will or the work of the devil. Some people with mental illnesses received care from their families, but most were jailed or confined in almshouses with the poor and infirm. By the mid-18th century, however, American physicians came to view mental illnesses as diseases of the brain, and advocated specialized facilities to treat the mentally ill. The Pennsylvania Hospital in Philadelphia, which opened in 1752, became the first hospital in the American colonies to admit people with mental illnesses, housing them in a separate ward. However, in the hospital’s early years, mentally ill patients were chained to the walls of dark, cold cells.
In the 1780s American physician Benjamin Rush made changes at the Pennsylvania Hospital that greatly improved conditions for mentally ill patients. Although he endorsed the continued use of restraints, punishment, and bleeding, he also arranged for heat and better ventilation in the wards, separation of violent patients from other patients, and programs that offered work, exercise, and recreation to patients. Between 1817 and 1828, following the examples of Tuke and Pinel, a number of institutions opened that devoted themselves exclusively to the care of mentally ill people. The first private mental hospital in the United States was the Asylum for the Relief of Persons Deprived of the Use of Their Reason (now Friends Hospital), opened by Quakers in 1817 in what is now Philadelphia. Other privately established institutions soon followed, and state-sponsored hospitals - in Kentucky, New York, Virginia, and South Carolina - opened beginning in 1824.
Nevertheless, circumstances for most mentally ill people in the United States, especially those who were poor, remained dreadful. In 1841 Dorothea Dix, a Boston schoolteacher, began a campaign to make the public aware of the plight of mentally ill people. By 1880, as a direct result of her efforts, 32 psychiatric hospitals for the poor had opened. Increasingly, society viewed psychiatric institutions as the most appropriate form of care for people with mental illnesses. However, by the late 19th century, conditions in these institutions had deteriorated. Overcrowded and understaffed, psychiatric hospitals had shifted their treatment approach from moral therapy to warehousing and punishment. In 1908 Clifford Whittingham Beers aroused new concern for mentally ill individuals with the publication of A Mind That Found Itself, an account of his experiences as a mental patient. In 1909 Beers founded the National Committee for Mental
Hygiene, which worked to prevent mental illness and ensure humane treatment of the mentally ill.
Following World War II (1939-1945), a movement emerged in the United States to reform the system of psychiatric hospitals, in which hundreds of thousands of mentally ill persons lived in isolation for years or decades. Many mental health professionals - seeing that large state institutions caused as much, if not more, harm to patients than mental illnesses themselves - came to believe that only patients with severe symptoms should be hospitalized. In addition, the development in the 1950s of antipsychotic drugs, which helped to control bizarre and violent behaviour, allowed more patients to be treated in the community. In combination, these factors led to the deinstitutionalisation movement: the release, over the next four decades, of hundreds of thousands of patients from state mental hospitals. In 1950, 513,000 patients resided in these institutions. By 1965 there were 475,000, and by 1990 state mental hospitals housed only 92,000 patients on any given night. Many patients who were released returned to their families, although many were transferred to questionable conditions in nursing homes or board-and-care homes. Many patients had no place to go and began to live on the streets.
The National Mental Health Act of 1946 created the National Institute of Mental Health as a centre for research and funding of research on mental illness. In 1955 Congress created a commission to investigate the state of mental health care, treatment, and prevention. In 1963, as a result of the commission’s findings, Congress passed the Community Mental Health Centres Act, which authorized the construction of community mental health centres throughout the country. Implementation of these centres was not as extensive as originally planned, and many people with severe mental illnesses failed to receive care of any kind.
One of the most important developments in the field of mental health in the United States has been the establishment of advocacy and support groups. The National Alliance for the Mentally Ill (NAMI), one of the most influential of these groups, was founded in 1972. NAMI’s goal is to improve the lives of people with severe mental illnesses and their families by eliminating discrimination in housing and employment and by improving access to essential treatments and programs.
During the 1980s, all levels of government in the United States cut back on funding for social services. For example, the Social Security Administration discontinued benefits for approximately 300,000 people between 1981 and 1983. Of these, an estimated 100,000 were people with mental illnesses. Although the government eventually restored Social Security benefits to many of these people, the interruption of services caused widespread hardship.
The emergence of managed care in the 1990s as a way to contain health care costs had a tremendous impact on mental health care in the United States. Health insurance companies and health maintenance organizations increasingly scrutinized the effectiveness of various psychotherapies and drug treatments and put stricter limits on mental health care. In response to these restrictions, Congress passed the Mental Health Parity Act of 1996. This law required private medical plans that offer mental health coverage to set equal yearly and lifetime payment limits for coverage of both mental and physical illnesses.
In 1997 the US of A, Equal Employment Opportunity Commission issued new guidelines intended to prevent discrimination against people with mental illnesses in the workplace. The rules, based on the Americans with Disabilities Act of 1990, prohibit employers from asking job applicants if they have a history of mental illness and require employers to provide reasonable accommodations to workers with mental illnesses.
Bipolar Disorder, mental illness in which a person’s mood alternates between extreme mania and depression. Bipolar disorder is also called manic-depressive illness. When manic, people with bipolar disorder feel intensely elated, self-important, energetic, and irritable. When depressed, they experience painful sadness, negative thinking, and indifference to things that used to bring them happiness.
Bipolar disorder is much less common than depression. In North America and Europe, about 1 percent of people experience bipolar disorder during their lives. Rates of bipolar disorder are similar throughout the world. In comparison, at least 8 percent of people experience serious depression during their lives. Bipolar disorder affects men and women about equally and at times more common in the higher socioeconomic classes. At least 15 percent of people with bipolar disorder commit suicide. This rate roughly equals the rate for people with major depression, the most severe form of depression.
Some research suggests that highly creative people - such as artists, composers, writers, and poets - show unusually high rates of bipolar disorder, and that periods of mania fuel their creativity. Famous artists and writers who probably suffered from bipolar disorder include poets Lord Byron and Anne Sexton, novelists Virginia Woolf and Ernest Hemingway, composers Peter Ilyich Tchaikovsky and Sergey Rachmaninoff, and painters Amedeo Modigliani and Jackson Pollock. Critics of this research note that many creative people do not suffer from bipolar disorder, and that most people with bipolar disorder are not especially creative.
Bipolar disorder usually begins in a person’s late teens or 20s. Men usually experience mania as the first mood episode, whereas women typically experience depression first. Episodes of mania and depression usually last from several weeks to several months. On average, people with untreated bipolar disorder experience four episodes of mania or depression over any ten-year period. Many people with bipolar disorder function normally between episodes. In ‘rapid-cycling’ bipolar disorder, however, which represents 5 to 15 percent of all cases, a person experiences four or more mood episodes within a year and may have little or no normal functioning in between episodes. In rare cases, swings between mania and depression occur over a period of days.
In another type of bipolar disorder, a person experiences major depression and hypomanic episodes, or episodes of milder mania. In a related disorder called cyclothymic disorder, a person’s mood alternates between mild depression and mild mania. Some people with cyclothymic disorder later develop full-blown bipolar disorder. Bipolar disorder may also follow a seasonal pattern, with a person typically experiencing depression in the fall and winter and mania in the spring or summer.
People in the depressive phase of bipolar disorder feel intensely sad or profoundly indifferent to work, activities, and people that once brought them pleasure. They think slowly, concentrate poorly, feel tired, and experience changes—usually an increase—in their appetite and sleep. They often feel a sense of worthlessness or helplessness. In addition, they may feel pessimistic or hopeless about the future and may think about or attempt suicide. In some cases of severe depression, people may experience psychotic symptoms, such as delusions (false beliefs) or hallucinations (false sensory perceptions).
In the manic phase of bipolar disorder, people feel intensely and inappropriately happy, self-important, and irritable. In this highly energized state they sleep less, have racing thoughts, and talk in rapid-fire speech that goes off in many directions. They have inflated self-esteem and confidence and may even have delusions of grandeur. Mania may make people impatient and abrasive, and when frustrated, physically abusive. They often behave in socially inappropriate ways, think irrationally, and show impaired judgment. For example, they may take aeroplane trips all over the country, make indecent sexual advances, and formulate grandiose plans involving indiscriminate investments of money. The self-destructive behaviour of mania includes excessive gambling, buying outrageously expensive gifts, abusing alcohol or other drugs, and provoking confrontations with obnoxious or combative behaviour.
Clinical depression is one of the most common forms of mental illness. Although depression can be treated with psychotherapy, many scientists believe there are biological causes for the disease. In this June 1998 Scientific American article, neurobiologist Charles B. Nemeroff discusses the connection between biochemical changes in the brain and depression.
The genes that a person inherits seem to have a strong influence on whether the person will develop bipolar disorder. Studies of twins provide evidence for this genetic influence. Among genetically identical twins where one twin has bipolar disorder, the other twin has the disorder in more than 70 percent of cases. Nevertheless, among pairs of fraternal twins, who have about half their genes in common, both twins have bipolar disorder in less than 15 percent of cases in which one twin has the disorder. The degree of genetic similarity seems to account for the difference between identical and fraternal twins. Further evidence for a genetic influence comes from studies of adopted children with bipolar disorder. These studies show that biological relatives of the children have a higher incidence of bipolar disorder than do people in the general population. Thus, bipolar disorder seems to run in families for genetic reasons.
Personal or work-related stress can trigger a manic episode, but this usually occurs in people with a genetic vulnerability. Other factors - such as prenatal development, childhood experiences, and social conditions - seem to have little influence in causing bipolar disorder. One study examined the children of identical twins in which only one member of each pair of twins had bipolar disorder. The study found that regardless of whether the parent had bipolar disorder or not, all of the children had the same high 10-percent rate of bipolar disorder. This observation clearly suggests that risk for bipolar illness come from genetic influence, not from exposure to a parent’s bipolar illness or from family problems caused by that illness.
Different therapies may shorten, delay, or even prevent the extreme moods caused by bipolar disorder. Lithium carbonate, a natural mineral salt, can help control both mania and depression in bipolar disorder. The drug generally takes two to three weeks to become effective. People with bipolar disorder may take lithium during periods of normal mood to delay or prevent subsequent episodes of mania or depression. Common side effects of lithium include nausea, increased thirst and urination, vertigo, loss of appetite, and muscle weakness. In addition, long-term use can impair functioning of the kidneys. For this reason, doctors do not prescribe lithium to bipolar patients with kidney disease. Many people find the side effects so unpleasant that they stop taking the medication, which often results in relapse.
From 20 to 40 percent of people do not respond to lithium therapy. For these people, two anticonvulsant drugs may help dampen severe manic episodes: carbamazepine (Tegretol) and valproate (Depakene). The use of traditional antidepressants to treat bipolar disorder carries risks of triggering a manic episode or a rapid-cycling pattern.
Anxiety, emotional state in which people feel uneasy, apprehensive, or fearful. People usually experience anxiety about events they cannot control or predict, or about events that seem threatening or dangerous. For example, students taking an important test may feel anxious because they cannot predict the test questions or feel certain of a good grade. People often use the words fear and anxiety to describe the same thing. Fear also describes a reaction to immediate danger characterized by a strong desire to escape the situation.
The physical symptoms of anxiety reflect a chronic ‘readiness’ to deal with some future threat. These symptoms may include fidgeting, muscle tension, sleeping problems, and headaches. Higher levels of anxiety may produce such symptoms as rapid heartbeat, sweating, increased blood pressure, nausea, and dizziness.
All people experience anxiety to some degree. Most people feel anxious when faced with a new situation, such as a first date, or when trying to do something well, such as give a public speech. A mild to moderate amount of anxiety in these situations is normal and even beneficial. Anxiety can motivate people to prepare for an upcoming event and can help keep them focussed on the task at hand.
However, too little anxiety or too much anxiety can cause problems. Individuals who feel no anxiety when faced with an important situation may lack alertness and focus. On the other hand, individuals who experience an abnormally high amount of anxiety often feel overwhelmed, immobilized, and unable to accomplish the task at hand. People with too much anxiety often suffer from one of the anxiety disorders, a group of mental illnesses. In fact, more people experience anxiety disorders than any other type of mental illness. A survey of people aged 15 to 54 in the United States found that about 17 percent of this population suffers from an anxiety disorder during any given year.
The fourth edition of the Diagnostic and Statistical Manual of Mental Disorders, a handbook for mental health professionals, describes a variety of anxiety disorders. These include generalized anxiety disorder, phobias, panic disorder, obsessive-compulsive disorder, and post-traumatic stress disorder.
People with generalized anxiety disorder feel anxious most of the time. They worry excessively about routine events or circumstances in their lives. Their worries often relate to finances, family, personal health, and relationships with others. Although they recognize their anxiety as irrational or out of proportion to actual events, they feel unable to control their worrying. For example, they may worry uncontrollably and intensely about money despite evidence that their financial situation is stable. Children with this disorder typically worry about their performance at school or about catastrophic events, such as tornadoes, earthquakes, and nuclear war.
People with generalized anxiety disorder often find that their worries interfere with their ability to function at work or concentrate on tasks. Physical symptoms, such as disturbed sleep, irritability, muscle aches, and tension, may accompany the anxiety. To receive a diagnosis of this disorder, individuals must have experienced its symptoms for at least six months.
Generalized anxiety disorder affects about 3 percent of people in the general population in any given year. From 55 to 66 percent of people with this disorder are female.
A phobia is an excessive, enduring fear of clearly defined objects or situations that interferes with a person’s normal functioning. Although they know their fear is irrational, people with phobias always try to avoid the source of their fear. Common phobias include fear of heights (acrophobia), fear of enclosed places (claustrophobia), fear of insects, snakes, or other animals, and fear of air travel. Social phobias involve a fear of performing, of critical evaluation, or of being embarrassed in front of other people.
Panic is an intense, overpowering surge of fear. People with panic disorder experience panic attacks - periods of quickly escalating, intense fear and discomfort accompanied by such physical symptoms as rapid heartbeat, trembling, shortness of breath, dizziness, and nausea. Because people with this disorder cannot predict when these attacks will strike, they develop anxiety about having additional panic attacks and may limit their activities outside the home.
In obsessive-compulsive disorder, people persistently experience certain intrusive thoughts or images (obsessions) or feel compelled to perform certain Behaviours (compulsions). Obsessions may include unwanted thoughts about inadvertently poisoning others or injuring a pedestrian while driving. Common compulsions include repetitive hand washing or such mental acts as repeated counting. People with this disorder often perform compulsions to reduce the anxiety produced by their obsessions. The obsessions and compulsions significantly interfere with their ability to function and may consume a great deal of time.
Post-traumatic stress disorder sometimes occurs after people experience traumatic or catastrophic events, such as physical or sexual assaults, natural disasters, accidents, and wars. People with this disorder relive the traumatic event through recurrent dreams or intrusive memories called flashbacks. They avoid things or places associated with the trauma and may feel emotionally detached or estranged from others. Other symptoms may include difficulty sleeping, irritability, and trouble concentrating.
Most anxiety disorders do not have an obvious cause. They result from a combination of biological, psychological, and social factors.
Studies suggest that anxiety disorders run in families. That is, children and close relatives of people with disorders are more likely than most to develop anxiety disorders. Some people may inherit genes that make them particularly vulnerable to anxiety. These genes do not necessarily cause people to be anxious, but the genes may increase the risk of anxiety disorders when certain psychological and social factors are also present.
Anxiety also appears to be related to certain brain functions. Chemicals in the brain called neurotransmitters enable neurons, or brain cells, to interconnect with each other. One neurotransmitter, gamma-amino butyric acid (GABA), appears to play a role in regulating one’s level of anxiety. Lower levels of GABA are associated with higher levels of anxiety. Some studies suggest that the neurotransmitters norepinephrine and serotonin play a role in panic disorder.
Psychologists have proposed a variety of models to explain anxiety. Austrian psychoanalyst Sigmund Freud suggested that anxiety results from internal, unconscious conflicts. He believed that a person’s mind represses wishes and fantasies about which the person feels uncomfortable. This repression, Freud believed, results in anxiety disorders, which he called neuroses.
More recently, behavioural researchers have challenged Freud’s model of anxiety. They believe one’s anxiety level relates to how much a person believes events can be predicted or controlled. Children who have little control over events, perhaps because of overprotective parents, may have little confidence in their ability to handle problems as adults. This lack of confidence can lead to increased anxiety.
Behavioural theorists also believe that children may learn anxiety from a role model, such as a parent. By observing their parent’s anxious response to difficult situations, the child may learn a similar anxious response. A child may also learn anxiety as a conditioned response. For example, an infant often startled by a loud noise while playing with a toy may become anxious just at the sight of the toy. Some experts suggest that people with a high level of anxiety misinterpret normal events as threatening. For instance, they may believe their rapid heartbeat suggests they are experiencing a panic attack when in reality it may be the result of exercise.
While some people may be biologically and psychologically predisposed to feel anxious, most anxiety is triggered by social factors. Many people feel anxious in response to stress, such as a divorce, starting a new job, or moving. Also, how a person expresses anxiety appears to be shaped by social factors. For example, many cultures accept the expression of anxiety and emotion in women, but expect more reserved emotional displays from men.
Mental health professionals use a variety of methods to help people overcome anxiety disorders. These include psychoactive drugs and psychotherapy, particularly behaviour therapy. Other techniques, such as exercise, hypnosis, meditation, and biofeedback, may also prove helpful.
Psychiatrists often prescribe benzodiazepines, a group of tranquillizing drugs, to reduce anxiety in people with high levels of anxiety. Benzodiazepines help to reduce anxiety by stimulating the GABA neurotransmitter system. Common benzodiazepines include alprazolam (Xanax), clonazepam (Klonopin), and diazepam (Valium). Two classes of antidepressant drugs - tricyclics and selective serotonin reuptake inhibitors (SSRIs) - also have proven effective in treating certain anxiety disorders.
Benzodiazepines can work quickly with few unpleasant side effects, but they can also be addictive. In addition, benzodiazepines can slow or impair motor behaviour or thinking and must be used with caution, particularly in elderly persons. SSRIs take longer to work than the benzodiazepines but are not addictive. Some people experience anxiety symptoms again when they stop taking the medications.
Therapists who attribute the cause of anxiety to unconscious, internal conflicts may use psychoanalysis to help people to a better understanding and resolve their conflicts. Other types of psychotherapy, such as cognitive-behavioural therapy, have proven effective in treating anxiety disorders. In cognitive-behavioural therapy, the therapist often educates the person about the nature of his or her particular anxiety disorder. Then, the therapist may help the person to challenge irrational thoughts, that lend toward anxiety. For example, to treat a person with a snake phobia, a therapist might gradually expose the person to snakes, beginning with pictures of snakes and progressing to rubber snakes and real snakes. The patient can use relaxation techniques acquired in therapy to overcome the fear of snakes.
Research has shown psychotherapy to be as effective or more effective than medications in treating many anxiety disorders. Psychotherapy may also provide more lasting benefits than medications when patients ends or terminates his or her treatment.
Panic Disorder the, mental illness in which a person experiences repeated, unexpected panic attacks and persistent anxiety about the possibility that the panic attacks will recur. A panic attack is a period of intense fear, apprehension, or discomfort. In panic disorder, the attacks usually occur without warning. Symptoms include a racing heart, shortness of breath, trembling, choking or smothering sensations, and fears of ‘going crazy,’ losing control, or dying from a heart attack. Panic attacks may last from a few seconds to several hours. Most peak within 10 minutes and end within 20 or 30 minutes.
About 2 percent of people in the United States suffer from panic disorder during any given year, and the condition affects more than twice as many women as men. People with panic disorder may experience panic attacks frequently, such as daily or weekly, or more sporadically. Additionally, panic attacks may occur as part of other anxiety disorders, such as phobias - in which a specific object or situation triggers the attack - and, more rarely, post-traumatic stress disorder.
People with panic disorder frequently develop agoraphobia, a fear of being in places or situations from which escape might be difficult if a panic attack occurs. People with agoraphobia typically fear situations such as travelling in a bus, train, car, or aeroplane, shopping at malls, going to theatres, crossing over bridges or through tunnels, and being alone in unfamiliar places. Therefore, they avoid these situations and may eventually become reluctant to leave their home. In addition, people with panic disorder appear to have an increased risk of alcoholism and drug dependence. Some studies implicate that by direct actions they have had a higher risk of depression and suicide.
Panic disorder: Both with and without agoraphobia, results from a combination of biological and psychological factors. Some individuals may inherit a vulnerability to stress and anxiety and the increased risk of experiencing panic attacks. In addition, certain physiological cues may trigger a panic attack. For example, if a person experiences a racing heart during a panic attack, he or she may begin to associate this sensation with panic attacks. A rapid heartbeat, even if caused by exercise, may then trigger future panic attacks.
Not everyone who experiences a panic attack develops panic disorder. For example, most people experience a rapid heartbeat after running but do not perceive the sensation as dangerous. Those who develop panic disorder tend to interpret their physical sensations as more terrible than they really are. Some psychologists believe that early childhood experiences of separation from important people, such as parents, increase the risk of developing panic disorder.
Mental health professionals usually treat panic disorder with medications, specialized psychotherapy, or a combination of both. Benzodiazepines, a group of tranquillizing drugs that includes alprazolam (Xanax) and diazepam (Valium), often reduce anxiety with few physical side effects. However, these medications can be addictive and may impair movement and concentration in some people. Some antidepressant drugs, such as imipramine (Tofranil), also reduce panic symptoms in some people but can produce side effects such as dizziness or dry mouth. Another class of drugs, selective serotonin reuptake inhibitors (SSRIs), appear to reduce panic symptoms with fewer side effects. SSRIs used to treat panic disorder include paroxetine (Paxil) and fluvoxamine (Luvox). Medication eliminates panic symptoms in 50 to 60 percent of patients. For many patients, however, panic attacks return when they stop taking the medication.
Research has shown that cognitive-behavioural therapy, a type of psychotherapy, eliminates panic attacks in 80 to 100 percent of patients. In this method, therapists help patients re-create the physical symptoms of a panic attack, teach them coping skills, and help them to alter their beliefs about the danger of these sensations. Patients with agoraphobia face their feared situations under the therapist’s supervision, using coping skills to overcome their strong anxiety. These coping skills may include physical relaxation techniques, such as deep breathing and muscle relaxation, as well as cognitive techniques that help people think rationally about anxiety-provoking situations. About 70 percent of panic disorder patients who also have moderate to severe agoraphobia benefit from this type of treatment.
Stress (psychology), is an unpleasant state of emotional and physiological arousal that people experience in situations that they perceive as dangerous or threatening to their well-being. The word stress means different things to different people. Some people define stress as events or situations that cause them to feel tension, pressure, or negative emotions such as anxiety and anger. Others view stress as the response to these situations. This response includes physiological changes - such as increased heart rate and muscle tension - as well as emotional and behavioural changes. However, most psychologists regard stress as a process involving a person’s interpretation and response to a threatening event.
Stress is a common experience. We may feel stress when we are very busy, have important deadlines to meet, or have too little time to finish all of our tasks. Often people experience stress because of problems at work or in social relationships, such as a poor evaluation by a supervisor or an argument with a friend. Some people may be particularly vulnerable to stress in situations involving the threat of failure or personal humiliation. Others have extreme fears of objects or - things associated with physical threats -such as snakes, illness, storms, or flying in an aeroplane and become stressed when they encounter or think about these perceived threats. Major life events, such as the death of a loved one, can cause severe stress.
Stress can have both positive and negative effects. Stress is a normal, adaptive reaction to threat. It signals danger and prepares us to take defensive action. Fear of things that pose realistic threats motivates us to deal with them or avoid them. Stress also motivates us to achieve and fuels creativity. Although stress may hinder performance on difficult tasks, moderate stress seems to improve motivation and performance on less complex tasks. In personal relationships, stress often leads to less cooperation and more aggression.
If not managed appropriately, stress can lead to serious problems. Exposure to chronic stress can contribute to both physical illnesses, such as heart disease, and mental illnesses, such as anxiety disorders. The field of health psychology focuses in part on how stress affects bodily functioning and on how people can use stress management techniques to prevent or minimize disease.
The circumstances that cause stress are called stressors. Stressors vary in severity and duration. For example, the responsibility of caring for a sick parent may be an ongoing source of major stress, whereas getting stuck in a traffic jam may cause mild, short-term stress. Some events, such as the death of a loved one, are stressful for everyone. However, in other situations, individuals may respond differently to the same event—what is a stressor for one person may not be stressful for another. For example, a student who is unprepared for a chemistry test and anticipates a bad grade may feel stress, whereas a classmate who studies in advance may feel confident of a good grade. For an event or situation to be a stressor for a particular individual, the person must appraise the situation as threatening and lack the coping resources to deal with it effectively.
Stressors can be classified into three general categories: catastrophic events, major life changes, and daily hassles. In addition, simply thinking about unpleasant past events or anticipating unpleasant future events can cause stress for many people.
A catastrophe is a sudden, often life-threatening calamity or disaster that pushes people to the outer limits of their coping capability. Catastrophes include natural disasters—such as earthquakes, tornadoes, fires, floods, and hurricanes—as well as wars, torture, automobile accidents, violent physical attacks, and sexual assaults. Catastrophes often continue to affect their victims’ mental health long after the event has ended. For example, in 1972 a dam burst and flooded the West Virginia mining town of Buffalo Creek, destroying the town. Two years after the disaster, most of the adult survivors continued to show emotional disturbances. Similarly, most of the survivors of concentration camps in World War II (1939-1945) continued to experience nightmares and other symptoms of severe emotional problems long after their release from the camps.
The most stressful events for adults involve major life changes, such as death of a spouse or family member, divorce, imprisonment, losing one’s job, and major personal disability or illness. For adolescents, the most stressful events are the death of a parent or a close family member, divorce of their parents, imprisonment of their mother or father, and major personal disability or illness. Sometimes, apparently positive events can have stressful components. For example, a woman who gets a job promotion may receive a higher salary and greater prestige, but she may also feel stress from supervising coworkers who were once peers. Getting married is usually considered a positive experience, but planning the wedding, deciding whom to invite, and dealing with family members may cause couples to feel stressed.
Much of the stress in our lives results from having to deal with daily hassles concerning our jobs, personal relationships, and everyday living circumstances. Many people experience the same hassles every day. Examples of daily hassles include living in a noisy neighbourhood, commuting to work in heavy traffic, disliking one’s fellow workers, worrying about owing money, waiting in a long line, and misplacing or losing things. When taken individually, these hassles may feel like only minor irritants, but cumulatively, over time, they can cause significant stress. The amounts of exposure people have to daily hassles is strongly related to their daily mood. Generally, the greater their exposure is to hassles, the worse is their mood. Studies have found that one’s exposure to daily hassles is more predictive of illness than is exposure to major life events.
A person who is stressed typically has anxious thoughts and difficulty concentrating or remembering. Stress can also change outward Behaviours. Teeth clenching, hand wringing, pacing, nail biting, and heavy breathing are common signs of stress. People also feel physically different when they are stressed. Butterflies in the stomach, cold hands and feet, dry mouth, and increased heart rate are all physiological effects of stress that we associate with the emotion of anxiety.
When a person appraises an event as stressful, the body undergoes a number of changes that heighten physiological and emotional arousal. First, the sympathetic division of the autonomic nervous system is activated. The sympathetic division prepares the body for action by directing the adrenal glands to secrete the hormones epinephrine (adrenaline) and norepinephrine (noradrenaline). In response, the heart begins to beat more rapidly, muscle tension increases, blood pressure rises, and blood flow is diverted from the internal organs and skin to the brain and muscles. Breathing speeds up, the pupils dilate, and perspiration increases. This reaction is sometimes called the fight-or-flight response because it energizes the body to confront either or flee from a threat.
Another part of the stress response involves the hypothalamus and the pituitary gland, parts of the brain that are important in regulating hormones and many other bodily functions. In times of stress, the hypothalamus directs the pituitary gland to secrete adrenocorticotropic hormone. This hormone, in turn, stimulates the outer layer, or cortex, of the adrenal glands to release glucocorticoids, primarily the stress hormone cortisol. Cortisol helps the body access fats and carbohydrates to fuel the fight-or-flight response.
Canadian scientist Hans Selye was one of the first people to study the stress response. As a medical student, Selye noticed that patients with quite different illnesses shared many of the same symptoms, such as muscle weakness, weight loss, and apathy. Selye believed these symptoms might be part of a general response by the body to stress. In the 1930s Selye studied the reactions of laboratory rats to a variety of physical stressors, such as heat, cold, poisons, strenuous exercise, and electric shock. He found that the different stressors all produced a similar response: enlargement of the adrenal glands, shrinkage of the thymus gland (a gland involved in the immune response), and bleeding stomach ulcers.
Selye proposed a three-stage model of the stress response, which he termed the general adaptation syndrome. The three stages in Selye’s model are alarm, resistance, and exhaustion. The alarm stage is a generalized state of arousal during the body’s initial response to the stressor. In the resistance stage, the body adapts to the stressor and continues to resist it with a high level of physiological arousal. When the stress persists for a long time, and the body is chronically overactive, resistance fails and the body moves to the exhaustion stage. In this stage, the body is vulnerable to disease and even death.
Physicians increasingly acknowledge that stress is a contributing factor in a wide variety of health problems. These problems include cardiovascular disorders such as hypertension (high blood pressure); coronary heart disease (coronary atherosclerosis, or narrowing of the heart’s arteries); and gastrointestinal disorders, such as ulcers. Stress also appears to be a risk factor in cancer, chronic pain problems, and many other health disorders. Researchers have clearly identified stress, and specifically a person's characteristic way of responding to stress, as a risk factor for cardiovascular diseases. The release of stress hormones has a cumulative negative effect on the heart and blood vessels. Cortisol, for example, increases blood pressure, which can damage the inside walls of blood vessels. It also increases the free fatty acids in the bloodstream, which in turn leads to plaque buildup on the lining of the blood vessels. As the blood vessels narrow over time it becomes increasingly difficult for the heart to pump sufficient blood through them.
People with certain personality types seem to be physiologically over responsive to stress and therefore more vulnerable to heart disease. For example, the so-called Type A personality is characterized by competitiveness, impatience, and hostility. When Type A people experience stress, their heart rate and blood pressure climb higher and recovery takes longer than with more easygoing people. The most ‘toxic’ personality traits of Type A people are frequent reactions of hostility and anger. These traits are correlated with an increased risk of coronary heart disease.
Stress also appears to influence the development of cancer, but the relationship is not as well established as it is for cardiovascular diseases. There is a moderate positive correlation between extent of exposure and life stressors and cancer n - he more stressors, the greater the likelihood of cancer. In addition, a tendency to cope with unpleasant events in a rigid, unemotional manner is associated with the development and progression of cancer.
Ordinarily the immune system is a marvel of precision. It protects the body from disease by seeking out and destroying foreign invaders, such as viruses and bacteria. Yet there is substantial evidence that stress suppresses the activity of the immune system, leaving an organism more susceptible to infectious diseases. An organism with a weakened immune system is also less able to control naturally occurring mutant cells that overproduce and lead to cancer.
Numerous studies have linked stress with decreased immune response. For example, when laboratory animals are physically restrained, exposed to inescapable electric shocks, or subjected to overcrowding, loud noises, or maternal separation, they show decreased immune system activity. Researchers have reported similar findings for humans. One study, for example, found weakened immune response in people whose spouses had just died. Other studies have documented weakened immune responses among students taking final examinations; people who are severely deprived of sleep; recently divorced or separated men and women; people caring for a family member with Alzheimer’s disease; and people who have recently lost their jobs.
Stress appears to depress immune function in two main ways. First, when people experience stress, they more often engage in Behaviours that have adverse effects on their health: cigarette smoking, using more alcohol or drugs, sleeping less, exercising less, and eating poorly. In addition, stress may alter the immune system directly through hormonal changes. Research indicates that glucocorticoids - hormones that are secreted by the adrenal glands during the stress response - actively suppress the body’s immune system.
At one time scientists believed the immune system functioned more or less as an independent system of the body. They now know that the immune system does not operate by itself, but interacts closely with other bodily systems. The field of psychoneuroimmunology focuses on the relationship between psychological influences (such as stress), the nervous system, and the immune system.
Stress influences mental health as well as physical health. People who experience a high level of stress for a long time - and who cope poorly with this stress - may become irritable, socially withdrawn, and emotionally unstable. They may also have difficulty concentrating and solving problems. Some people under intense and prolonged stress may start to suffer from extreme anxiety, depression, or other severe emotional problems. Anxiety disorders caused by stress may include generalized anxiety disorder, phobias, panic disorder, and obsessive-compulsive disorder. People who survive catastrophes sometimes develop an anxiety disorder called post-traumatic stress disorder. They reexperience the traumatic event again and again in dreams and in disturbing memories or flashbacks during the day. They often seem emotionally numb and may be easily startled or angered.
Coping with stress means using thoughts and actions to deal with stressful situations and lower our stress levels. Many people have a characteristic way of coping with stress based on their personality. People who cope well with stress tend to believe they can personally influence what happens to them. They usually make more positive statements about themselves, resist frustration, remain optimistic, and persevere even under extremely adverse circumstances. Most important, they choose the appropriate strategies to cope with the stressors they confront. Conversely, people who cope poorly with stress tend to have an opposite or opposing personality characteristics, such as lower self-esteem and a pessimistic outlook on life.
Psychologists distinguish two broad types of coping strategies: problem-focussed coping and emotion-focussed coping. The goal of both strategies is to control one’s stress level. In problem-focussed coping, people try to short-circuit negative emotions by taking some action to modify, avoid, or minimize the threatening situation. They change their behaviour to deal with the stressful situation. In emotion-focussed coping, people try to moderate directly or eliminate unpleasant emotions. Examples of emotion-focussed coping include rethinking the situation in a positive way, relaxation, denial, and unrealistic thinking.
To understand these strategies, consider the example of a pre-med student in college who faces three difficult final examinations in a single week. She knows she must get top grades in order to have a chance at acceptance to medical school. This situation is a potential source of stress. To manage she could organize a study group and master the course materials systematically (problem-focussed coping), or she could decide that she needs to relax and collect herself for an hour or so (emotion-focussed coping) before proceeding with an action plan (problem-focussed coping). She might also decide to watch television for hours on end to prevent having to think about or study for her exams (emotion-focussed coping).
Overall, problem-focussed coping is the most effective coping strategy when people have realistic opportunities to change aspects of their situation and reduce stress. Emotion-focussed coping is most useful as a short-term strategy. It can help reduce one’s arousal level before engaging in problem-solving and taking action, and it can help people deal with stressful situations in which there are few problem-focussed coping options.
Support from friends, family members, and others who care for us goes a long way in helping us to get by in times of trouble. Social support systems provide us with emotional sustenance, tangible resources and aid, and information when we are in need. People with social support feel cared about and valued by others and feel a sense of belonging to a larger social network.
A large body of research has linked social support to good health and a superior ability to cope with stress. For example, one long-term study of several thousand California residents found that people with extensive social ties lived longer than those with few close social contacts. Another study found that heart-attack victims who lived alone were nearly twice as likely to have another heart attack as those who lived with someone. Even the perception of social support can help people cope with stress. Studies have found that people’s appraisal of the availability of social support is more closely related to how well they deal with stressors than the actual amount of support they receive or the size of their social network.
Research also suggests that the companionship of animals can help lower stress. For example, one study found that in times of stress, people with pet dogs made fewer visits to the doctor than those without pets.
Biofeedback is a technique in which people learn voluntary control of stress-related physiological responses, such as skin temperature, muscle tension, blood pressure, and heart rate. Normally, people cannot control these responses voluntarily. In biofeedback training, people are connected to an instrument or machine that measures a particular physiological response, such as heart rate, and feeds that measurement back to them in an understandable way. For example, the machine might beep with each heartbeat or display the number of heartbeats per minute on a digital screen. Next, individuals learn to be sensitive to subtle changes inside their body that affect the response system being measured. Gradually, they learn to produce changes in that response system - for example, voluntarily to lower their heart rate. Typically individuals use different techniques and proceed by trial and error until they discover a way to produce the desired changes.
Scientists do not understand the mechanisms by which biofeedback works. Nonetheless, it has become a widely used and generally accepted technique for producing relaxation and lowering physiological arousal in patients with stress-related disorders. One use of biofeedback is in the treatment of tension headaches. By learning to lower muscle tension in the forehead, scalp, and neck, many tension headache sufferers can find long-term relief.
In addition to biofeedback, two other major methods of relaxation are progressive muscular relaxation and meditation. Progressive muscular relaxation involves systematically tensing and then relaxing different groups of skeletal (voluntary) muscles, while directing one’s attention toward the contrasting sensations produced by the two procedures. After practising progressive muscular relaxation, individuals become increasingly sensitive to rising tension levels and can produce the relaxation response during everyday activities (often by repeating a cue word, such as calm, to themselves).
Meditation, in addition to teaching relaxation, is designed to achieve subjective goals such as contemplation, wisdom, and altered states of consciousness. Some forms have a strong Eastern religious and spiritual heritage based in Zen Buddhism and yoga. Other varieties emphasize a particular lifestyle for practitioners. One of the commonalities for giving shape to mediating meditations, is that the properties of transcendental meditation involves in the focussing attentions on or upon the repeating mantra, which is a word, sound, or phrase thought to have particularly calming properties.
Both progressive muscle relaxation and meditation reliably reduce stress-related arousal. They have been used successfully to treat a range of stress-related disorders, including hypertension, migraine and tension headaches, and chronic pain.
Aerobic exercise - such as running, walking, biking, and skiing - can help keep stress levels down. Because aerobic exercise increases the endurance of the heart and lungs, an aerobically fit individual will have a lower heart rate at rest and lower blood pressure, less reactivity to stressors, and quicker recovery from stressors. In addition, studies show that people who exercise regularly have higher self-esteem and suffer less from anxiety and depression than comparable people who are not aerobically fit. The American College of Sports Medicine recommends exercising three to four times a week for at least 20 minutes to reduce the risk of cardiovascular disease.
Neurophysiology is the study of how nerve cells, or neurons, receive and transmit information. Two types of phenomena are involved in processing nerve signals: electrical and chemical. Electrical events propagate a signal within a neuron, and chemical processes transmit the signal from one neuron to another neuron or to a muscle cell.
A neuron is a long cell that has a thick central area containing the nucleus; it also has one long process called an axon and one or more short, bushy processes called dendrites. Dendrites is receive impulses from other neurons. (The exceptions are sensory neurons, such as those that transmit information about temperature or touch, in which the signal is generated by specialized receptors in the skin.) These impulses are propagated electrically along the cell membrane to the end of the axon. At the tip of the axon the signal is chemically transmitted to an adjacent neuron or muscle cell.
The propagation of a nerve cell is like all other cells, for which a neuron contains charged ions: Potassium and sodium (positively charged) and chlorine (negatively charged). Neurons differ from other cells in that they are able to produce a nerve impulse. A neuron is polarized - that is, it has an overall negative charge inside the cell membrane because of the high concentration of chlorine ions and low concentration of potassium and sodium ions. The concentration of these same ions is exactly reversed outside the cell. This charge differential represents stored electrical energy, sometimes referred to as membrane potential or resting potential. The negative charge inside the cell is maintained by two features. The first is the selective permeability of the cell membrane, which is more permeable to potassium than sodium. The second feature is sodium pumps within the cell membrane that actively pump sodium out of the cell. When depolarization occurs, this charge differential across the membrane is reversed, and a nerve impulse is produced.
Depolarization is a rapid change in the permeability of the cell membrane. When sensory information or any other kind of stimulating current is received by the neuron, the membrane permeability is changed, allowing a sudden influx of sodium ions into the cell. The high concentration of sodium, or action potential, changes the overall charge within the cell from negative to positive. The local change in ion concentration triggers similar reactions along the membrane, propagating the nerve impulse. After a brief period called the refractory period, during which the ionic concentration returned to resting potential, the neuron can repeat this process.
Nerve impulses travel at different speeds, depending on the cellular composition of a neuron. Where speed of impulse is important, as in the nervous system, axons are insulated with a membranous substance called myelin. The insulation provided by myelin maintains the ionic charge over long distances. Nerve impulses are propagated at specific points along the myelin sheath; these points are called the nodes of Ranvier. Examples of myelinated axons are those in sensory nerve fibres and nerves connected to skeletal muscles. In non-myelinated cells, the nerve impulse is propagated more diffusely.
When the electrical signal reaches the tip of an axon, it stimulates small presynaptic vesicles in the cell. These vesicles contain chemicals called neurotransmitters, which are released into the microscopic space between neurons (the synaptic cleft). The neurotransmitters attach to specialized receptors on the surface of the adjacent neuron. This stimulus causes the adjacent cell to depolarize and propagate an action potential of its own. The duration of a stimulus from a neurotransmitter is limited by the breakdown of the chemicals in the synaptic cleft and the reuptake by the neuron that produced them. Formerly, each neuron was thought to make only one transmitter, but recent studies have shown that some cells make two or more.
Once the brain reaches maturity, the number of neurons does not increase, and any neurons that are damaged are permanently disabled. Nonetheless, the plasticity of the brain can greatly benefit people with damage to the brain and nervous system. Organisms can compensate for loss by strengthening old neural connections and sprouting new ones. That is why people who suffer strokes are often able to recover their lost speech and motor abilities.
Neurotransmitters is a chemical made by neurons, or nerve cells. Neurons send out neurotransmitters as chemical signals to activate or inhibit the function of neighbouring cells.
Within the central nervous system, which consists of the brain and the spinal cord, neurotransmitters pass from neuron to neuron. In the peripheral nervous system, which is made up of the nerves that run from the central nervous system to the rest of the body, the chemical signals pass between a neuron and an adjacent muscle or gland cell.
Nine chemical compounds—belonging to three chemical families—are widely recognized as neurotransmitters. In addition, certain other body chemicals, including adenosine, histamine, enkephalins, endorphins, and epinephrine, have neurotransmitter like properties. Experts believe that there are many more neurotransmitters as yet undiscovered.
The first of the three families is composed of amines, a group of compounds containing molecules of carbon, hydrogen, and nitrogen. Surrounded by the amine neurotransmitters, are the acetylcholine, norepinephrine, dopamine, and serotonin. Where Acetylcholine is the most widely used neurotransmitter in the body, and neurons that leave the central nervous system (for example, those running to skeletal muscle) use acetylcholine as their neurotransmitter; neurons that run to the heart, blood vessels, and other organs may use acetylcholine or norepinephrine. Dopamine is involved in the movement of muscles, and it controls the secretion of the pituitary hormone prolactin, which triggers milk production in nursing mothers.
The second neurotransmitter family is composed of amino acids, organic compounds containing both an amino group (NH2) and a carboxylic acid group (COOH). Amino acids that serve as neurotransmitters include glycine, glutamic and aspartic acids, and gamma-amino butyric acid (GABA). Glutamic acid and GABA are the most abundant neurotransmitters within the central nervous system, and especially in the cerebral cortex, which is largely responsible for such higher brain functions as thought and interpreting sensations.
The third neurotransmitter family is composed of peptides, which are compounds that contain at least 2, and sometimes as many as 100 amino acids. Peptide neurotransmitters are poorly understood, but scientists know that the peptide neurotransmitter called substance P influences the sensation of pain.
Overall, each neuron uses only a single compound as its neurotransmitter. However, some neurons outside the central nervous system are able to release both an amine and a peptide neurotransmitter.
Neurotransmitters are manufactured from precursor compounds like amino acids, glucose, and the dietary amine-called choline. Neurons modify the structure of these precursor compounds in a series of reactions with enzymes. Neurotransmitters that come from amino acids include serotonin, which is derived from tryptophan; dopamine and norepinephrine, which are derived from tyrosine; and glycine, which is derived from threonine. Among the neurotransmitters made from glucose are glutamate, aspartame, and GABA. Choline serves as the precursor for acetylcholine.
In the nervous system, a message-carrying impulse travels from one end of a nerve cell to the other by means of an electrical impulse. When it reaches the terminal end of a nerve cell, the impulse triggers tiny sacs called presynaptic vessicles to release their contents, chemical messengers called neurotransmitters. The neurotransmitters float across the synapse, or gap between adjacent nerve cells. When they reach the neighbouring nerve cell, the neurotransmitters fit into specialized receptor sites much as a key fits into a lock, causing that nerve cell to ‘fire,’ or generate an electric message-carrying impulse. As the message continues through the nervous system, the presynaptic cell absorbs the excess neurotransmitters, and repackages them in presynaptic vessicles in a process called neurotransmitter reuptake.
Transmitters are released into a microscopic gap, called a synapse, that separates the transmitting neuron from the cell receiving the chemical signal. The cell that generates the signal is called the presynaptic cell, while the receiving cell is termed the postsynaptic cell.
After their release into the synapse, neurotransmitters combine chemically with highly specific protein molecules, termed receptors, that are embedded in the surface membranes of the postsynaptic cell. When this combination occurs, the voltage, or electrical force, of the postsynaptic cell is either increased (excited) or decreased (inhibited).
When a neuron is in its resting state, its voltage is about -70 millivolts. An excitatory neurotransmitter alters the membrane of the postsynaptic neuron, making it possible for ions (electrically charged molecules) to move back and forth across the neuron’s membranes. This flow of ions makes the neuron’s voltage rise toward zero. If enough excitatory receptors have been activated, the postsynaptic neuron responds by firing, generating a nerve impulse that causes its own neurotransmitter to be released into the next synapse. An inhibitory neurotransmitter causes different ions to pass back and forth across the postsynaptic neuron’s membrane, lowering the nerve cell’s voltage to -80 or -90 millivolts. The drop in voltage makes it less likely that the postsynaptic cell will fire.
If the postsynaptic cell is a muscle cell rather than a neuron, an excitatory neurotransmitter will cause the muscle to contract. If the postsynaptic cell is a gland cell, an excitatory neurotransmitter will cause the cell to secrete its contents.
While most neurotransmitters interact with their receptors to create new electrical nerve impulses that energize or inhibit the adjoining cell, some neurotransmitter interactions do not generate or suppress nerve impulses. Instead, they interact with a second type of receptor that changes the internal chemistry of the postsynaptic cell by either causing or blocking the formation of chemicals called second messenger molecules. These second messengers regulate the postsynaptic cell’s biochemical processes and enable it to conduct the maintenance necessary to continue synthesizing neurotransmitters and conducting nerve impulses. Examples of second messengers, which are formed and entirely contained within the postsynaptic cell, include cyclic adenosine monophosphate, diacylglycerol, and inositol phosphates.
Once neurotransmitters have been secreted into synapses and have passed on their chemical signals, the presynaptic neuron clears the synapse of neurotransmitter molecules. For example, acetylcholine is broken down by the enzyme acetylcholinesterase into choline and acetate. Neurotransmitters like dopamine, serotonin, and GABA are removed by a physical process called reuptake. In reuptake, a protein in the presynaptic membrane acts as a sort of sponge, causing the neurotransmitters to reenter the presynaptic neuron, where they can be broken down by enzymes or repackaged for reuse.
Neurotransmitters are known to be involved in a number of disorders, including Alzheimer’s disease. Victims of Alzheimer’s disease suffer from loss of intellectual capacity, disintegration of personality, mental confusion, hallucinations, and aggressive—even violent - behaviour. These symptoms are the result of progressive degeneration in many types of neurons in the brain. Forgetfulness, one of the earliest symptoms of Alzheimer’s disease, is partly caused by the destruction of neurons that normally release the neurotransmitter acetylcholine. Medications that increase brain levels of acetylcholine have helped restore short-term memory and reduce mood swings in some Alzheimer’s patients.
Neurotransmitters also play a role in Parkinson disease, which slowly attacks the nervous system, causing symptoms that worsen over time. Fatigue, mental confusion, a mask like facial expression, stooping posture, shuffling gait, and problems with eating and speaking are among the difficulties suffered by Parkinson victims. These symptoms have been partly linked to the deterioration and eventual death of neurons that run from the base of the brain to the basal ganglia, a collection of nerve cells that manufacture the neurotransmitter dopamine. The reasons why such neurons die are yet to be understood, but the related symptoms can be alleviated. L-dopa, or levodopa, widely used to treat Parkinson disease, acts as a supplementary precursor for dopamine. It causes the surviving neurons in the basal ganglia to increase their production of dopamine, thereby compensating to some extent for the disabled neurons.
Many other effective drugs have been shown to act by influencing neurotransmitter behaviour. Some drugs work by interfering with the interactions between neurotransmitters and intestinal receptors. For example, belladonna decreases intestinal cramps in such disorders as irritable bowel syndrome by blocking acetylcholine from combining with receptors. This process reduces nerve signals to the bowel wall, which prevents painful spasms.
Other drugs block the reuptake process. One well-known example is the drug fluoxetine (Prozac), which blocks the reuptake of serotonin. Serotonin then remains in the synapse for a longer time, and its ability to act as a signal is prolonged, which contributes to the relief of depression and the control of obsessive-compulsive behaviours
Roderick MacKinnon, born in 1956, an American biomedical researcher and cowinner of the 2003 Nobel Prize in chemistry for his discoveries involving ion channels, the pores that govern the passage of molecules into and out of cells. Every second in each of the billions of cells in the human body, millions of ions, such as potassium and sodium, shuttle back and forth through these special portals in the cellular membrane. This action underlies a range of physiological processes, including muscle contraction and the communication of impulses between nerve cells. MacKinnon and his colleagues were the first to show the detailed structure of one type of ion channel.
Born in 1956, MacKinnon grew up in Burlington, Massachusetts, outside Boston. He earned his bachelor’s degree in biochemistry from Brandeis University in Waltham, Massachusetts, in 1978, and his medical degree from Tufts University School of Medicine in Boston in 1982. After beginning a career in medicine, MacKinnon turned to biomedical research. Postdoctoral fellowships at Harvard University in Cambridge, Massachusetts, and Brandeis ultimately led to a professorship in the Department of Neurobiology at Harvard Medical School in 1989. In 1996 MacKinnon moved to Rockefeller University in New York City, where he became professor of molecular Neurobiology and biophysics.
To study an ion channel - in this case, a particular cellular protein involved in the transport - of potassium. As MacKinnon chose a difficult method known as X-ray crystallography. This method involves forming the protein into a crystal and then using X rays to determine the protein’s structure. Many scientists doubted that the approach would work, but in 1998 MacKinnon and his team achieved success, presenting a detailed three-dimensional picture of the potassium channel.
In subsequent research, MacKinnon and his colleagues discovered more about the chemical workings of ion channels. This work helped to explain, for example, how such a pore permits the passage of millions of potassium ions per second while largely blocking the passage of sodium ions. Increased knowledge of these protein pores will be important for the design of future drugs because the malfunctioning of ion channels has been linked to heart disease and cystic fibrosis, among other illnesses.
Schizophrenia, is a severe mental illness characterized by a variety of symptoms, including loss of contact with reality, bizarre behaviour, disorganized thinking and speech, decreased emotional expressiveness, and social withdrawal. Usually only some of these symptoms occur in any one person. The term schizophrenia comes from Greek words meaning ‘split mind.’ belief, schizophrenia does not refer to a person with a split personality or multiple personality, to observers, schizophrenia may seem like madness or insanity.
Perhaps more than any other mental illness, schizophrenia has a debilitating effect on the lives of the people who suffer from it. A person with schizophrenia may have difficulty telling the difference between real and unreal experiences, logical and illogical thoughts, or appropriate and inappropriate behaviour. Schizophrenia seriously impairs a person’s ability to work, go to school, enjoy relationships with others, or take care of oneself. In addition, people with schizophrenia frequently require hospitalization because they pose a danger to themselves. About 10 percent of people with schizophrenia commit suicide, and many others attempt suicide. Once people develop schizophrenia, they usually suffer from the illness for the rest of their lives. Although there is no cure, treatment can help many people with schizophrenia lead productive lives.
Schizophrenia also carries an enormous cost to society. People with schizophrenia occupy about one-third of all beds in psychiatric hospitals in the United States. In addition, people with schizophrenia account for at least 10 percent of the homeless population in the United States. The National Institute of Mental Health has estimated that schizophrenia costs the United States tens of billions of dollars each year in direct treatment, social services, and lost productivity.
Approximately 1 percent of people develop schizophrenia at some time during their lives. Experts estimate that about 1.8 million people in the United States have schizophrenia. The prevalence of schizophrenia is the same regardless of sex, race, and culture. Although women are just as likely as men to develop schizophrenia, women tend to experience the illness less severely, with fewer hospitalizations and better social functioning in the community.
Schizophrenia usually develops in late adolescence or early adulthood, between the ages of 15 and 30. Much less commonly, schizophrenia develops later in life. The illness may begin abruptly, but it usually develops slowly over months or years. Mental health professionals diagnose schizophrenia based on an interview with the patient in which they determine whether the person has experienced specific symptoms of the illness.
Symptoms and functioning in people with schizophrenia tend to vary over time, sometimes worsening and other times improving. For many patients the symptoms gradually become less severe as they grow older. About 25 percent of people with schizophrenia become symptom-free later in their lives.
A variety of symptoms characterize schizophrenia. The most prominent include symptoms of psychosis - such as delusions and hallucinations - as well as bizarre behaviour, strange movements, and disorganized thinking and speech. Many people with schizophrenia do not recognize that their mental functioning is disturbed.
Some people with schizophrenia experience delusions of persecution—false beliefs that other people are plotting against them. This interview between a patient with schizophrenia and his therapist illustrates the paranoia that can affect people with this illness.
Delusions are false beliefs that appear obviously untrue to other people. For example, a person with schizophrenia may believe that he is the king of England when he is not. People with schizophrenia may have delusions that others, such as the police or the FBI, are plotting against them or spying on them. They may believe that aliens are controlling their thoughts or that their own thoughts are being broadcast to the world so that other people can hear them.
People with schizophrenia may also experience hallucinations (false sensory perceptions). People with hallucinations see, hear, smell, feel, or taste things that are not really there. Auditory hallucinations, such as hearing voices when no one else is around, are especially common in schizophrenia. These hallucinations may include two or more voices conversing with each other, voices that continually comment on the person’s life, or voices that command the person to do something.
People with schizophrenia often behave bizarrely. They may talk to themselves, walk backward, laugh suddenly without explanation, make funny faces, or masturbate in public. In rare cases, they maintain a rigid, bizarre pose for hours on end. Alternately, they may engage in constant random or repetitive movements.
People with schizophrenia sometimes talk in incoherent or nonsensical ways, which suggests confused or disorganized thinking. In conversation they may jump from topic to topic or string together loosely associated phrases. They may combine words and phrases in meaningless ways or make up new words. In addition, they may show poverty of speech, in which they talk less and more slowly than other people, fail to answer questions or reply only briefly, or suddenly stop talking in the middle of speech.
Another common characteristic of schizophrenia is social withdrawal. People with schizophrenia may avoid others or act as though others do not exist. They often show decreased emotional expressiveness. For example, they may talk in a low, monotonous voice, avoid eye contact with others, and display a blank facial expression. They may also have difficulties experiencing pleasure and may lack interest in participating in activities.
Other symptoms of schizophrenia include difficulties with memory, attention span, abstract thinking, and planning ahead. People with schizophrenia commonly have problems with anxiety, depression, and suicidal thoughts. In addition, people with schizophrenia are much more likely to abuse or become dependent upon drugs or alcohol than other people. The use of alcohol and drugs often worsens the symptoms of schizophrenia, resulting in relapses and hospitalizations.
Schizophrenia appears to result not from a single cause, but from a variety of factors. Most scientists believe that schizophrenia is a biological disease caused by genetic factors, an imbalance of chemicals in the brain, structural brain abnormalities, or abnormalities in the prenatal environment. In addition, stressful life events may contribute to the development of schizophrenia in those who are predisposed to the illness.
Research shows that the more genetically related a person is to someone with schizophrenia, the greater the risk that person has of developing the illness. For example, children of one parent with schizophrenia have a 13 percent chance of developing the illness, whereas children of two parents with schizophrenia have a 46 percent chance of developing the disorder.
Research suggests that the genes one inherits strongly influence one’s risk of developing schizophrenia. Studies of families have shown that the more closely one is related to someone with schizophrenia, the greater the risk one has of developing the illness. For example, the children of one parent with schizophrenia have about a 13 percent chance of developing the illness, and children of two parents with schizophrenia have about a 46 percent chance of eventually developing schizophrenia. This increased risk occurs even when such children are adopted and raised by mentally healthy parents. In comparison, children in the general population have only about a 1 percent chance of developing schizophrenia.
Some evidence suggests that schizophrenia may result from an imbalance of chemicals in the brain called neurotransmitters. These chemicals enable neurons (brain cells) to communicate with each other. Some scientists suggest that schizophrenia results from excess activity of the neurotransmitter dopamine in certain parts of the brain or from an abnormal sensitivity to dopamine. Support for this hypothesis comes from antipsychotic drugs, which reduce psychotic symptoms in schizophrenia by blocking brain receptors for dopamine. In addition, amphetamines, which increase dopamine activity, intensify psychotic symptoms in people with schizophrenia. Despite these findings, many experts believe that excess dopamine activity alone cannot account for schizophrenia. Other neurotransmitters, such as serotonin and norepinephrine, may play important roles as well.
Magnetic resonance imaging (MRI) reveals structural differences between a normal adult brain, left, and the brain of a person with schizophrenia, right. The schizophrenic brain has enlarged ventricles (fluid-filled cavities), shown in light gray. However, not all people with schizophrenia show this abnormality.
Brain imaging techniques, such as magnetic resonance imaging and positron-emission tomography, have led researchers to discover specific structural abnormalities in the brains of people with schizophrenia. For example, people with chronic schizophrenia tend to have enlarged brain ventricles (cavities in the brain that contain cerebrospinal fluid). They also have a smaller overall volume of brain tissue compared to mentally healthy people. Other people with schizophrenia show abnormally low activity in the frontal lobe of the brain, which governs abstract thought, planning, and judgment. Research has identified possible abnormalities in many other parts of the brain, including the temporal lobes, basal ganglia, thalamus, Hippocampus, and superior temporal gyrus. These defects may partially explain the abnormal thoughts, perceptions, and Behaviours that characterize schizophrenia.
Evidence suggests that factors in the prenatal environment and during birth can increase the risk of a person later developing schizophrenia. These events are believed to affect the brain development of the fetus during a critical period. For example, pregnant women who have been exposed to the influenza virus or who have poor nutrition have a slightly increased chance of giving birth to a child who later develops schizophrenia. In addition, obstetric complications during the birth of a child - for example, delivery with forceps - can slightly increase the chances of the child later developing schizophrenia.
Although scientists favour a biological cause of schizophrenia, stress in the environment may affect the onset and course of the illness. Stressful life circumstances—such as growing up and living in poverty, the death of a loved one, an important change in jobs or relationships, or chronic tension and hostility at home—can increase the chances of schizophrenia in a person biologically predisposed to the disease. In addition, stressful events can trigger a relapse of symptoms in a person who already has the illness. Individuals who have effective skills for managing stress may be less susceptible to its negative effects. Psychological and social rehabilitation can help patients develop more effective skills for dealing with stress.
Although there is no cure for schizophrenia, effective treatment exists that can improve the long-term course of the illness. With many years of treatment and rehabilitation, significant numbers of people with schizophrenia experience partial or full remission of their symptoms.
Treatment of schizophrenia usually involves a combination of medication, rehabilitation, and treatment of other problems the person may have. Antipsychotic drugs (also called neuroleptics) are the most frequently used medications for treatment of schizophrenia. Psychological and social rehabilitation programs may help people with schizophrenia function in the community and reduce stress related to their symptoms. Treatment of secondary problems, such as substance abuse and infectious diseases, is also an important part of an overall treatment program.
Antipsychotic medications, developed in the mid-1950s, can dramatically improve the quality of life for people with schizophrenia. The drugs reduce or eliminate psychotic symptoms such as hallucinations and delusions. The medications can also help prevent these symptoms from returning. Common antipsychotic drugs include risperidone (Risperdal), olanzapine (Zyprexa), clozapine (Clozaril), quetiapine (Seroquel), haloperidol (Haldol), thioridazine (Mellaril), chlorpromazine (Thorazine), fluphenazine (Prolixin), and trifluoperazine (Stelazine). People with schizophrenia usually must take medication for the rest of their lives to control psychotic symptoms. Antipsychotic medications appear to be less effective at treating other symptoms of schizophrenia, such as social withdrawal and apathy.
Antipsychotic drugs help reduce symptoms in 80 to 90 percent of people with schizophrenia. However, those who benefit often stop taking medication because they do not understand that they are ill or because of unpleasant side effects. Minor side effects include weight gain, dry mouth, blurred vision, restlessness, constipation, dizziness, and drowsiness. Other side effects are more serious and debilitating. These may include muscle spasms or cramps, tremors, and tardive dyskinesia, an irreversible condition marked by uncontrollable movements of the lips, mouth, and tongue. Newer drugs, such as clozapine, olanzapine, risperidone, and quetiapine, tend to produce fewer of these side effects. However, clozapine can cause agranulocytosis, a significant reduction in white blood cells necessary to fight infections. This condition can be fatal if not detected early enough. For this reason, people taking clozapine must have weekly tests to monitor their blood.
Because many patients with schizophrenia continue to experience difficulties despite taking medication, psychological and social rehabilitation is often necessary. A variety of methods can be effective. Social skills training helps people with schizophrenia learn specific behaviours for functioning in society, such as making friends, purchasing items at a store, or initiating conversations. Behavioural training methods can also help them learn self-care skills such as personal hygiene, money management, and proper nutrition. In addition, cognitive-behavioural therapy, a type of psychotherapy, can help reduce persistent symptoms such as hallucinations, delusions, and social withdrawal.
Family intervention programs can also benefit people with schizophrenia. These programs focus on helping family members understand the nature and treatment of schizophrenia, how to monitor the illness, and how to help the patient make progress toward personal goals and greater independence. They can also lower the stress experienced by everyone in the family and help prevent the patient from relapsing or being rehospitalized.
Because many patients have difficulty obtaining or keeping jobs, supported employment programs that help patients find and maintain jobs are a helpful part of rehabilitation. In these programs, the patient works alongside people without disabilities and earns competitive wages. An employment specialist (or vocational specialist) helps the person maintain their job by, for example, training the person in specific skills, helping the employer accommodate the person, arranging transportation, and monitoring performance. These programs are most effective when the supported employment is closely integrated with other aspects of treatment, such as medication and monitoring of symptoms.
Some people with schizophrenia are vulnerable to frequent crises because they do not regularly go to mental health centres to receive the treatment they need. These individuals often relapse and face rehospitalization. To ensure that such patients take their medication and receive appropriate psychological and social rehabilitation, assertive community treatment (ACT) programs have been developed that deliver treatment to patients in natural settings, such as in their homes, in restaurants, or on the street.
People with schizophrenia often have other medical problems, so an effective treatment program must attend to these as well. One of the most common associated problems is substance abuse. Successful treatment of substance abuse in patients with schizophrenia requires careful coordination with their mental health care, so that the same clinicians are treating both disorders at the same time.
The high rate of substance abuse in patients with schizophrenia contributes to a high prevalence of infectious diseases, including hepatitis B and C and the human immunodeficiency virus (HIV). Assessment, education, and treatment or management of these illnesses is critical for the long-term health of patients.
Other problems frequently associated with schizophrenia include housing instability and homelessness, legal problems, violence, trauma and post-traumatic stress disorder, anxiety, depression, and suicide attempts. Close monitoring and psychotherapeutic interventions are often helpful in addressing these problems.
Several other psychiatric disorders are closely related to schizophrenia. In schizoaffective disorder, a person shows symptoms of schizophrenia combined with either mania or severe depression. Schizophreniform disorder refers to an illness in which a person experiences schizophrenic symptoms for more than one month but fewer than six months. In schizotypal personality disorder, a person engages in odd thinking, speech, and behaviour, but usually does not lose contact with reality. Sometimes mental health professionals refer to these disorders together as schizophrenia-spectrum disorders.
The signals conveying everything that human beings sense and think, and every motion they make, follow nerve pathways in the human body as waves of ions (atoms or groups of atoms that carry electric charges). Australian physiologist Sir John Eccles discovered many of the intricacies of this electrochemical signalling process, particularly the pivotal step in which a signal is conveyed from one nerve cell to another.
How does one nerve cell transmit the nerve impulse to another cell? Electron microscopy and other methods show that it does so by means of special extensions that deliver a squirt of transmitter substance
The human brain is the most highly organized form of matter known, and in complexity the brains of the other higher animals are not greatly inferior. For certain purposes it is expedient to regard the brain as being analogous to a machine. Even if it is so regarded, however, it is a machine of a totally different kind from those made by man. In trying to understand the workings of his own brain man meets his highest challenge. Nothing is given; there are no operating diagrams, no maker's instructions.
The first step in trying to understand the brain is to examine its structure in order to discover the components from which it is built and how they are related to one another. After that one can attempt to understand the mode of operation of the simplest components. These two modes of investigation - the morphological and the physiological - have now become complementary. In studying the nervous system with today's sensitive electrical devices, however, it is all too easy to find physiological events that cannot be correlated with any known anatomical structure. Conversely, the electron microscope reveals many structural details whose physiological significance is obscure or unknown.
At the close of the past century the Spanish anatomist Santiago Ramón y Cajal showed how all parts of the nervous system are built up of individual nerve cells of many different shapes and sizes. Like other cells, each nerve cell has a nucleus and a surrounding cytoplasm. Its outer surface consists of numerous fine branches - the dendrites - that receive nerve impulses from other nerve - cells, and one relatively long branch - the axonthat transmits nerve impulses. Near its end the axon divides into branches that terminate at the dendrites or bodies of other nerve cells. The axon can be as short as a fraction of a millimetre or as long as a metre, depending on its place and function. It has many of the properties of an electric cable and is uniquely specialized to conduct the brief electrical waves called nerve impulses. In very thin axons these impulses travel at less than one metre per second; in others, for example in the large axons of the nerve cells that activate muscles, they travel as fast as 100 metres per second.
The electrical impulse that travels along the axon ceases abruptly when it comes to the point where the axon's terminal fibres make contact with another nerve cell. These junction points were given the name ‘synapses’ by Sir Charles Sherrington, who laid the foundations of what is sometimes called synaptology. If the nerve impulse is to continue beyond the synapse, it must be regenerated afresh on the other side. As recently as 15 years ago some physiologists held that transmission at the synapse was predominantly, if not exclusively, an electrical phenomenon. Now, however, there is abundant evidence that transmission is effectuated by the release of specific chemical substances that trigger a regeneration of the impulse. In fact, the first strong evidence showing that a transmitter substance acts across the synapse was provided more than 40 years ago by Sir Henry Dale and Otto Loewi.
It has been estimated that the human central nervous system, which of course includes the spinal cord as well as the brain itself, consists of about 10 billion (1010) nerve cells. With rare exceptions each nerve cell receives information directly in the form of impulses from many other nerve cells—often hundreds—and transmits information to a like number. Depending on its threshold of response, a given nerve cell may fire an impulse when stimulated by only a few incoming fibres or it may not fire until stimulated by many incoming fibres. It has long been known that this threshold can be raised or lowered by various factors. Moreover, it was conjectured some 60 years ago that some of the incoming fibres must inhibit the firing of the receiving cell rather than excite it. The conjecture was subsequently confirmed, and the mechanism of the inhibitory effect has now been clarified. This mechanism and its equally fundamental counterpart - nerve-cell excitation - are the subject of this article.
At the level of anatomy there are some clues to indicate how the fine axon terminals impinging on a nerve cell can make the cell regenerate a nerve impulse of its own … a nerve cell and its dendrites are covered by fine branches of nerve fibres that terminate in knoblike structures. These structures are the synapses.
The electron microscope has revealed structural details of synapses that fit in nicely with the view that a chemical transmitter is involved in nerve transmission. Enclosed in the synaptic knob are many vesicles, or tiny sacs, which appear to contain the transmitter substances that induce synaptic transmission. Between the synaptic knob and the synaptic membrane of the adjoining nerve cell is a remarkably uniform space of about 20 millimicrons that is termed the synaptic cleft. Many of the synaptic vesicles are concentrated adjacent to this cleft; it seems plausible that the transmitter substance is discharged from the nearest vesicles into the cleft, where it can act on the adjacent cell membrane. This hypothesis is supported by the discovery that the transmitter is released in packets of a few thousand molecules.
The study of synaptic transmission was revolutionized in 1951 by the introduction of delicate techniques for recording electrically from the interior of single nerve cells. This is done by inserting into the nerve cell an extremely fine glass pipette with a diameter of .5 micron—about a fifty-thousandth of an inch. The pipette is filled with an electrically conducting salt solution such as concentrated potassium chloride. If the pipette is carefully inserted and held rigidly in place, the cell membrane appears to seal quickly around the glass, thus preventing the flow of a short-circuiting current through the puncture in the cell membrane. Impaled in this fashion, nerve cells can function normally for hours. Although there is no way of observing the cells during the insertion of the pipette, the insertion can be guided by using as clues the electric signals that the pipette picks up when close to active nerve cells.
When my colleagues and I in New Zealand and later at the John Curtin School of Medical Research in Canberra first employed this technique, we chose to study the large nerve cells called motoneurons, which lie in the spinal cord and whose function is to activate muscles. This was a fortunate choice: intracellular investigations with motoneurons have proved to be easier and more rewarding than those with any other kind of mammalian nerve cell.
We soon found that when the nerve cell responds to the chemical synaptic transmitter, the response depends in part on characteristic features of ionic composition that are also concerned with the transmission of impulses in the cell and along its axon. When the nerve cell is at rest, its physiological makeup resembles that of most other cells in that the water solution inside the cell is quite different in composition from the solution in which the cell is bathed. The nerve cell is able to exploit this difference between external and internal composition and use it in quite different ways for generating an electrical impulse and for synaptic transmission.
The composition of the external solution is well established because the solution is essentially the same as blood from which cells and proteins have been removed. The composition of the internal solution is known only approximately. Indirect evidence indicates that the concentrations of sodium and chloride ions outside the cell are respectively some 10 and 14 times higher than the concentrations inside the cell. In contrast, the concentration of potassium ions inside the cell is about 30 times higher than the concentration outside.
How can one account for this remarkable state of affairs? Part of the explanation is that the inside of the cell is negatively charged with respect to the outside of the cell by about 70 millivolts. Since like charges repel each other, this internal negative charge tends to drive chloride ions (Cl-) outward through the cell membrane and, at the same time, to impede their inward movement. In fact, a potential difference of 70 millivolts is just sufficient to maintain the observed disparity in the concentration of chloride ions inside the cell and outside it; chloride ions diffuse inward and outward at equal rates. A drop of 70 millivolts across the membrane therefore defines the ‘equilibrium potential’ for chloride ions.
To obtain a concentration of potassium ions (K+) that is 30 times higher inside the cell than outside would require that the interior of the cell membrane be about 90 millivolts negative with respect to the exterior. Since the actual interior is only 70 millivolts negative, it falls short of the equilibrium potential for potassium ions by 20 millivolts. Evidently the thirtyfold concentration can be achieved and maintained only if there is some auxiliary mechanism for ‘pumping’ potassium ions into the cell at a rate equal to their spontaneous net outward diffusion.
The pumping mechanism has the still more difficult task of pumping sodium ions (Na+) out of the cell against a potential gradient of 130 millivolts. This figure is obtained by adding the 70 millivolts of internal negative charge to the equilibrium potential for sodium ions, which is 60 millivolts of internal positive charge. If it were not for this postulated pump, the concentration of sodium ions inside and outside the cell would be almost the reverse of what is observed.
In their classic studies of nerve-impulse transmission in the giant axon of the squid, A. L. Hodgkin, A. F. Huxley and Bernhard Katz of Britain demonstrated that the propagation of the impulse coincides with abrupt changes in the permeability of the axon membrane. When a nerve impulse has been triggered in some way, what can be described as a gate opens and lets sodium ions pour into the axon during the advance of the impulse, making the interior of the axon locally positive. The process is self-reinforcing in that the flow of some sodium ions through the membrane opens the gate further and makes it easier for others to follow. The sharp reversal of the internal polarity of the membrane constitutes the nerve impulse, which moves like a wave until it has travelled the length of the axon. In the wake of the impulse the sodium gate closes and a potassium gate opens, thereby restoring the normal polarity of the membrane within a millisecond or less.
With this understanding of the nerve impulse in hand, one is ready to follow the electrical events at the excitatory synapse. One might guess that if the nerve impulse results from an abrupt inflow of sodium ions and a rapid change in the electrical polarity of the axon's interior, something similar must happen at the body and dendrites of the nerve cell in order to generate the impulse in the first place. Indeed, the function of the excitatory synaptic terminals on the cell body and its dendrites is to depolarize the interior of the cell membrane essentially by permitting an inflow of sodium ions. When the depolarization reaches a threshold value, a nerve impulse is triggered.
As a simple instance of this phenomenon we have recorded the depolarization that occurs in a single motoneuron activated directly by the large nerve fibres that enter the spinal cord from special stretch-receptors known as annulospiral endings. These receptors in turn are located in the same muscle that is activated by the motoneuron under study. Thus the whole system forms a typical reflex arc, such as the arc responsible for the patellar reflex, or ‘knee jerk.’
To conduct the experiment we anaesthetize an animal (most often a cat) and free by dissection a muscle nerve that contains these large nerve fibres. By applying a mild electric shock to the exposed nerve one can produce a single impulse in each of the fibres; since the impulses travel to the spinal cord almost synchronously they are referred to collectively as a volley. The number of impulses contained in the volley can be reduced by reducing the stimulation applied to the nerve. The volley strength is measured at a point just outside the spinal cord and is displayed on an oscilloscope. About half a millisecond after detection of a volley there is a wavelike change in the voltage inside the motoneuron that has received the volley. The change is detected by a microelectrode inserted in the motoneuron and is displayed on another oscilloscope.
What we find is that the negative voltage inside the cell becomes progressively less negative as more of the fibres impinging on the cell are stimulated to fire. This observed depolarization is in fact a simple summation of the depolarizations produced by each individual synapse. When the depolarization of the interior of the motoneuron reaches a critical point, a ‘spike’ suddenly appears on the second oscilloscope, showing that a nerve impulse has been generated. During the spike the voltage inside the cell changes from about 70 millivolts negative to as much as 30 millivolts positive. The spike regularly appears when the depolarization, or reduction of membrane potential, reaches a critical level, which is usually between 10 and 18 millivolts. The only effect of a further strengthening of the synaptic stimulus is to shorten the time needed for the motoneuron to reach the firing threshold. The depolarizing potentials produced in the cell membrane by excitatory synapses are called excitatory postsynaptic potentials, or EPSP's.
Through one barrel of a double-barrelled microelectrode one can apply a background current to change the resting potential of the interior of the cell membrane, either increasing it or decreasing it. When the potential is made more negative, the EPSP rises more steeply to an earlier peak. When the potential is made less negative, the EPSP rises more slowly to a lower peak. Finally, when the charge inside the cell is reversed so as to be positive with respect to the exterior, the excitatory synapses give rise to an EPSP that is actually the reverse of the normal one.
These observations support the hypothesis that excitatory synapses produce what amounts virtually to a short circuit in the synaptic membrane potential. When this occurs, the membrane no longer acts as a barrier to the passage of ions but lets them flow through in response to the differing electric potential on the two sides of the membrane. In other words, the ions are momentarily allowed to travel freely down their electrochemical gradients, which means that sodium ions flow into the cell and, to a lesser degree, potassium ions flow out. It is this net flow of positive ions that creates the excitatory postsynaptic potential. The flow of negative ions, such as the chloride ion, is apparently not involved. By artificially altering the potential inside the cell one can establish that there is no flow of ions, and therefore no EPSP, when the voltage drop across the membrane is zero.
How is the synaptic membrane converted from a strong ionic barrier into an ion-permeable state? It is currently accepted that the agency of conversion is the chemical transmitter substance contained in the vesicles inside the synaptic knob. When a nerve impulse reaches the synaptic knob, some of the vesicles are caused to eject the transmitter substance into the synaptic cleft. The molecules of the substance would take only a few microseconds to diffuse across the cleft and become attached to specific receptor sites on the surface membrane of the adjacent nerve cell.
Presumably the receptor sites are associated with fine channels in the membrane that are opened in some way by the attachment of the transmitter-substance molecules to the receptor sites. With the channels thus opened, sodium and potassium ions flow through the membrane thousands of times more readily than they normally do, thereby producing the intense ionic flux that depolarizes the cell membrane and produces the EPSP. In many synapses the current flows strongly for only about a millisecond before the transmitter substance is eliminated from the synaptic cleft, either by diffusion into the surrounding regions or as a result of being destroyed by enzymes. The latter process is known to occur when the transmitter substance is acetylcholine, which is destroyed by the enzyme acetylcholinesterase.
The substantiation of this general picture of synaptic transmission requires the solution of many fundamental problems. Since we do not know the specific transmitter substance for the vast majority of synapses in the nervous system we do not know if there are many different substances or only a few. The only one identified with reasonable certainty in the mammalian central nervous system is acetylcholine. We know practically nothing about the mechanism by which a presynaptic nerve impulse causes the transmitter substance to be injected into the synaptic cleft. Nor do we know how the synaptic vesicles not immediately adjacent to the synaptic cleft are moved up to the firing line to replace the emptied vesicles. It is conjectured that the vesicles contain the enzyme systems needed to recharge themselves. The entire process must be swift and efficient: the total amount of transmitter substance in synaptic terminals is enough for only a few minutes of synaptic activity at normal operating rates. There are also knotty problems to be solved on the other side of the synaptic cleft. What, for example, is the nature of the receptor sites? How are the ionic channels in the membrane opened up?
Let us turn now to the second type of synapse that has been identified in the nervous system. These are the synapses that can inhibit the firing of a nerve cell even though it may be receiving a volley of excitatory impulses. When inhibitory synapses are examined in the electron microscope, they look very much like excitatory synapses. (There are probably some subtle differences, but they need not concern us here.) Microelectrode recordings of the activity of single motoneurons and other nerve cells have now shown that the inhibitory postsynaptic potential (IPSP) is virtually a mirror image of the EPSP. Moreover, individual inhibitory synapses, like excitatory synapses, have a cumulative effect. The chief difference is simply that the IPSP makes the cell's internal voltage more negative than it is normally, which is in a direction opposite to that needed for generating a spike discharge.
By driving the internal voltage of a nerve cell in the negative direction inhibitory synapses oppose the action of excitatory synapses, which of course drive it in the positive direction. Hence if the potential inside a resting cell is 70 millivolts negative, a strong volley of inhibitory impulses can drive the potential to 75 or 80 millivolts negative. One can easily see that if the potential is made more negative in this way the excitatory synapses find it more difficult to raise the internal voltage to the threshold point for the generation of a spike. Thus the nerve cell responds to the algebraic sum of the internal voltage changes produced by excitatory and inhibitory synapses.
If, as in the experiment described earlier, the internal membrane potential is altered by the flow of an electric current through one barrel of a double-barrelled microelectrode, one can observe the effect of such changes on the inhibitory postsynaptic potential. When the internal potential is made less negative, the inhibitory postsynaptic potential is deepened. Conversely, when the potential is made more negative, the IPSP diminishes; it finally reverses when the internal potential is driven below minus 80 millivolts.
One can therefore conclude that inhibitory synapses share with excitatory synapses the ability to change the ionic permeability of the synaptic membrane. The difference is that inhibitory synapses enable ions to flow freely down an electrochemical gradient that has an equilibrium point at minus 80 millivolts rather than at zero, as is the case for excitatory synapses. This effect could be achieved by the outward flow of positively charged ions such as potassium or the inward flow of negatively charged ions such as chloride, or by a combination of negative and positive ionic flows such that the interior reaches equilibrium at minus 80 millivolts.
In an effort to discover the permeability changes associated with the inhibitory potential my colleagues and I have altered the concentration of ions normally found in motoneurons and have introduced a variety of other ions that are not normally present. This can be done by impaling nerve cells with micropipettes that are filled with a salt solution containing the ion to be injected. The actual injection is achieved by passing a brief current through the micropipette.
If the concentration of chloride ions within the cell is in this way increased as much as three times, the inhibitory postsynaptic potential reverses and acts as a depolarizing current; that is, it resembles an excitatory potential. On the other hand, if the cell is heavily injected with sulfate ions, which are also negatively charged, there is no such reversal. This simple test shows that under the influence of the inhibitory transmitter substance, which is still unidentified, the subsynaptic membrane becomes permeable momentarily to chloride ions but not to sulfate ions. During the generation of the IPSP the outflow of chloride ions is so rapid that it more than outweighs the flow of other ions that generate the normal inhibitory potential.
My colleagues have now tested the effect of injecting motoneurons with more than 30 kinds of negatively charged ion. With one exception the hydrated ions (ions bound to water) to which the cell membrane is permeable under the influence of the inhibitory transmitter substance are smaller than the hydrated ions to which the membrane is impermeable. The exception is the formate ion (HCO2-), which may have an ellipsoidal shape and so be able to pass through membrane pores that block smaller spherical ions.
Apart from the formate ion all the ions to which the membrane is permeable have a diameter not greater than 1.14 times the diameter of the potassium ion; that is, they are less than 2.9 angstrom units in diameter. Comparable investigations in other laboratories have found the same permeability effects, including the exceptional behaviour of the formate ion, in fishes, toads and snails. It may well be that the ionic mechanism responsible for synaptic inhibition is the same throughout the animal kingdom.
The significance of these and other studies is that they strongly indicate that the inhibitory transmitter substance opens the membrane to the flow of potassium ions but not to sodium ions. It is known that the sodium ion is somewhat larger than any of the negatively charged ions, including the formate ion, that are able to pass through the membrane during synaptic inhibition. It is not possible, however, to test the effectiveness of potassium ions by injecting excess amounts into the cell because the excess is immediately diluted by an osmotic flow of water into the cell.
As indicated, the concentration of potassium ions inside the nerve cell is about 30 times greater than the concentration outside, and to maintain this large difference in concentration without the help of a metabolic pump the inside of the membrane would have to be charged 90 millivolts negative with respect to the exterior. This implies that if the membrane were suddenly made porous to potassium ions, the resulting outflow of ions would make the inside potential of the membrane even more negative than it is in the resting state, and that is just what happens during synaptic inhibition. The membrane must not simultaneously become porous to sodium ions, because they exist in much higher concentration outside the cell than inside and their rapid inflow would more than compensate for the potassium outflow. In fact, the fundamental difference between synaptic excitation and synaptic inhibition is that the membrane freely passes sodium ions in response to the former and largely excludes the passage of sodium ions in response to the latter.
This fine discrimination between ions that are not very different in size must be explained by any hypothesis of synaptic action. It is most unlikely that the channels through the membrane are created afresh and accurately maintained for a thousandth of a second every time a burst of transmitter substance is released into the synaptic cleft. It is more likely that channels of at least two different sizes are built directly into the membrane structure. In some way the excitatory transmitter substance would selectively unplug the larger channels and permit the free inflow of sodium ions. Potassium ions would simultaneously flow out and thus would tend to counteract the large potential change that would be produced by the massive sodium inflow. The inhibitory transmitter substance would selectively unplug the smaller channels that are large enough to pass potassium and chloride ions but not sodium ions.
To explain certain types of inhibition other features must be added to this hypothesis of synaptic transmission. In the simple hypothesis chloride and potassium ions can flow freely through pores of all inhibitory synapses. It has been shown, however, that the inhibition of the contraction of heart muscle by the vagus nerve is due almost exclusively to potassium-ion flow. On the other hand, in the muscles of crustaceans and in nerve cells in the snail's brain synaptic inhibition is due largely to the flow of chloride ions. This selective permeability could be explained if there were fixed charges along the walls of the channels. If such charges were negative, they would repel negatively charged ions and prevent their passage; if they were positive, they would similarly prevent passage of positively charged ions. One can now suggest that the channels opened by the excitatory transmitter are negatively charged and so do not permit the passage of the negatively charged chloride ion, even though it is small enough to move through the channel freely.
One might wonder if a given nerve cell can have excitatory synaptic action at some of its axon terminals and inhibitory action at others. The answer is no. Two different kinds of nerve cell are needed, one for each type of transmission and synaptic transmitter substance. This can readily be demonstrated by the effect of strychnine and tetanus toxin in the spinal cord; they specifically prevent inhibitory synaptic action and leave excitatory action unaltered. As a result the synaptic excitation of nerve cells is uncontrolled and convulsions result. The special types of cell responsible for inhibitory synaptic action are now being recognized in many parts of the central nervous system.
This account of communication between nerve cells is necessarily oversimplified, yet it shows that some significant advances are being made at the level of individual components of the nervous system. By selecting the most favourable situations we have been able to throw light on some details of nerve-cell behaviour. We can be encouraged by these limited successes. But the task of understanding in a comprehensive way how the human brain operates staggers its own imagination.
Anatomy (Greek anatome, ‘dissection’) is the branch of natural science dealing with the structural organization of living things. It is an old science, having its beginnings in prehistoric times. For centuries anatomical knowledge consisted largely of observations of dissected plants and animals. The proper understanding of structure, however, implies a knowledge of function in the living organism. Anatomy is therefore almost inseparable from physiology, which is sometimes called functional anatomy. As one of the basic life sciences, anatomy is closely related to medicine and to other branches of biology.
It is convenient to subdivide the study of anatomy in several different ways. One classification is based on the type of organisms studied, the major subdivisions being plant anatomy and animal anatomy. Animal anatomy is further subdivided into human anatomy and comparative anatomy, which seeks out similarities and differences among animal types. Anatomy can also be subdivided into biological processes - for example, developmental anatomy, the study of embryos, and pathological anatomy, the study of diseased organs. Other subdivisions, such as surgical anatomy and anatomical art, are based on the relationship of anatomy to other branches of activity under the general heading of applied anatomy. Still another way to subdivide anatomy is by the techniques employed - for example, microanatomy, which concerns itself with observations made with the help of the microscope.
The human skeleton consists of more than 200 bones bound together by tough and relatively inelastic connective tissues called ligaments. The different parts of the body vary greatly in their degree of movement. Thus, the arm at the shoulder is freely movable, whereas the knee joint is definitely limited to a hinge like action. The movements of individual vertebrae are extremely limited; the bones composing the skull are immovable. Movements of the bones of the skeleton are effected by contractions of the skeletal muscles, to which the bones are attached by tendons. These muscular contractions are controlled by the nervous system.
The nervous system is composed of the central nervous system and the peripheral nervous system. The central nervous system, which includes the brain and spinal cord, processes and coordinates all incoming sensory information and outgoing motor commands, and it is also the seat of complex brain functions such as memory, intelligence, learning, and emotion. The peripheral nervous system includes all neural tissue outside of the central nervous system. It is responsible for providing sensory, or afferent, information to the central nervous system and carrying motor, or efferent, commands out to the body’s tissues. Voluntary motor commands, such as moving muscles to walk or talk, are controlled by the somatic nervous system, while involuntary motor commands, such as digestion and heart beat, are controlled by the autonomic nervous system. The autonomic nervous system is further divided into two systems. The sympathetic nervous system, sometimes called the ‘fight or flight’ system, increases alertness, stimulates tissue, and prepares the body for quick responses to unusual situations. In contrast, the parasympathetic nervous system, sometimes called the ‘rest and repose’ system, conserves energy and controls sedentary activities, such as digestion.
Nervous systems range in complexity from the jellyfish’s network of nerve cells to the central and peripheral systems of humans. Common to many animals is the nervous structure of the earthworm, which consists of a cerebral ganglion, a main nerve cord, and branching pairs of lateral nerves. In some cases, as in insects, the cerebral ganglion acts as a primitive brain, controlling and coordinating various basic functions.
The nervous system has two divisions: the somatic, which allows voluntary control over skeletal muscle, and the autonomic, which is involuntary and controls cardiac and smooth muscle and glands. The autonomic nervous system has two divisions: the sympathetic and the parasympathetic. Many, but not all, of the muscles and glands that distribute nerve impulses to the larger interior organs possess a double nerve supply; in such cases the two divisions may exert opposing effects. Thus, the sympathetic system increases heartbeat, and the parasympathetic system decreases heartbeat. The two nervous systems are not always antagonistic, however. For example, both nerve supplies to the salivary glands excite the cells of secretion. Furthermore, a single division of the autonomic nervous system may both excite and inhibit a single effector, as in the sympathetic supply to the blood vessels of skeletal muscle. Finally, the sweat glands, the muscles that cause involuntary erection or bristling of the hair, the smooth muscle of the spleen, and the blood vessels of the skin and skeletal muscle are actuated only by the sympathetic division.
Voluntary movement of head, limbs, and body is caused by nerve impulses arising in the motor area of the cortex of the brain and carried by cranial nerves or by nerves that emerge from the spinal cord to connect with skeletal muscles. The reaction involves both excitation of nerve cells stimulating the muscles involved and inhibition of the cells that stimulate opposing muscles. A nerve impulse is an electrical change within a nerve cell or fibre; measured in millivolts, it lasts a few milliseconds and can be recorded by electrodes.
The human brain has three major structural components: the large dome-shaped cerebrum (top), the smaller somewhat spherical cerebellum (lower right), and the brainstem (centre). Prominent in the brainstem are the medulla oblongata (the egg-shaped enlargement at centre) and the thalamus (between the medulla and the cerebrum). The cerebrum is responsible for intelligence and reasoning. The cerebellum helps to maintain balance and posture. The medulla is involved in maintaining involuntary functions such as respiration, and the thalamus acts as a relay centre for electrical impulses travelling to and from the cerebral cortex. Lack of blood flow to any part of the brain results in a stroke, permanent damage that interferes with the functions of the affected part of the brain.
Movement may occur also in direct response to an outside stimulus; thus, a tap on the knee causes a jerk, and a light shone into the eye makes the pupil contract. These involuntary responses are called reflexes. Various nerve terminals called receptors constantly send impulses into the central nervous system. These are of three classes: exteroceptors, which are sensitive to pain, temperature, touch, and pressure; interoceptors, which react to changes in the internal environment; and proprioceptors, which respond to variations in movement, position, and tension. These impulses terminate in special areas of the brain, as do those of special receptors concerned with sight, hearing, smell, and taste.
Whereas most major nerves emerge from the spinal cord, the 12 pairs of cranial nerves project directly from the brain. All but 1 pair relay motor or sensory information (or both); the tenth, or vagus nerve, affects visceral functions such as heart rate, vasoconstriction, and contraction of the smooth muscle found in the walls of the trachea, stomach, and intestine.
Muscular contractions do not always cause actual movement. A small fraction of the total number of fibres in most muscles are usually contracting. This serves to maintain the posture of a limb and enables the limb to resist passive elongation or stretch. This slight continuous contraction is called muscle tone.
The human circulatory system is composed of the muscular heart and an intricate network of elastic blood vessels known as arteries, veins, and capillaries. These structures work together to circulate blood throughout the body, in the process delivering life-preserving oxygen and nutrients to tissue cells while also removing waste products.
In passing through the system, blood pumped by the heart follows a winding course through the right chambers of the heart, into the lungs, where it picks up oxygen, and back into the left chambers of the heart. From these it is pumped into the main artery, the aorta, which branches into increasingly smaller arteries until it passes through the smallest, known as arterioles. Beyond the arterioles, the blood passes through a vast amount of tiny, thin-walled structures called capillaries. Here, the blood gives up its oxygen and its nutrients to the tissues and absorbs from them carbon dioxide and other waste products of metabolism. The blood completes its circuit by passing through small veins that join to form increasingly larger vessels until it reaches the largest veins, the inferior and superior venae cavae, which return it to the right side of the heart. Blood is propelled mainly by contractions of the heart; contractions of skeletal muscle also contribute to circulation. Valves in the heart and in the veins ensure its flow in one direction.
Scanning electron micrograph of a normal T lymphocyte. T lymphocytes are specialized white blood cells that identify and destroy invading organisms such as bacteria and viruses. Some T lymphocytes directly destroy invading organisms, whereas other T lymphocytes regulate the immune system by directing immune responses.
The body defends itself against foreign proteins and infectious microorganisms by means of a complex dual system that depends on recognizing a portion of the surface pattern of the invader. The two parts of the system are termed cellular immunity, in which lymphocytes are the effective agent, and humoral immunity, based on the action of antibody molecules.
The highly complex human immune system works to hinder outside invaders such as bacteria and viruses from causing harm to the body. In this 1973 Scientific American article, Danish immunologist Neils Kaj Jerne described the different components of the immune system and explained how they work together. The article considered such questions as why the body develops antibodies for specific viruses and bacteria, and how lymphocytes (a type of white blood cell) complement one another in fighting infection and other types of disease. For his contributions to the study of the immune system, Jerne shared the 1984 Nobel Prize in physiology or medicine.
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When particular lymphocytes recognize a foreign molecular pattern (termed an antigen), they release antibodies in great numbers; other lymphocytes store the memory of the pattern for future release of antibodies should the molecule reappear. Antibodies attach themselves to the antigen and in that way mark them for destruction by other substances in the body’s defence arsenal. These are primarily complement, a complex of enzymes that make holes in foreign cells, and phagocytes, cells that engulf and digest foreign matter. They are drawn to the area by chemical substances released by activated lymphocytes.
The challenge faced by the human immune system is to recognize and destroy ‘one selves’ - substances such as foreign microbes and damaged or abnormal cells - while recognizing and leaving healthy cells and tissues intact. In order to do so, the body must produce and develop a complex array of immune cells. In an article for Scientific American, immunologists Irving Weissman and Max Cooper provide an overview of what scientists have learned about the development of the immune system.
Lymphocytes, which resemble blood plasma in composition, are manufactured in the bone marrow and multiply in the thymus and spleen. They circulate in the bloodstream, penetrating the walls of the blood capillaries to reach the cells of the tissues. From there they migrate to an independent network of capillaries that is comparable to and almost as extensive as that of the blood’s circulatory system. The capillaries join to form larger and larger vessels that eventually link up with the bloodstream through the jugular and subclavian veins; valves in the lymphatic vessels ensure flow in one direction. Nodes at various points in the lymphatic network act as stations for the collection and manufacture of lymphocytes; they may become enlarged during an infectious disease. In anatomy, the network of lymphatic vessels and the lymph nodes are together called the lymphatic system; its function as the vehicle of the immune system was not recognized until the 1960s.
Air travels to the lungs though a series of tubes and airways. The two branches of the trachea, called bronchi, subdivide within the lobes into smaller and smaller air vessels. They terminate in alveoli, tiny air sacs surrounded by capillaries. When the alveoli inflate with inhaled air, oxygen diffuses into the blood in the capillaries to be pumped by the heart to the tissues of the body, and carbon dioxide diffuses out of the blood into the lungs, where it is exhaled.
Respiration is carried on by the expansion and contraction of the lungs; the process and the rate at which it proceeds are controlled by a nervous centre in the brain.
In the lungs, oxygen enters tiny capillaries, where it combines with hemoglobin in the red blood cells and is carried to the tissues. Simultaneously, carbon dioxide, which entered the blood in its passages through the tissues, passes through capillaries into the air contained within the lungs. Inhaling draws into the lungs air that is higher in oxygen and lower in carbon dioxide; exhaling forces from the lungs air that is high in carbon dioxide and low in oxygen. Changes in the size and gross capacity of the chest are controlled by contractions of the diaphragm and of the muscles between the ribs.
Located on the left side of the body, under the diaphragm, the stomach is a muscular, saclike organ that connects the esophagus and small intestine. Its main function is to break down food. Cells in the stomach lining secrete enzymes, hydrochloric acid, and other chemicals to continue the digestive process begun in the mouth and produce mucus to keep these substances from digesting the lining itself.
The energy required for maintenance and proper functioning of the human body is supplied by food. After it is broken into fragments by chewing and mixed with saliva, digestion begins. The food passes down the gullet into the stomach, where the process is continued by the gastric and intestinal juices. Thereafter, the mixture of food and secretions, called chyme, is pushed down the alimentary canal by peristalsis, rhythmic contractions of the smooth muscle of the gastrointestinal system. The contractions are initiated by the parasympathetic nervous system; such muscular activity can be inhibited by the sympathetic nervous system. Absorption of nutrients from chyme occurs mainly in the small intestine; unabsorbed food and secretions and waste substances from the liver pass to the large intestines and are expelled as feces. Water and water-soluble substances travel via the bloodstream from the intestines to the kidneys, which absorb all the constituents of the blood plasma except its proteins. The kidneys return most of the water and salts to the body, while excreting other salts and waste products, along with excess water, as urine.
Called the master gland, the pituitary secretes hormones that control the activity of other endocrine glands and regulate various biological processes. Its secretions include growth hormone (which stimulates cellular activity in bone, cartilage, and other structural tissue); thyroid stimulating hormone (which causes the thyroid to release metabolism-regulating hormones); antidiuretic hormone (which causes the kidney to excrete less water in the urine); and prolactin (which stimulates milk production and breast development in females). The pituitary gland is influenced both neurally and hormonally by the hypothalamus.
In addition to the integrative action of the nervous system, control of various body functions is exerted by the endocrine glands. An important part of this system, the pituitary, lies at the base of the brain. This master gland secretes a variety of hormones, including the following: (1) a hormone that stimulates the thyroid gland and controls its secretion of thyroxine, which dictates the rate at which all cells utilize oxygen; (2) a hormone that controls the secretion in the adrenal gland of hormones that influence the metabolism of carbohydrates, sodium, and potassium and control the rate at which substances are exchanged between blood and tissue fluid; (3) substances that control the secretion in the ovaries of estrogen and progesterone and the creation in the testicles of testosterone; (4) the somatotropic, or growth, hormone, which controls the rate of development of the skeleton and large interior organs through its effect on the metabolism of proteins and carbohydrates; and (5) an insulin inhibitor—a lack of insulin causes diabetes mellitus.
The posterior lobe of the pituitary secretes vasopressin, which acts on the kidney to control the volume of urine; a lack of vasopressin causes diabetes insipidus, which results in the passing of large volumes of urine. The posterior lobe also elaborates oxytocin, which causes contraction of smooth muscle in the intestines and small arteries and is used to bring about contractions of the uterus in childbirth. Other glands in the endocrine system are the pancreas, which secretes insulin, and the parathyroid, which secretes a hormone that regulates the quantity of calcium and phosphorus in the blood.
The bones of the human female pelvis form a bowl-shaped cavity that supports the weight of a developing fetus and encloses the organs of the female reproductive tract. Two ovaries, the female gonads, produce mature eggs. Leading away from the ovaries are the fallopian tubes, or oviducts, the site of fertilization. The uterus, a muscular organ with an expandable neck called the cervix, houses the developing fetus, which leaves the woman's body through the vagina, or birth canal.
The organs of the male reproductive system enable a man to have sexual intercourse and to fertilize female sex cells (eggs) with sperm. The gonads, called testicles, produce sperm. Sperm pass through a long duct called the vas deferens to the seminal vesicles, a pair of sacs that lies behind the bladder. These sacs produce seminal fluid, which mixes with sperm to produce semen. Semen leaves the seminal vesicles and travels through the prostate gland, which produces additional secretions that are added to semen. During male orgasm the penis ejaculates semen.
Reproduction is accomplished by the union of male sperm and the female ovum. In coitus, the male organ ejaculates more than 250 million sperm into the vagina, from which some make their way to the uterus. Ovulation, the release of an egg into the uterus, occurs approximately every 28 days; during the same period the uterus is prepared for the implantation of a fertilized ovum by the action of estrogens. If a male cell fails to unite with a female cell, other hormones cause the uterine wall to slough off during menstruation. From puberty to menopause, the process of ovulation, and preparation, and menstruation is repeated monthly except for periods of pregnancy. The duration of pregnancy is about 280 days. After childbirth, prolactin, a hormone secreted by the pituitary, activates the production of milk.
The skin is an organ of double-layered tissue stretched over the surface of the body and protecting it from drying or losing fluid, from harmful external substances, and from extremes of temperature. The inner layer, called the dermis, contains sweat glands, blood vessels, nerve endings (sense receptors), and the bases of hair and nails. The outer layer, the epidermis, is only a few cells thick; it contains pigments, pores, and ducts, and its surface is made of dead cells that it sheds from the body. (Hair and nails are adaptations arising from the dead cells.) The sweat glands excrete waste and cool the body through evaporation of fluid droplets; the blood vessels of the dermis supplement temperature regulation by contracting to preserve body heat and expanding to dissipate it. Separate kinds of receptors convey pressure, temperature, and pain. Fat cells in the dermis insulate the body, and oil glands lubricate the epidermis.
Belgian anatomist and physician Andreas Vesalius helped establish the foundations of modern anatomy in the 16th century by dissecting human cadavers and publishing his results. He served as physician to Holy Roman Emperor Charles V and his son, Philip II, king of Spain. This portrait is by Dutch-born English painter Peter Lely.
The oldest known systematic study of anatomy is contained in an Egyptian papyrus dating from about 1600 bc. The treatise reveals knowledge of the larger viscera but little concept of their functions. About the same degree of knowledge is reflected in the writings of the Greek physician Hippocrates in the 5th century bc. In the 4th century bc Aristotle greatly increased anatomical knowledge of animals. The first real progress in the science of human anatomy was made in the following century by the Greek physicians Herophilus and Erasistratus, who dissected human cadavers and were the first to distinguish many functions, including those of the nervous and muscular systems. Little further progress was made by the ancient Romans or by the Arabs. The Renaissance first influenced the science of anatomy in the latter half of the 16th century.
In his notebooks, Renaissance artist Leonardo da Vinci recorded insights into topics as diverse as flying machines, hydrodynamics, and the human soul. He proposed a course of study for an aspiring painter, which included a thorough investigation of human anatomy. In this excerpt, Leonardo describes the measurements that led to the creation of one of his best-known drawings—a man inscribed within the dimensions of a square and a circle. Editor Irma A. Richter provides introductions to the following passages on human structure and movement from Leonardo’s notebooks.
Modern anatomy began with the publication in 1543 of the work of the Belgian anatomist Andreas Vesalius. Before the publication of this classical work anatomists had been so bound by tradition that the writings of authorities of more than 1000 years earlier, such as the Greek physician Galen, who had been restricted to the dissection of animals, were accepted in lieu of actual observation. Vesalius and other Renaissance anatomists, however, based their descriptions on their own observations of human corpses, thus setting the pattern for subsequent study in anatomy.
For many years anatomists (even those of the modern era) were concerned mainly with the accumulation of a vast amount of information known as descriptive morphology. Descriptive morphology has been supplemented, and to a certain extent supplanted, by the development of experimental morphology, which attempts to identify the hereditary and environmental determinants in morphology and their relationships by controlled-environment and grafting experiments on embryos. Ideally, anatomical investigation consists of a combination of descriptive and experimental approaches. Present-day anatomy involves scrutiny of the structure of organisms at many levels of observation. For example, the anatomist studies the cells and tissues of organisms with the unaided eye, with simple and compound lenses, with various kinds of microscopes, and by chemical methods of analysis.
The 17th-century invention of the compound microscope led to the development of microscopic anatomy, which is divided into histology, the study of tissues, and cytology, the study of cells. Under the leadership of the Italian anatomist Marcello Malpighi, the study of the microscopic structure of animals and plants flourished during the 17th century. Many great anatomists of the period were reluctant to accept microscopic anatomy as part of their science. By contrast, modern anatomy is studied usually with the aim of correlating the structure of organisms as seen by the naked eye with their structure as revealed by more refined methods of observation.
Pathological anatomy was established as a branch of the science by the Italian physician Giovanni Morgagni, and in the late 18th century comparative anatomy was systematized by the French naturalist Georges Cuvier.
In the late 18th and early 19th centuries restrictive legislation limiting the use of unclaimed human bodies for the study of anatomy and surgery gave rise in England and the United States to an era of body snatching. The scandals arising from this practice forced the repeal of the English restrictions in 1832 and the enactment of more advanced legislation.
Microscopic anatomy developed rapidly in the 19th century. During the second half of the century many basic facts about the fine structure of organisms were discovered, largely as a result of greatly improved optical microscopes and of new methods that made cells and tissues easy to study with this instrument. The method of microtomy, the cutting of tissue into thin, practically transparent slices, was perfected. Microtomy was rendered incomparably more valuable by the application to the tissue slices of various types of dyes and stains that make it much easier to see various parts of the cell.
Knowledge of microscopic anatomy was greatly expanded during the 20th century as a result of the development of microscopes that provided much greater resolution and magnification than had conventional instruments, thus revealing formerly unclear or invisible detail; and expanded laboratory techniques helped facilitate observation. The ultraviolet microscope allows the observer to see more because the wavelengths of its probing rays are shorter than those of visible light (the resolving power of a microscope is inversely proportional to the wavelength of the light used). It also is used to emphasize particular details through selective absorption of certain ultraviolet wavelengths. The electron microscope gives even greater magnification and resolution. These tools have opened up formerly unexplored fields of anatomical investigation. Other modern microscopes have made visible unstained and living materials that would be invisible under the conventional microscope. Two examples are the phase-contrast microscope and the interference microscope. Through utilization of ordinary light beams, both these instruments differentiate parts of living, unstained cells.
The discovery of X rays by the German physicist Wilhelm Conrad Roentgen enabled anatomists to study tissues and organ systems in living animals. The first X-ray photograph, taken in 1896, was of a human hand. Today’s techniques permit three-dimensional X-ray photographs of the soft tissues of the viscera after ingestion of special opaque fluids, and of ‘slices’ of the body with computer-aided X-ray beams. The latter is called computerized tomography, or CT scanning. Other noninvasive techniques that have been developed include the use of ultrasonic waves for imaging soft tissues and the application of nuclear magnetic resonance systems to research and diagnosis.
Another 20th-century technique of anatomical investigation is tissue culture, which involves the cultivation of cells and tissues of complex organisms outside the body. The technique permits the isolation of living units so that the investigator can directly observe the processes of growth, multiplication, and differentiation of cells. Tissue culture thus has added a new dimension to anatomical science.
The closely related techniques of histochemistry and cytochemistry are concerned with the investigation of chemical activities of tissues and cells. For example, the presence of certain colours within cells indicates that particular chemical reactions have occurred. In addition, the density of the colour reaction may serve as an index of the intensity of the reaction. Histochemical methods have been particularly successful in the study of enzymes, catalytic substances that control and direct many of the cell’s activities. Much knowledge of enzymes was gained in studies carried out after removal of the enzymes from their cells of origin, but not until the advent of histochemistry could the anatomist see through the microscope which cells carry specific enzymes or gauge how active these enzymes are in different cells under various conditions.
An important technique of histochemistry involves the use of radioactive isotopes of various chemical elements that are present in cells and tissues or compounds ‘tagged’ or ‘labelled’ with radioactive isotopes are administered to living materials, permitting the investigator to trace the pathways taken by these substances through the various tissues. The degree of concentration and dilution of elements within specific cellular constituents may be estimated by measuring the radiations emanating from these tissues. The technique of labelling compounds with radioactive isotopes makes it possible to study the distribution and concentration of isotopes in tissue slices similar to those studied routinely under the microscope. This study, called autoradiography, is accomplished by bringing the radioactive tissue slices into contact with photographic films and emulsions that are sensitive to radiation.
Another technique of localizing chemical compounds within tissue slices is microincineration: the heating of microscopic sections to the point at which the organic materials present are destroyed and only the mineral skeleton remains. The remaining minerals can then be identified by special chemical and microscopic procedures. Thus, microincineration provides still another way of locating specific chemical elements within particular cell or tissue components.
Another development in the field of histochemistry is microspectrophotometry, a precise method of colour analysis. In this process the colours within a tissue slice are analysed with a spectrophotometer, an instrument that measures the intensity of each colour as a function of wavelength. Microspectrophotometry can be used to estimate the characteristics of unstained cells and tissues by measuring their absorption of particular wavelengths. Another application permits precise judgments to be made concerning the nature and intensity of colour reactions. These judgments provide, in turn, accurate information about the location and intensity of chemical reactions in the components of living organisms.
States of Consciousness. No simple, agreed-upon definition of consciousness exists. Attempted definitions tend to be tautological (for example, consciousness defined as awareness) or merely descriptive (for example, consciousness described as sensations, thoughts, or feelings). Despite this problem of definition, the subject of consciousness has had a remarkable history. At one time the primary subject matter of psychology, consciousness as an area of study suffered an almost total demise, later reemerging to become a topic of current interest.
French thinker René Descartes applied rigorous scientific methods of deduction to his exploration of philosophical questions. Descartes is probably best known for his pioneering work in philosophical skepticism. Author Tom Sorell examines the concepts behind Descartes’s work Meditationes de Prima Philosophia (1641; Meditations on First Philosophy), focussing on its unconventional use of logic and the reactions it aroused.
Most of the philosophical discussions of consciousness arose from the mind-body issues posed by the French philosopher and mathematician René Descartes in the 17th century. Descartes asked: Is the mind, or consciousness, independent of matter? Is consciousness extended (physical) or unextended (nonphysical)? Is consciousness determinative, or is it determined? English philosophers such as John Locke equated consciousness with physical sensations and the information they provide, whereas European philosophers such as Gottfried Wilhelm Leibniz and Immanuel Kant gave a more central and active role to consciousness.
The philosopher who most directly influenced subsequent exploration of the subject of consciousness was the 19th-century German educator Johann Friedrich Herbart, who wrote that ideas had quality and intensity and that they may inhibit or facilitate one another. Thus, ideas may pass from ‘states of reality’ (consciousness) to ‘states of tendency’ (unconsciousness), with the dividing line between the two states being described as the threshold of consciousness. This formulation of Herbart clearly presages the development, by the German psychologist and physiologist Gustav Theodor Fechner, of the psychophysical measurement of sensation thresholds, and the later development by Sigmund Freud of the concept of the unconscious.
The experimental analysis of consciousness dates from 1879, when the German psychologist Wilhelm Max Wundt started his research laboratory. For Wundt, the task of psychology was the study of the structure of consciousness, which extended well beyond sensations and included feelings, images, memory, attention, duration, and movement. Because early interest focussed on the content and dynamics of consciousness, it is not surprising that the central methodology of such studies was introspection; that is, subjects reported on the mental contents of their own consciousness. This introspective approach was developed most fully by the American psychologist Edward Bradford Titchener at Cornell University. Setting his task as that of describing the structure of the mind, Titchener attempted to detail, from introspective self-reports, the dimensions of the elements of consciousness. For example, taste was ‘dimensionalized’ into four basic categories: sweet, sour, salt, and bitter. This approach was known as structuralism.
By the 1920s, however, a remarkable revolution had occurred in psychology that was to essentially remove considerations of consciousness from psychological research for some 50 years: Behaviourism captured the field of psychology. The main initiator of this movement was the American psychologist John Broadus Watson. In a 1913 article, Watson stated, ‘I believe that we can write a psychology and never use the terms consciousness, mental states, mind . . . imagery and the like.’ Psychologists then turned almost exclusively to behaviour, as described in terms of stimulus and response, and consciousness was totally bypassed as a subject. A survey of eight leading introductory psychology texts published between 1930 and the 1950s found no mention of the topic of consciousness in five texts, and in two it was treated as a historical curiosity.
Beginning in the late 1950s, however, interest in the subject of consciousness returned, specifically in those subjects and techniques relating to altered states of consciousness: sleep and dreams, meditation, biofeedback, hypnosis, and drug-induced states. Much of the surge in sleep and dream research was directly fuelled by a discovery relevant to the nature of consciousness. A physiological indicator of the dream state was found: At roughly 90-minute intervals, the eyes of sleepers were observed to move rapidly, and at the same time the sleepers' brain waves would show a pattern resembling the waking state. When people were awakened during these periods of rapid eye movement, they almost always reported dreams, whereas if awakened at other times they did not. This and other research clearly indicated that sleep, once considered a passive state, was instead an active state of consciousness.
During the 1960s, an increased search for ‘higher levels’ of consciousness through meditation resulted in a growing interest in the practices of Zen Buddhism and Yoga from Eastern cultures. A full flowering of this movement in the United States was seen in the development of training programs, such as Transcendental Meditation, that were self-directed procedures of physical relaxation and focussed attention. Biofeedback techniques also were developed to bring body systems involving factors such as blood pressure or temperature under voluntary control by providing feedback from the body, so that subjects could learn to control their responses. For example, researchers found that persons could control their brain-wave patterns to some extent, particularly the so-called alpha rhythms generally associated with a relaxed, meditative state. This finding was especially relevant to those interested in consciousness and meditation, and a number of ‘alpha training’ programs emerged.
Another subject that led to increased interest in altered states of consciousness was hypnosis, which involves a transfer of conscious control from the subject to another person. Hypnotism has had a long and intricate history in medicine and folklore and has been intensively studied by psychologists. Much has become known about the hypnotic state, relative to individual suggestibility and personality traits; the subject has now largely been demythologized, and the limitations of the hypnotic state are fairly well known. Despite the increasing use of hypnosis, however, much remains to be learned about this unusual state of focussed attention.
Finally, many people in the 1960s experimented with the psychoactive drugs known as hallucinogens, which produce disorders of consciousness. The most prominent of these drugs are lysergic acid diethylamide, or LSD; mescaline; and psilocybin; the latter two have long been associated with religious ceremonies in various cultures. LSD, because of its radical thought-modifying properties, was initially explored for its so-called mind-expanding potential and for its psychotomimetic effects (imitating psychoses). Little positive use, however, has been found for these drugs, and their use is highly restricted.
Scientists have long considered the nature of consciousness without producing a fully satisfactory definition. In the early 20th century American philosopher and psychologist William James suggested that consciousness is a mental process involving both attention to external stimuli and short-term memory. Later scientific explorations of consciousness mostly expanded upon James’s work. In this article from a 1997 special issue of Scientific American, Nobel laureate Francis Crick, who helped determine the structure of DNA, and fellow biophysicist Christof Koch explain how experiments on vision might deepen our understanding of consciousness.
As the concept of a direct, simple linkage between environment and behaviour became unsatisfactory in recent decades, the interest in altered states of consciousness may be taken as a visible sign of renewed interest in the topic of consciousness. That persons are active and intervening participants in their behaviour has become increasingly clear. Environments, rewards, and punishments are not simply defined by their physical character. Memories are organized, not simply stored. An entirely new area called cognitive psychology has emerged that centres on these concerns. In the study of children, increased attention is being paid to how they understand, or perceive, the world at different ages. In the field of animal behaviour, researchers increasingly emphasize the inherent characteristics resulting from the way a species has been shaped to respond adaptively to the environment. Humanistic psychologists, with a concern for self-actualization and growth, have emerged after a long period of silence. Throughout the development of clinical and industrial psychology, the conscious states of persons in terms of their current feelings and thoughts were of obvious importance. The role of consciousness, however, was often de-emphasizes in favour of unconscious needs and motivations. Trends can be seen, however, toward a new emphasis on the nature of states of consciousness.
Scientists have long considered the nature of consciousness without producing a fully satisfactory definition. In the early 20th century American philosopher and psychologist William James suggested that consciousness is a mental process involving both attention to external stimuli and short-term memory. Later scientific explorations of consciousness mostly expanded upon James’s work, and the laureate Francis Crick, who helped determine the structure of DNA, and fellow biophysicist Christof Koch explain how experiments on vision might deepen our understanding of consciousness.
The overwhelming question in neurobiology today is the relation between the mind and the brain. Everyone agrees that what we know as mind is closely related to certain aspects of the behaviour of the brain, not to the heart, as Aristotle thought. Its most mysterious aspect is consciousness or awareness, which can take many forms, from the experience of pain to self-consciousness. In the past the mind (or soul) was often regarded, as it was by Descartes, as something immaterial, separate from the brain but interacting with it in some way. A few neuroscientists, such as Sir John Eccles, still assert that the soul is distinct from the body. But most neuroscientists now believe that all aspects of mind, including its most puzzling attribute - consciousness or awareness - are likely to be explainable in a more materialistic way as the behaviour of large sets of interacting neurons. As William James, the father of American psychology, said a century ago, consciousness is not a thing but a process.
Exactly what the process is, however, has yet to be discovered. For many years after James penned The Principles of Psychology, consciousness was a taboo concept in American psychology because of the dominance of the behaviorist movement. With the advent of cognitive science in the mid-1950s, it became possible once more for psychologists to consider mental processes as opposed to merely observing behaviour. In spite of these changes, until recently most cognitive scientists ignored consciousness, as did almost all neuroscientists. The problem was felt to be either purely ‘philosophical’ or too elusive to study experimentally. It would not have been easy for a neuroscientists to get a grant just to study consciousness.
In our opinion, such timidity is ridiculous, so a few years ago we began to think about how best to attack the problem scientifically. How to explain mental events as being caused by the firing of large sets of neurons? Although there are those who believe such an approach is hopeless, we feel it is not productive to worry too much over aspects of the problem that cannot be solved scientifically or, more precisely, cannot be solved solely by using existing scientific ideas. Radically new concepts may indeed be needed - recall the modifications of scientific thinking forced on us by quantum mechanics. The only sensible approach is to press the experimental attack until we are confronted with dilemmas that call for new ways of thinking.
There are many possible approaches to the problem of consciousness. Some psychologists feel that any satisfactory theory should try to explain as many aspects of consciousness as possible, including emotion, imagination, dreams, mystical experiences and so on. Although such an all-embracing theory will be necessary in the long run, we thought it wiser to begin with the particular aspect of consciousness that is likely to yield most easily. What this aspect may be is a matter of personal judgment. We selected the mammalian visual system because humans are very visual animals and because so much experimental and theoretical work has already been done on it.
It is not easy to grasp exactly what we need to explain, and it will take many careful experiments before visual consciousness can be described scientifically. We did not attempt to define consciousness itself because of the dangers of premature definition. (If this seems like a copout, try defining the word ‘gene’ - you will not find it easy.) Yet the experimental evidence that already exists provides enough of a glimpse of the nature of visual consciousness to guide research. In this article, we will attempt to show how this evidence opens the way to attack this profound and intriguing problem.
Visual theorists agree that the problem of visual consciousness is ill posed. The mathematical term ‘ill posed’ means that additional constraints are needed to solve the problem. Although the main function of the visual system is to perceive objects and events in the world around us, the information available to our eyes is not sufficient by itself to provide the brain with its unique interpretation of the visual world. The brain must use past experience (either its own or that of our distant ancestors, which is embedded in our genes) to help interpret the information coming into our eyes. An example would be the derivation of the three-dimensional representation of the world from the two-dimensional signals falling onto the retinas of our two eyes or even onto one of them.
Visual theorists also would agree that seeing is a constructive process, one in which the brain has to carry out complex activities (sometimes called computations) in order to decide which interpretation to adopt of the ambiguous visual input. ‘Computation’ implies that the brain acts to form a symbolic representation of the visual world, with a mapping (in the mathematical sense) of certain aspects of that world onto elements in the brain.
Ray Jackendoff of Brandeis University postulates, as do most cognitive scientists, that the computations carried out by the brain are largely unconscious and that what we become aware of is the result of these computations. But while the customary view is that this awareness occurs at the highest levels of the computational system, Jackendoff has proposed an intermediate-level theory of consciousness.
What we see, Jackendoff suggests, relates to a representation of surfaces that are directly visible to us, together with their outline, orientation, colour, texture and movement. (This idea has similarities to what the late David C. Marr of the Massachusetts Institute of Technology called a ‘2 1/2-dimensional sketch.’ It is more than a two-dimensional sketch because it conveys the orientation of the visible surfaces. It is less than three-dimensional because depth information is not explicitly represented.) In the next stage this sketch is processed by the brain to produce a three-dimensional representation. Jackendoff argues that we are not visually aware of this three-dimensional representation.
An example may make this process clearer. If you look at a person whose back is turned to you, you can see the back of the head but not the face. Nevertheless, your brain infers that the person has a face. We can deduce as much because if that person turned around and had no face, you would be very surprised.
The viewer-entered representation that corresponds to the visible back of the head is what you are vividly aware of. What your brain infers about the front would come from some kind of three-dimensional representation. This does not mean that information flows only from the surface representation to the three-dimensional one; it almost certainly flows in both directions. When you imagine the front of the face, what you are aware of is a surface representation generated by information from the three-dimensional model.
It is important to distinguish between an explicit and an implicit representation. An explicit representation is something that is symbolized without further processing. An implicit representation contains the same information but requires further processing to make it explicit. The pattern of coloured dots on a television screen, for example, contains an implicit representation of objects (say, a person's face), but only the dots and their locations are explicit. When you see a face on the screen, there must be neurons in your brain whose firing, in some sense, symbolizes that face.
We call this pattern of firing neurons an active representation. A latent representation of a face must also be stored in the brain, probably as a special pattern of synaptic connections between neurons. For example, you probably have a representation of the Statue of Liberty in your brain, a representation that usually is inactive. If you do think about the Statue, the representation becomes active, with the relevant neurons firing away.
An object, incidentally, may be represented in more than one way—as a visual image, as a set of words and their related sounds, or even as a touch or a smell. These different representations are likely to interact with one another. The representation is likely to be distributed over many neurons, both locally and more globally. Such a representation may not be as simple and straightforward as uncritical introspection might indicate. There is suggestive evidence, partly from studying how neurons fire in various parts of a monkey's brain and partly from examining the effects - of certain types of brain damage in humans, that different aspects of a face and of the implications of a face - may be represented in different parts of the brain.
First, there is the representation of a face as a face: two eyes, a nose, a mouth and so on. The neurons involved are usually not too fussy about the exact size or position of this face in the visual field, nor are they very sensitive to small changes in its orientation. In monkeys, there are neurons that respond best when the face is turning in a particular direction, while others seem to be more concerned with the direction in which the eyes are gazing.
Then there are representations of the parts of a face, as separate from those for the face as a whole. Further, the implications of seeing a face, such as that person's sex, the facial expression, the familiarity or unfamiliarity of the face, and in particular whose face it is, may each be correlated with neurons firing in other places.
What we are aware of at any moment, in one sense or another, is not a simple matter. We have suggested that there may be a very transient form of fleeting awareness that represents only rather simple features and does not require an attentional mechanism. From this brief awareness the brain constructs a viewer-entered representation - what we see vividly and clearly - that does require attention. This in turn probably leads to three-dimensional object representations and thence to more cognitive ones.
Representations corresponding to vivid consciousness are likely to have special properties. William James thought that consciousness had involved both attention and short-term memory. Most psychologists today would agree with this view. Jackendoff writes that consciousness is ‘enriched’ by attention, implying that whereas attention may not be essential for certain limited types of consciousness, it is necessary for full consciousness. Yet it is not clear exactly which forms of memory are involved. Is long-term memory needed? Some forms of acquired knowledge are so embedded in the machinery of neural processing that they are almost certainly used in becoming aware of something. On the other hand, there is evidence from studies of brain-damaged patients that the ability to lay down new long-term episodic memories is not essential for consciousness to be experienced.
It is difficult to imagine that anyone could be conscious if he or she had no memory whatsoever of what had just happened, even an extremely short one. Visual psychologists talk of iconic memory, which lasts for a fraction of a second, and working memory (such as that used to remember a new telephone number) that lasts for only a few seconds unless it is rehearsed. It is not clear whether both of these are essential for consciousness. In any case, the division of short-term memory into these two categories may be too crude.
If these complex processes of visual awareness are localized in parts of the brain, which processes are likely to be where? Many regions of the brain may be involved, but it is almost certain that the cerebral neocortex plays a dominant role. Visual information from the retina reaches the neocortex mainly by way of a part of the thalamus (the lateral geniculate nucleus); another significant visual pathway from the retina is to the superior colliculus, at the top of the brain stem.
The cortex in humans consists of two intricately folded sheets of nerve tissue, one on each side of the head. These sheets are connected by a large tract of about half a billion axons called the corpus callosum. It is well known that if the corpus callosum is cut, as is done for certain cases of intractable epilepsy, one side of the brain is not aware of what the other side is seeing. In particular, the left side of the brain (in a right-handed person) appears not to be aware of visual information received exclusively by the right side. This shows that none of the information required for visual awareness can reach the other side of the brain by travelling down to the brain stem and, from there, back up. In a normal person, such information can get to the other side only by using the axons in the corpus callosum.
A different part of the brain—the hippocampal system—is involved in one-shot, or episodic, memories that, over weeks and months, it passes on to the neocortex. This system is so placed that it receives inputs from, and projects to, many parts of the brain. Thus, one might suspect that the hippocampal system is the essential seat of consciousness. This is not the case: evidence from studies of patients with damaged brains shows that this system is not essential for visual awareness, although naturally a patient lacking one is severely handicapped in everyday life because he cannot remember anything that took place more than a minute or so in the past.
In broad terms, the neocortex of alert animals probably acts in two ways. By building on crude and somewhat redundant wiring, produced by our genes and by embryonic processes, the neocortex draws on visual and other experience to slowly ‘rewire’ itself to create categories (or ‘features’) it can respond to. A new category is not fully created in the neocortex after exposure to only one example of it, although some small modifications of the neural connections may be made.
The second function of the neocortex (at least of the visual part of it) is to respond extremely rapidly to incoming signals. To do so, it uses the categories it has learned and tries to find the combinations of active neurons that, on the basis of its past experience, are most likely to represent the relevant objects and events in the visual world at that moment. The formation of such coalitions of active neurons may also be influenced by biases coming from other parts of the brain: for example, signals telling it what best to attend to or high-level expectations about the nature of the stimulus.
Consciousness, as James noted, is always changing. These rapidly formed coalitions occur at different levels and interact to form even broader coalitions. They are transient, lasting usually for only a fraction of a second. Because coalitions in the visual system are the basis of what we see, evolution has seen to it that they form as fast as possible; otherwise, no animal could survive. The brain is handicapped in forming neuronal coalitions rapidly because, by computer standards, neurons act very slowly. The brain compensates for this proportional deliberation as partly by using very many neurons, simultaneously and in parallel, and partly by arranging the system in a roughly hierarchical manner.
If visual awareness at any moment corresponds to sets of neurons firing, then the obvious question is: Where are these neurons located in the brain, and in what way are they firing? Visual awareness is highly unlikely to occupy all the neurons in the neocortex that are firing above their background rate at a particular moment. We would expect that, theoretically, at least some of these neurons would be involved in doing computations—trying to arrive at the best coalitions—whereas others would express the results of these computations, in other words, what we see.
Fortunately, some experimental evidence can be found to back up this theoretical conclusion. A phenomenon called binocular rivalry may help identify the neurons whose firing symbolizes awareness. This phenomenon can be seen in dramatic form in an exhibit prepared by Sally Duensing and Bob Miller at the Exploratorium in San Francisco.
Binocular rivalry occurs when each eye has a different visual input relating to the same part of the visual field. The early visual system on the left side of the brain receives an input from both eyes but sees only the part of the visual field to the right of the fixation point. The converse is true for the right side. If these two conflicting inputs are rivalrous, one sees not the two inputs superimposed but first one input, then the other, and so on in alternation.
In the exhibit, called ‘The Cheshire Cat,’ viewers put their heads in a fixed place and are told to keep the gaze fixed. By means of a suitably placed mirror, one of the eyes can look at another person's face, directly in front, while the other eye sees a blank white screen to the side. If the viewer waves a hand in front of this plain screen at the same location in his or her visual field occupied by the face, the face is wiped out. The movement of the hand, being visually very salient, has captured the brain's attention. Without attention the face cannot be seen. If the viewer moves the eyes, the face reappears.
In some cases, only part of the face disappears. Sometimes, for example, one eye, or both eyes, will remain. If the viewer looks at the smile on the person's face, the face may disappear, leaving only the smile. For this reason, the effect has been called the Cheshire Cat effect, after the cat in Lewis Carroll's Alice's Adventures in Wonderland.
Although it is very difficult to record activity in individual neurons in a human brain, such studies can be done in monkeys. A simple example of binocular rivalry has been studied in a monkey by Nikos K. Logothetis and Jeffrey D. Schall, both then at MIT They trained a macaque to keep its eyes still and to signal whether it is seeing upward or downward movement of a horizontal grating. To produce rivalry, upward movement is projected into one of the monkey's eyes and downward movement into the other, so that the two images overlap in the visual field. The monkey signals that it sees up and down movements alternatively, just as humans would. Even though the motion stimulus coming into the monkey's eyes is always the same, the monkey's percept changes every second or so.
Cortical area MT (which some researchers prefer to label V5) is an area mainly concerned with movement. What do the neurons in MT do when the monkey's percept is sometimes up and sometimes down? (The researchers studied only the monkey's first response.) The simplified - answer the actual data are rather more messy - is that whereas the firing of some of the neurons correlates with the changes in the percept, for others the average firing rate is relatively unchanged and independent of which direction of movement the monkey is seeing at that moment. Thus, it is unlikely that the firing of all the neurons in the visual neocortex at one particular moment corresponds to the monkey's visual awareness. Exactly which neurons do correspond to awareness remains to be discovered.
We have postulated that when we clearly see something, there must be neurons actively firing that stand for what we see. This might be called the activity principle. Here, too, there is some experimental evidence. One example is the firing of neurons in a specific cortical visual area in response to illusory contours. Another and perhaps more striking case is the filling in of the blind spot. The blind spot in each eye is caused by the lack of photoreceptors in the area of the retina where the optic nerve leaves the retina and projects to the brain. Its location is about 15 degrees from the fovea (the visual centre of the eye). Yet if you close one eye, you do not see a hole in your visual field.
Philosopher Daniel C. Dennett of Tufts University is unusual among philosophers in that he is interested both in psychology and in the brain. This interest is much to be welcomed. In a recent book, Consciousness Explained, he has argued that it is wrong to talk about filling in. He concludes, correctly, that ‘an absence of information is not the same as information about an absence.’ From this general principle he argues that the brain does not fill in the blind spot but rather ignores it.
Dennett's argument by itself, however, does not establish that filling in does not occur; it only suggests that it might not. Dennett also states that ‘your brain has no machinery for [filling in] at this location.’ This statement is incorrect. The primary visual cortex lacks a direct input from one eye, but normal ‘machinery’ is there to deal with the input from the other eye. Ricardo Gattass and his colleagues at the Federal University of Rio de Janeiro have shown that in the macaque some of the neurons in the blind-spot area of the primary visual cortex do respond to input from both eyes, probably assisted by inputs from other parts of the cortex. Moreover, in the case of simple filling in, some of the neurons in that region respond as if they were actively filling in.
Thus, Dennett's claim about blind spots is incorrect. In addition, psychological experiments by Vilayanur S. Have Ramachandran shown that what is filled in can be quite complex depending on the overall context of the visual scene. How, he argues, can your brain be ignoring something that is in fact commanding attention?
Filling in, therefore, is not to be dismissed as nonexistent or unusual. It probably represents a basic interpolation process that can occur at many levels in the neocortex. It is, incidentally, a good example of what is meant by a constructive process.
How can we discover the neurons whose firing symbolizes a particular percept? William T. Newsome and his colleagues at Stanford University have done a series of brilliant experiments on neurons in cortical area MT of the macaque's brain. By studying a neuron in area MT, we may discover that it responds best to very specific visual features having to do with motion. A neuron, for instance, might fire strongly in response to the movement of a bar in a particular place in the visual field, but only when the bar is oriented at a certain angle, moving in one of the two directions perpendicular to its length within a certain range of speed.
It is technically difficult to excite just a single neuron, but it is known that neurons that respond to roughly the same position, orientation and direction of movement of a bar tend to be located near one another in the cortical sheet. The experimenters taught the monkey a simple task in movement discrimination using a mixture of dots, some moving randomly, the rest all in one direction. They showed that electrical stimulation of a small region in the right place in cortical area MT would bias the monkey's motion discrimination, almost always in the expected direction.
Thus, the stimulation of these neurons can influence the monkey's behaviour and probably its visual percept. Such experiments do not, however, show decisively that the firing of such neurons is the exact neural correlate of the percept. The correlate could be only a subset of the neurons being activated. Or perhaps the real correlate is the firing of neurons in another part of the visual hierarchy that are strongly influenced by the neurons activated in area MT.
These same reservations apply also to cases of binocular rivalry. Clearly, the problem of finding the neurons whose firing symbolizes a particular percept is not going to be easy. It will take many careful experiments to track them down even for one kind of percept.
It seems obvious that the purpose of vivid visual awareness is to feed into the cortical areas concerned with the implications of what we see; from there the information shuttles on the one hand to the hippocampal system, to be encoded (temporarily) into long-term episodic memory, and on the other to the planning levels of the motor system. But is it possible to go from a visual input to a behavioural output without any relevant visual awareness?
That such a process can happen is demonstrated by the remarkable class of patients with ‘blindsight.’ These patients, all of whom have suffered damage to their visual cortex, can point with fair accuracy at visual targets or track them with their eyes while vigorously denying seeing anything. In fact, these patients are as surprised as their doctors by their abilities. The amount of information that ‘gets through,’ however, is limited: blindsight patients have some ability to respond to wavelength, orientation and motion, yet they cannot distinguish a triangle from a square.
It is naturally of great interest to know which neural pathways are being used in these patients. Investigators originally suspected that the pathway ran through the superior colliculus. Recent experiments suggest that a direct albeit weak connection may be involved between the lateral geniculate nucleus and other visual areas in the cortex. It is unclear whether an intact primary visual cortex region is essential for immediate visual awareness. Conceivably the visual signal in blindsight is so weak that the neural activity cannot produce awareness, although it remains strong enough to get through to the motor system.
Normal-seeing people regularly respond to visual signals without being fully aware of them. In automatic actions, such as swimming or driving a car, complex but stereotypical actions occur with little, if any, associated visual awareness. In other cases, the information conveyed is either very limited or very attenuated. Thus, while we can function without visual awareness, our behaviour without it is rather restricted.
Clearly, it takes a certain amount of time to experience a conscious percept. It is difficult to determine just how much time is needed for an episode of visual awareness, but one aspect of the problem that can be demonstrated experimentally is that signals received close together in time are treated by the brain as simultaneous.
A disk of red light is flashed for, say, 20 milliseconds, followed immediately by a 20-millisecond flash of green light in the same place. The subject reports that he did not see a red light followed by a green light. Instead he saw a yellow light, just as he would have if the red and the green light had been flashed simultaneously. Yet the subject could not have experienced yellow until after the information from the green flash had been processed and integrated with the preceding red one.
Experiments of this type led psychologist Robert Efron, now at the University of California at Davis, to conclude that the processing period for perception is about 60 to 70 milliseconds. Similar periods are found in experiments with tones in the auditory system. It is always possible, however, that the processing times may be different in higher parts of the visual hierarchy and in other parts of the brain. Processing is also more rapid in trained, compared with naive, observers.
Because it appears to be involved in some forms of visual awareness, it would help if we could discover the neural basis of attention. Eye movement is a form of attention, since the area of the visual field in which we see with high resolution is remarkably small, roughly the area of the thumbnail at arm's length. Thus, we move our eyes to gaze directly at an object in order to see it more clearly. Our eyes usually move three or four times a second. Psychologists have shown, however, that there appears to be a faster form of attention that moves around, in some sense, when our eyes are stationary.
The exact psychological nature of this faster attentional mechanism is at present controversial. Several neuroscientists, however, including Robert Desimone and his colleagues at the National Institute of Mental Health, have shown that the rate of firing of certain neurons in the macaque's visual system depends on what the monkey is attending to in the visual field. Thus, attention is not solely a psychological concept; it also has neural correlates that can be observed. A number of researchers have found that the pulvinar, a region of the thalamus, appears to be involved in visual attention. We would like to believe that the thalamus deserves to be called ‘the organ of attention,’ but this status has yet to be established.
The major problem is to find what activity in the brain corresponds directly to visual awareness. It has been speculated that each cortical area produces awareness of only those visual features that are ‘columnar,’ or arranged in the stack or column of neurons perpendicular to the cortical surface. Thus, the primary visual cortex could code for orientation and area MT for motion. So far experientialists have not found one particular region in the brain where all the information needed for visual awareness appears to come together. Dennett has dubbed such a hypothetical place ‘The Cartesian Theatre.’ He argues on theoretical grounds that it does not exist.
Awareness seems to be distributed not just on a local scale, but more widely over the neocortex. Vivid visual awareness is unlikely to be distributed over every cortical area because some areas show no response to visual signals. Awareness might, for example, be associated with only those areas that connect back directly to the primary visual cortex or alternatively with those areas that project into one another's layer 4. (The latter areas are always at the same level in the visual hierarchy.)
The key issue, then, is how the brain forms its global representations from visual signals. If attention is indeed crucial for visual awareness, the brain could form representations by attending to just one object at a time, rapidly moving from one object to the next. For example, the neurons representing all the different aspects of the attended object could all fire together very rapidly for a short period, possibly in rapid bursts.
This fast, simultaneous firing might not only excite those neurons that symbolized the implications of that object but also temporarily strengthen the relevant synapses so that this particular pattern of firing could be quickly recalled—a form of short-term memory. If only one representation needs to be held in short-term memory, as in remembering a single task, the neurons involved may continue to fire for a period.
A problem arises if it is necessary to be aware of more than one object at exactly the same time. If all the attributes of two or more objects were represented by neurons firing rapidly, their attributes might be confused. The colour of one might become attached to the shape of another. This happens sometimes in very brief presentations.
Some time ago Christoph von der Malsburg, now at the Ruhr-Universität Bochum, suggested that this difficulty would be circumvented if the neurons associated with any one object all fired in synchrony (that is, if their times of firing were correlated) but out of synchrony with those representing other objects. Recently two groups in Germany reported that there does appear to be correlated firing between neurons in the visual cortex of the cat, often in a rhythmic manner, with a frequency in the 35- to 75-hertz range, sometimes called 40-hertz, or g, oscillation.
Von der Malsburg's proposal prompted us to suggest that this rhythmic and synchronized firing might be the neural correlate of awareness and that it might serve to bind together activity concerning the same object in different cortical areas. The matter is still undecided, but at present the fragmentary experimental evidence does rather little to support such an idea. Another possibility is that the 40-hertz oscillations may help distinguish figure from ground or assist the mechanism of attention.
Are there some particular types of neurons, distributed over the visual neocortex, whose firing directly symbolizes the content of visual awareness? One very simplistic hypothesis is that the activities in the upper layers of the cortex are largely unconscious ones, whereas the activities in the lower layers (layers 5 and 6) mostly correlate with consciousness. We have wondered whether the pyramidal neurons in layer 5 of the neocortex, especially the larger ones, might play this latter role.
These are the only cortical neurons that project right out of the cortical system (that is, not to the neocortex, the thalamus or the claustrum). If visual awareness represents the results of neural computations in the cortex, one might expect that what the cortex sends elsewhere would symbolize those results. Moreover, the neurons in layer 5 show a rather unusual propensity to fire in bursts. The idea that layer 5 neurons may directly symbolize visual awareness is attractive, but it still is too early to tell whether there is anything in it.
Visual awareness is clearly a difficult problem. More work is needed on the psychological and neural basis of both attention and very short-term memory. Studying the neurons when a percept changes, even though the visual input is constant, should be a powerful experimental paradigm. We need to construct neurobiological theories of visual awareness and test them using a combination of molecular, neurobiological and clinical imaging studies.
We believe that once we have mastered the secret of this simple form of awareness, we may be close to understanding a central mystery of human life: how the physical events occurring in our brains while we think and act in the world relate to our subjective sensations—that is, how the brain relates to the mind.
Postscript: There have been several relevant developments since this article was first published. It now seems likely that there are rapid ‘on-line’ systems for stereotyped motor responses such as hand or eye movement. These systems are unconscious and lack memory. Conscious seeing, on the other hand, seems to be slower and more subject to visual illusions. The brain needs to form a conscious representation of the visual scene that it then can use for many different actions or thoughts. Exactly how all these pathways work and how they interact is far from clear.
There have been more experiments on the behaviour of neurons that respond to bistable visual percepts, such as binocular rivalry, but it is probably too early to draw firm conclusions from them about the exact neural correlates of visual consciousness. We have suggested on theoretical grounds based on the neuroanatomy of the macaque monkey that primates are not directly aware of what is happening in the primary visual cortex, even though most of the visual information flows through it. This hypothesis is supported by some experimental evidence, but it is still controversial.
