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The biology of the Brain

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Brain cells. The human brain has from 10 billion to 100 billion neurons. All of these neurons are present within a few months after birth. After a person reaches about 20 years of age, some neurons die or disappear each day. In general, neurons that die are not replaced during a person's lifetime. Over a lifetime, however, this loss equals less than 10 per cent of all the neurons. 

The brain's billions of neurons connect with one another in complex networks. All physical and mental functioning depends on the establishment and maintenance of neuron networks. A person's habits and skills--such as nail-biting or playing a musical instrument--become embedded within the brain in frequently activated neuron networks. When a person stops performing an activity, the neural networks for the activity fall into disuse and may eventually disappear. 

As in all other cells, a thin membrane forms the outermost layer of each neuron. However, a neuron's membrane is highly specialized to carry nerve impulses. Each neuron consists of a cell body and a number of tubelike fibres. The longest fibre, called the axon, carries nerve impulses from the cell body to other neurons. Short, branching fibres called dendrites pick up impulses from the axons of other neurons and transmit them to the cell body. The structure where any branch of one neuron transmits a nerve impulse to a branch of another neuron is called a synapse. Each neuron may form synapses with thousands of other nerve cells. 
 

Some axons have a coating of fatty material called myelin. The myelin insulates the fibre and speeds the transmission of impulses along its surface. Myelin is white, and tightly packed axons covered with it form white matter. The neuron cell bodies and the axons without myelin sheaths make up the grey matter of the brain. The cerebral cortex is made up of grey matter, and most of the rest of the cerebrum consists of white matter. 

The neurons are surrounded by glia, cells whose name comes from a Greek word for glue. Glial cells have traditionally been thought of as a supportive framework for the neurons. The glia also perform many other important tasks. For example, certain glia keep the brain free of injured and diseased neurons by engulfing and digesting them. Other glia produce the myelin sheaths that insulate some axons. Research using cells grown in laboratories also indicates that glia, like neurons, may transmit some nerve impulses. 

How the brain is protected 

The hard, thick bones of the skull shield the brain from blows that could otherwise seriously injure it. In addition, three protective membranes called meninges cover the brain. The outermost membrane is the tough dura mater, which lines the inner surface of the skull. A thinner membrane, the arachnoid, lies just beneath the dura mater. The delicate pia mater directly covers the brain. It follows the folds of the brain's surface and contains blood vessels that carry blood to and from the cerebral cortex. A clear liquid called cerebrospinal fluid separates the pia mater and the arachnoid. This fluid forms a thin cushioning layer between the soft tissues of the brain and the hard bones of the skull. 

The blood-brain barrier safeguards brain tissues from the damage that could result from contact with certain large molecules carried in the bloodstream. Substances in the blood reach body tissues by passing through the thin walls of tiny blood vessels called capillaries. Much of this flow occurs through the spaces between the cells that make up the capillary walls. In brain capillaries, the cells are more tightly packed than in other capillaries, and the passage of substances from blood to brain cells is carefully restricted. The brain needs some kinds of large molecules for nutrition, however. The capillary walls have certain enzymes and other properties that enable these particular molecules to pass through. 

The work of the brain 

The structure of our brain determines how we experience the world. Our experiences, in turn, influence how our neurons develop and connect with one another. Individual brains can differ significantly, depending on a person's background and experience. The fingers activate the same general area of the sensory cortex in everyone's brain. But this area is larger in people who use their fingers particularly often--for example, people who play stringed instruments, or people who read Braille (an alphabet of small raised dots developed for the blind). 

Scientists have also found evidence that the brains of men and women differ. The corpus callosum--the thick band of nerve fibres connecting the cerebral hemispheres--is larger in women. Careful examinations of brains after death have shown that women have about 10 per cent more neurons in the cortex than men. Studies of men and women reading or thinking about words also show differences. These studies have found that men generally use only their left cerebral hemisphere for processing language, but women use both hemispheres. 

Researchers are not sure if these physical differences in men's and women's brains mean that men and women think differently. Some evidence suggests that the sexes may have different mental strengths. Psychological testing consistently shows that men, on average, perform better than women on spatial tasks, such as visualizing objects in three dimensions. Women, on the other hand, do better than men on tests involving writing, reading, and vocabulary. But this average difference in ability is small. Many individual men are better at language than the average for women, and many women have better spatial skills than the average for men. 

Scientists have developed many methods to study how the brain works. Experiments with animals have revealed a great deal about the workings of various areas of the brain. Scientists have also learned much about the normal activity of the brain by observing injured brains. Damage to a specific part of the brain causes predictable problems in speech, movement, or mental ability. 

Surgeons have mapped the functions of many areas of the cerebral cortex by electrically stimulating the brain during brain . Brain operations do not require that the patients be unconscious because the brain feels no pain directly. Thus, the patients can tell the surgeons what they experience when particular brain areas are stimulated. 

Brain surgery has revealed that certain functions of the cerebrum occur chiefly in one hemisphere or the other. Surgeons treat some cases of epilepsy by cutting the corpus callosum. This operation produces a condition called the split brain, in which no communication occurs between the cerebral hemispheres. Studies of split-brain patients suggest that the left hemisphere largely controls our ability to use language, mathematics, and logic. The right hemisphere is the main centre for musical ability, the recognition of faces and complicated visual patterns, and the expression of emotion. 

Positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) are safe new technologies that enable scientists to study healthy, living brains at work. These technologies do not require any physical contact with the brain. They produce images similar to X rays that show which parts of the brain are active while a person performs a particular mental or physical task. PET shows the parts of the brain that are using the most glucose (a form of sugar), and fMRI shows the parts where high oxygen levels indicate increased activity. 

In receiving sensory messages. Sensory messages are received and interpreted primarily in the cerebral cortex. Various parts of the body send nerve impulses to the thalamus, which routes them to the appropriate areas of the cerebral cortex. An area of the sensory cortex called the somatosensory cortex receives messages that it interprets as bodily sensations, such as touch and temperature. It lies in the parietal lobe of each hemisphere along the central fissure. Each part of the somatosensory cortex receives and interprets impulses from a specific part of the body. 

Other specialized areas of the cerebrum receive the sensory impulses of seeing, hearing, taste, and smell. Impulses from the eyes travel to the visual cortex in the occipital lobes. Portions of the temporal lobes receive messages from the ears. The area for taste lies buried in the lateral fissure, and the centre of smell is on the underside of the frontal lobes. 

In controlling movement. Some reflex actions do not involve the brain. If a person touches a hot stove, for example, pain impulses flash to the spinal cord, which immediately sends back a message to withdraw the hand. However, the brain plays the major role in controlling our conscious movements as well as those we are unaware of. The basal ganglia are groups of neurons that lie at the base of the cerebrum. The basal ganglia help control well-learned movement sequences involved in such activities as walking or eating. Areas in the brain stem control the movements of the body's involuntary muscles, which line the walls of the stomach, intestines, and blood vessels. 

The cerebral cortex and the cerebellum together largely regulate voluntary movements. The motor cortex in each cerebral hemisphere sends nerve impulses to the particular muscles used in an activity, such as writing or throwing a ball. The motor cortex lies in the frontal lobe in front of the central fissure. Each area of the motor cortex controls the movements of a specific part of the body. The largest areas control those parts of the body that make the most complicated and precise movements. Thus, a large area controls the lips and tongue, which make complex movements in speaking. Much smaller areas control the relatively simple movements made by such parts as the back and shoulders. 

The major motor pathways to the body cross over in the brain stem. The motor cortex of the left hemisphere thus controls movements on the right side of the body. Similarly, the right motor cortex directs movements on the left side of the body. More than 90 per cent of all people are right-handed because the left motor cortex, which directs the right hand, is dominant over the right motor cortex, which directs the left hand. 

The cerebellum coordinates the muscle movements ordered by the motor cortex. Nerve impulses alert the cerebellum when the motor cortex orders a part of the body to perform a certain action. Almost instantly, impulses from that part of the body inform the cerebellum of how the action is being carried out. The cerebellum compares the movement with the intended movement and then signals the motor cortex to make any necessary corrections. In this way, the cerebellum ensures that the body moves smoothly and efficiently. 

In the use of language. In the late 1800's, scientists observed that damage to particular parts of the brain caused the same language disabilities in most patients. Damage to the left frontal lobe in Broca's area, named in honour of the French surgeon Pierre Paul Broca, destroyed the ability to speak. Damage to the left temporal lobe in Wernicke's area, named in honour of the German neurologist Carl Wernicke, caused difficulty understanding language. These observations led many scientists to think that the brain processed words in an orderly relay through a series of language-related areas. But new imaging technologies such as PET and fMRI enable scientists to observe the brain directly while people speak, listen, read, and think. PET and fMRI studies show that language processing is extremely complex. Language areas are spread widely through the brain, and different types of language tasks activate these areas in many sequences and patterns. 

In regulating body processes. The main control centres for body processes are in the brain stem. Nerve centres in the medulla regulate such body functions as breathing, heartbeat, and blood flow. Other areas within the brain stem control swallowing and the movements of the stomach and intestines. 

The hypothalamus also has nerve centres that control certain body processes. Most of these centres maintain constant conditions within the body. For example, some centres regulate the amount of water in the body. Certain neurons detect changes in the level of water in the body's blood and tissues and relay this information to the hypothalamus. If the water level is too low, the hypothalamus produces the sensation of thirst, which causes the person to drink water. At the same time, the hypothalamus sends messages that cause the kidneys to reduce the amount of water they remove from the body. If the water level becomes too high, the messages from the hypothalamus eliminate thirst and increase the amount of water removed by the kidneys. Other centres in the hypothalamus operate on the same principle in regulating hunger and body temperature. 

A slender stalk of tissue connects the hypothalamus with the master gland of the body, the pituitary. The hypothalamus indirectly regulates many body processes by controlling the pituitary's production and release of chemical messengers called hormones. Among other functions, these hormones regulate the body's rate of growth and its sexual and reproductive processes. 

In producing emotions. The emotions we experience involve many areas of the brain as well as other body organs. A group of brain structures called the limbic system plays a central role in the production of emotions. This system includes portions of the temporal lobes, parts of the hypothalamus and thalamus, other structures. 

An emotion may be provoked by a thought in the cerebral cortex or by messages from the sense organs. In either case, nerve impulses are produced that reach the limbic system. These impulses stimulate different areas of the system, depending on the kind of sensory message or thought. For example, the impulses might activate parts of the system that produce pleasant feelings involved in such emotions as joy and love. Or the impulses might stimulate areas that produce unpleasant feelings associated with anger or fear. 

In thinking and remembering. Scientists have only an elementary understanding of the extraordinarily complicated processes of thinking and remembering. Thinking involves processing information over circuits in the association cortex and other parts of the brain. These circuits enable the brain to combine information stored in the memory with information gathered by the senses. Scientists are just beginning to understand the brain's simplest circuits. Forming abstract ideas and studying difficult subjects must require circuits of astonishing complexity. 

The frontal lobes of the cerebrum play a key role in many thinking processes that distinguish human beings from other animals. The frontal lobes are particularly important for abstract thinking, for imagining the likely consequences of actions, and for understanding another person's feelings or motives. Injury or abnormal development of the frontal lobes can result in the loss of these abilities. 

Some aspects of human thinking--such as religious or philosophical beliefs--are still beyond scientists' understanding and may always be. Scientists also have much to learn about the physical basis of memory. Certain structures of the limbic system appear to play major roles in storing and retrieving memories. These structures include the amygdaloid complex and the hippocampus, both in the temporal lobe. People who suffer damage to these structures may lose the ability to form new memories, though they may retain information about events occurring before the damage. These people can learn new physical skills, but when performing them do not remember having done the activities before. 

Evidence suggests that memories may be formed through the establishment of new brain circuits or the alteration of existing circuits. Either process would involve changes at the synapses--that is, at the structures where impulses pass from one neuron to another. These changes may be controlled by glycoproteins or other large molecules at the synapses. Extensive research will be required to verify this general explanation of memory formation and to discover the specific details of the processes involved. 

The chemistry of the brain 

As in all other cells, many complex chemical processes occur within the neurons of the brain. However, some chemical processes occur only within and among neurons. Scientists are especially interested in gaining a fuller understanding of these processes and how they relate to the transmission of nerve impulses. 

A nerve impulse is an electrical and chemical process controlled by the nerve cell membrane. The process involves ions (electrically charged atoms) of chemical elements, such as sodium and potassium. The membrane, which has pores, maintains varying concentrations of these ions inside the neuron and in its surrounding fluids. As the membrane selectively allows ions to enter and leave the cell, an electric charge--the nerve impulse--travels along the neuron. For more details about this process. The rest of this section discusses the chemicals that transmit impulses from neuron to neuron. 

The brain's chemical messengers. Certain chemicals called neurotransmitters make it possible for a nerve impulse to travel from the axon of one neuron to the dendrite of another. An impulse cannot be transmitted electrically across the synaptic cleft, the tiny gap between the axon and the dendrite. Instead, when an impulse reaches the end of the axon, it triggers the release of neurotransmitter molecules from the cell. These molecules cross the synaptic cleft and attach themselves to sites called receptors on the dendrite of the other neuron. This action alters the electrical activity of the receiving neuron in one of two ways. Some transmitters stimulate the neuron to produce a nerve impulse. Others tend to prevent the neuron from producing an impulse. 

Neurons may manufacture more than one neurotransmitter, and their membrane surfaces may contain receptors for more than a single transmitter. A neuron may "learn" from past experience and change the proportions of its various neurotransmitters and receptors. Thus, the brain has great flexibility and can alter its response to situations encountered over spans of time ranging from seconds to decades. 

The brain produces many kinds of chemicals that are used as neurotransmitters. The most common ones include acetylcholine, dopamine, noradrenaline, and serotonin. The chemicals are not distributed evenly throughout the brain. Each is found only or primarily in specific areas. For example, the cell bodies of neurons that contain dopamine are in the midbrain of the brain stem. The axons of these cells reach into other areas, including the frontal lobes of the cerebrum and an area near the centre of the brain called the corpus striatum. These dopamine pathways function in the regulation of emotions and in the control of complex movements. 

During the 1970's, researchers discovered that morphine and related drugs relieve pain by attaching to receptors in certain regions of the brain. This discovery suggested that the brain produces its own painkillers that attach to these same receptors. Further research led to the discovery of endorphins and encephalins, two neurotransmitters that bind to these receptors. 

In the 1980's, researchers found that receptors exist in families. Each member or subtype of a family is responsible for a specific function. For example, scientists have discovered more than a dozen receptor subtypes for serotonin. This knowledge has led to development of drugs that affect specific serotonin receptors, such as migraine drugs and certain antidepressants. Scientists believe that the discovery of additional receptor subtypes will result in the development of drugs that work with increased precision in the treatment of thought, mood, and behaviour disorders. 

Brain chemistry and mental illness. All the brain's functions depend on the normal action of neurotransmitters. An excess or deficiency of a specific transmitter or group of transmitters may lead to a serious disorder in thought, mood, or behaviour. For example, studies have suggested that chemical imbalances in the brain play a significant role in several types of mental illnesses. There is some evidence that the brain produces too much dopamine in a severe mental illness called schizophrenia. This excess of dopamine may create emotional disturbances and cause a person to see things and hear sounds that do not exist. 

Disturbances in brain chemistry may also be involved in bipolar disorder, also known as manic-depressive disorder. A person with this mental illness has alternate periods of mania (extreme joy and overactivity) and depression (sadness). Some research suggests that an excess of dopamine, noradrenaline, and serotonin causes mania and that a deficiency of the same chemicals causes depression. 

How drugs affect brain chemistry. Psychiatrists treat some mental illnesses with drugs that restore the brain's normal chemical activity. For example, many tranquillizers that relieve the symptoms of schizophrenia block the brain's receptors for dopamine. However, it seems unlikely that a single neurotransmitter is responsible for schizophrenia or other complex mental illnesses, such as bipolar disorder and depression. These disorders probably result from chemical disturbances involving several neurotransmitters. For example, some drugs that have proved successful in treating depression influence noradrenaline, while others influence serotonin. Still others affect both of these neurotransmitters. 

Certain drugs produce a feeling of well-being or reduce tension and worry by temporarily altering the normal chemistry of the brain. For example, amphetamines increase mental activity by causing brain cells to release an excessive amount of dopamine. Abuse of amphetamines can create mental disturbances like those that occur in some forms of schizophrenia. 

A person's senses, emotions, thought processes, and judgment can be altered dramatically and dangerously by hallucinogenic drugs. These drugs include mescaline, psilocybin, and LSD (lysergic acid diethylamide). Each of these drugs structurally resembles one or more neurotransmitters. Mescaline resembles dopamine and noradrenaline, and LSD and psilocybin resemble serotonin. Scientists think a hallucinogenic drug may produce its effects by combining with the brain's receptors for the natural transmitter that it resembles. Hallucinogenic drugs may produce disturbances in brain chemistry that last long after their contact with the brain. For example, scientists believe that the drug called MDMA, commonly known as Ecstasy, may cause permanent damage to neurons that release serotonin. This damage may produce harmful effects on mood, thoughts, sleep, and motivation. 
 

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