The biology of the Brain
...Continued from Page 1
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
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
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
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
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
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
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
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
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
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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