The biology of the Brain
...Continued from Page 2
Disorders of the brain
Injuries, diseases, and inherited disorders can damage the brain. However,
the seriousness of brain damage depends chiefly on the area of the brain
involved rather than on the cause of the damage. Disorders that destroy
brain cells are especially serious because the body cannot replace the lost
cells. In some cases, however, undamaged areas of the brain may eventually
take over control of some functions formerly carried out by the damaged
Modern instruments and
techniques enable doctors to diagnose brain disorders earlier and more
accurately than in the past. For example, an instrument called an
electroencephalograph (EEG) measures the patterns of electrical activity
produced by the brain. Differences from normal EEG patterns may indicate
damage to the brain and also help locate the area of the damage.
Computer-assisted EEG's can record and organize vast amounts of
The brain's responses to specific
visual, auditory, and touch stimuli can be measured. Scientists can diagnose
disorders by comparing the responses with average results obtained from
large numbers of people. Another important technique is computerized
tomography (CT). It involves X-raying the brain in detail from many angles.
A computer then analyses the X-ray data and constructs a cross-sectional
image of the brain on a TV screen. Magnetic resonance imaging (MRI) uses
magnetic fields and radio waves to produce images of the brain's structure.
Injuries are the leading cause of brain damage among people under 50 years
of age. A blow to the head may cause temporary unconsciousness without
permanent damage. Severe injuries to the head may cause more serious brain
damage. Head injuries before, during, or shortly after birth may cause
cerebral palsy. There are several types of cerebral palsy, all of which
involve lack of control of muscle movements.
Stroke is the most common serious disorder of the brain. A stroke occurs
when the blood supply to part of the brain is cut off. Nerve cells in the
affected areas die, and the victim may lose the ability to carry out
functions controlled by those areas. Many stroke victims suffer paralysis on
one side of the body. Other symptoms include difficulty in speaking or in
understanding language. Most strokes result from damage to the blood vessels
caused by hypertension (high blood pressure) or arteriosclerosis (hardening
of the arteries). Some victims of massive strokes die, but many other stroke
victims survive and recover at least partially.
Tumours are abnormal growths that can cause severe brain damage. The effects
of a tumour depend on its size and location. A tumour may destroy brain
cells in the area surrounding it. As the tumour grows, it also creates
pressure, which may damage other areas of the brain or at least interfere
with their normal function. Symptoms of a tumour include headache, seizures,
unusual sleepiness, a change in personality, or disturbances in sense
perception or speech.
Surgery cures some tumours. For cancerous tumours, doctors may combine
surgery with drugs or radiation. One type of radiation, called stereotactic
radiosurgery, is sometimes used as an alternative to traditional surgery. In
stereotactic radiosurgery, doctors use computers and a CT scan or MRI to
produce a three-dimensional image of the brain. Beams of radiation are then
focused precisely on a target, which may be a tumour or a blood-vessel
malformation. The individual beams are either too brief or too weak to harm
areas of the brain in the path of the radiation. But their combined effect
will destroy the target. These procedures are quick and painless and allow
patients to resume moderate activity the same day.
Infectious diseases. A number of diseases caused by bacteria or viruses can
damage the brain. The most common of these infectious diseases are
encephalitis and meningitis, either of which may be caused by bacteria or
viruses. Encephalitis is an inflammation of the brain. Meningitis is an
inflammation of the meninges, the membranes that cover the brain and spinal
cord. Chorea is a disease of the brain that mainly affects children from 7
to 15 years old. Most cases of chorea occur with rheumatic fever and may be
caused by the same bacteria which cause that disease. A virus disease called
poliomyelitis attacks the brain and spinal cord. Vaccines to prevent polio
were developed in the 1950's.
Genetic disorders. Our genes (the hereditary materials in cells) carry
instructions for the development of our entire bodies, including the brain.
These instructions are extremely complex, and so errors occasionally occur.
These errors can lead to serious defects in the structure and functioning of
the brain. Some infants are mentally retarded at birth because genetic
errors caused the brain to develop improperly during the mother's pregnancy.
In Down syndrome, for example, an extra chromosome is present. Chromosomes
are structures in the cell nucleus that contain the genes. The extra
chromosome causes mental retardation as ell as physical defects. Another
disorder that causes mental retardation is fragile-X syndrome. This disorder
results from an abnormality on the X chromosome, one of the chromosomes that
determine a person's sex.
Some children suffer severe brain damage after birth because of an inherited
deficiency of an enzyme that the body needs to use foods properly. For
example, a child who has phenylketonuria (PKU) lacks an enzyme needed to
convert a certain amino acid (protein part) into a form the body can use.
This amino acid, phenylalanine, accumulates in the blood and damages
developing brain tissues. A diet low in phenylalanine can prevent brain
damage in PKU victims.
Some genetic errors damage the brain only later in life. Huntington's
disease, for example, strikes most victims during middle age. It causes
various areas of the cerebrum and basal ganglia to wither away. Involuntary,
jerky movements are the main early symptoms of this disease, but it
eventually leads to incurable mental disintegration.
Scientists believe that genetic factors play an important role in most cases
of Alzheimer's disease. This disease most commonly strikes after age 60. It
is characterized by an increasingly severe loss of memory and other mental
abilities. Most people with Alzheimer's disease eventually cannot care for
themselves and become bedridden.
Heredity also plays a role in some types of mental illness. Many children of
schizophrenics apparently inherit a tendency to develop schizophrenia.
Studies have also revealed an inherited tendency to develop bipolar
disorder. These tendencies may involve inherited defects in brain chemistry.
Researchers continue to study these tendencies and how they interact with
environmental conditions to produce mental illness.
Other brain disorders include: (1) epilepsy, (2) multiple sclerosis (MS),
and (3) Parkinson's disease. Scientists do not know the cause of these
Epilepsy. Victims of epilepsy suffer seizures that occur when many nerve
cells in one area of the brain release abnormal bursts of impulses. A
seizure may cause temporary uncontrolled muscle movements or
unconsciousness. Defects in genes cause some cases of epilepsy, but the
cause of most cases is not known. Doctors treat epilepsy with drugs that
reduce the number of seizures or prevent them entirely.
Multiple sclerosis develops when axons in parts of the brain and spinal cord
lose their myelin sheaths. As a result, the axons cannot carry nerve
impulses properly. Symptoms vary depending on what brain areas are affected,
but they may include double vision, loss of balance, and weakness in an arm
or leg. No cure is yet known. Drugs can relieve some of the symptoms. Some
of these drugs help slow the loss of myelin.
Parkinson's disease is characterized by slowness of movement, muscle
rigidity, and trembling. These conditions result in part from the
destruction of the nerve pathways that use dopamine as a transmitter.
Treatment with the drug L-dopa replaces the missing dopamine and so can
relieve the symptoms of Parkinson's disease, though it cannot cure the
disease. Some researchers have treated Parkinson's disease by transplanting
dopamine-producing brain tissue from fetuses into part of the basal ganglia,
which help control body movement. This procedure is risky, and its
usefulness has not yet been proved. In addition, it has aroused controversy
on moral grounds because the fetal cells are obtained during abortions.
The brain in animals
Most invertebrates (animals without a backbone) do not have a well-developed
brain. Instead, they have clusters of nerve cells, called ganglia, that
coordinate the activities of the body. All vertebrates (animals with a
backbone) have some kind of brain. Scientific evidence suggests the complex
brain in higher animals evolved (developed gradually) through the ages.
In invertebrates. The more advanced invertebrates, such as worms and
insects, have some type of relatively simple brain. An earthworm, for
example, has in its head region a large pair of ganglia that control the
worm's behaviour on the basis of information received from the sense organs.
An insect has a more complex brain that consists of three pairs of ganglia.
The ganglia receive information from the sense organs and control such
complex activities as feeding and flying.
Octopuses have the most highly developed brain among invertebrates. Their
brain is divided into several parts, the largest of which is the optic lobe.
The optic lobe processes information from an octopus's eyes, which resemble
the eyes of vertebrates in structure and function.
In vertebrates, the brain can be divided into three main regions: (1) the
forebrain, (2) the midbrain, and (3) the hindbrain. The midbrain is the most
highly developed region in primitive vertebrates, such as fish and
amphibians. In contrast, the forebrain, or cerebrum, makes up only a small
part of the brain in these animals. As increasingly complex vertebrates
evolved, two major changes occurred in the brain. The size and importance of
the cerebrum increased enormously, and the relative size and importance of
the midbrain decreased. The hindbrain consists of the medulla and the
cerebellum. Its structure and function are basically the same in all
vertebrates, though the cerebellum is larger and more complex in advanced
Among fish and amphibians, the midbrain consists chiefly of two optic lobes.
These lobes serve not only as the centre of vision but also as the major
area for coordinating sensory and motor impulses. A fish's cerebrum is
composed of two small, smooth swellings that serve mainly as the centre of
smell. In amphibians, the cerebrum is slightly larger and is covered by a
In reptiles, some functions of the midbrain have been taken over by the
cerebrum. A reptile's cerebrum is larger and more complex than that of a
fish or amphibian. Within the cerebrum are basal ganglia. These small
bundles of neurons form a major area where information is analysed,
processed, and stored. Some advanced reptiles have a small area of cerebral
cortex that differs from the cortex in lower vertebrates. This area, called
the neocortex (new cortex), functions as an important area for information
processing and storage.
Birds have a cerebrum larger than that of fish, amphibians, and reptiles.
But unlike some advanced reptiles, birds lack a neocortex. Instead, the
dominant part of their brain consists of large, highly developed basal
ganglia, which fill most of the interior of the cerebrum. The basal ganglia
serve as the main centre for processing and storing information and give
birds an impressive ability to learn new behaviour. They apparently also
store the instructions for the many instinctive behaviour patterns of birds.
Birds also have a well-developed cerebellum, which coordinates all the
sensory and motor impulses involved in flying.
The brain reaches its highest level of development in mammals. The neocortex
forms nearly all the cerebral cortex of the mammalian brain, and the
midbrain serves mainly as a relay centre. The most primitive mammals, such
as moles and shrews, have a relatively small cerebrum with a smooth cerebral
cortex. More advanced mammals, such as horses and cats, have a larger
cerebrum covered by a cortex with many ridges and grooves. These
indentations increase the surface area of the brain. Whales and dolphins
have a large, highly developed brain. The brain in chimpanzees and other
apes is even more highly developed. It resembles the human brain more
closely than does the brain in any other species of animals.
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