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The biology of the Brain

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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 areas. 

 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 disorders. 

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 animals. 

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 cortex. 

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.

Additional resources 


Barmeier, Jim. The Brain. Lucent Books, San Diego California, U.S.A., 1996. 

Lambert, Mark. The Brain and the Nervous System. MacMillan, London, 1988. 

Lees, Dave and Liles, Eleanor. You and Your Mind. Longman, Harlow, Essex, U.K., 1989. One of the vols. in the Science at Work series. 

Parker, Steve. The Brain and Nervous System. Franklin Watts, London, 1989. 

Silverstein, A. and V. World of the Brain. William Morrow, New York, 1986. 

Albrecht, Karl. Brain Power: Learn to Improve Your Thinking Skills. Prentice Hall, Hemel Hempstead, Hertfordshire, U.K., 1980. 

Berger, Melvin. Exploring the Mind and Brain. Thomas Y. Crowell, New York, 1983. 

Blakemore, Colin. The Mind Machine. British Broadcasting Association, London, 1988. Based on the BBC TV series of the same name. 

Kilvington, Kenneth A. The Science of Mind. Massachusetts Institute of Technology Press, Cambridge, Massachussetts, U.S.A., 1989. 

Llinas, Rodolfo R. Workings of the Brain. W.H. Freeman, Oxford, 1990. 

Ornstein, R. and Thompson, R.F. The Amazing Brain. Chatto and Windus, London, 1984. 

Turkington, Carol. The Brain Encyclopedia. Facts On File, New York, 1996.

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