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The BrainThe Brain

The brain is literally the nerve centre of our bodies. More sophisticated than any computer, it sits neatly within our skulls controlling and directing everything we do or think whether we are awake or asleep.

No part of the body is more important or complicated than the brain. It lies at the centre of a complex network of nerves that runs through the spinal cord. It's our brains -- bigger and more sophisticated than those of any other animal -- that make us what we are. They control literally everything we think and do, from walking and talking to feelings like love, jealousy, sadness and every other emotion.

GENERAL CONTROLS

The brain and the spinal cord make up what is known as the central nervous system. This controls basic bodily functions such as breathing, temperature and heart-rate, as well as conscious actions such as eating, talking and moving.

The CNS is constantly receiving messages in the form of nerve impulses from our senses, which inform the brain at all times of where we are and what is going on around us. The brain interprets this information and , when we decide to act on it, sends instructions down other nerve cells called motor nerves. Sometimes, however, in situations of danger or where survival is at stake, there simply isn't time for a message to be interpreted by the brain. In this case, the message goes only to the spinal cord which responds with a reflex action. Examples of these are blinking, reactions to pain and sexual responses. When the doctor tests the knee jerk reflex, he is checking on the speed of your reflexes, which can indicate potential damage to the spinal cord.

All the messages flashing to and from our brains are transmitted by minute electrical impulses, which travel through special nerve cells known as neurons. This electrical activity can be measured by an electroencephalogram, or EEG.

Strangely, the left side of the brain controls the right hand side of the body and the right side of the brain controls the left hand side.

The brain cells form a mass of jelly-like tissue encased in three layers of protective membranes called meninges and bathed in a fluid called, cerbrospinal fluid. Four arteries in. the neck supply the brain with the blood without which it cannot survive. In fact, the brain receives about one fifth of the heart's output of blood.

MAPPING THE BRAIN
The brain is divided into three different regions: the hindbrain, midbrain and forebrain. Each of these is divided into separate sections, which are responsible for different functions and linked to other parts of the brain.

The largest structure in the hindbrain is the cerebellum, which is in charge of balance and co-ordination. The brain stem, which lies at the base of the cerebellum, is the oldest, most primitive part of the hindbrain. It is here that all incoming and outgoing messages come together and cross over to the opposite side of the brain. The brain stem is concerned with the vital functions of life itself. Activities such as our heart rate, breathing, blood pressure, consciousness, swallowing, coughing, vomiting and hiccuping are all controlled by this part of the brain.

CONSCIOUSNESS
The reticular activating system, which is part of the brain stem, is the area of the brain that controls consciousness. It sifts through the mass of incoming information and decides which is important enough to alert other regions of the brain.

It does so by controlling the amount of electrical activity each part of the brain receives.

Just beyond the brain stem, in the midbrain, is an area which controls eye movements. Beyond this, the forebrain begins. The egg-shaped thalamus is located here; this acts like a relay station for incoming sensory information.

Beneath this is the hypothalamus. This tiny region, which has close links with the pituitary gland, is involved with bodily functions such as hunger and thirst, regulation of body temperature, aggression, sexual behaviour and sleep.

CENTRE OF THE EMOTIONS
Encircling the thalamus is a part of the brain called the limbic system, which is made up a number of structures, including the septum pelucidum, hippocampus and amygdala. Another very ancient part of the brain, the limbic system is concerned with the emotions, and parts of the memory.

This system has close links with the sense of smell, which is one reason why a particular smell or odour can evoke a forgotten memory more intensely than anything else. The limbic system also relays messages from the body's internal organs to other parts of the brain, which explains why such sensations are tinged with emotion and goes some way to account for phenomena such as 'nervous stomach aches'.

The largest part of the whole brain is a structure known as the cerebrum, which is located in the forebrain. The cerebrum is more developed in humans than in any other animal and is vital to thought, memory, consciousness and higher mental activities. The cerebrum is divided into two halves, or hemispheres, which are joined by a thick bundle of nerve fibres called the corpus callosum.

VARIOUS FUNCTIONS
The two hemispheres, although identical in appearance, have completely different functions. For example, higher mental processes such as speech and writing are controlled from the dominant cerebral hemisphere.

Whether you are right or left handed, the left hemisphere is usually dominant in writing, The non-dominant right brain controls visual/spatial orientation and tasks such as map-reading, navigation and so on. It's also thought to be the more intuitive side of the brain, which is used in artistic and creative thoughts and tasks.

The outer surface of the cerebrum, known as the cerebral cortex, consists of 3mm (0.1 in) thick, wrinkled layer of grey matter, with nerve cells arranged in six different layers.

This part of the brain is so developed that, in order to fit inside the skull, it has to fold over into an area 30 times smaller than its actual size, This is the region of the brain that is concerned with conscious thought, sensation, movements, speech, and complex activities such as writing. Sensory information reaching this part of the brain undergoes detailed analysis before any action is initiated. So, memory, thought and decision making all come into play.

THE FOUR LOBES
Each of the two hemispheres of the cortex are divided into four distinct lobes -- occipital, parietal, temporal and frontal - named after the bones of the skull that lie over them. Each of these is concerned with different senses: the occipital lobes are concerned with vision, the parietal with touch, the temporal with hearing and smell, and the frontal lobes with movement and complicated thought.

Specific parts of each lobe are devoted to receiving sensory messages from different areas of the body, so for example, there is a tiny area in the parietal lobe devoted to sensation from the knee, while, a larger area is devoted to sensation from the thumb. This is the reason why the thumb is more sensitive than the knee. The frontal lobes, concerned as they are with thought and action, are also central to our personality.

Beneath the cortex, there are tracts of nerve fibres, made up of white matter, which connect the various areas of the cortex to each other and also to nerve centres in the centre of the forebrain and brain stem. Deeper still within the hemispheres are groups of cells called basal ganglia, which are connected to the brain stem and cerebellum.

MEMORY BANKS
Above all, our brains are concerned with memory and learning. Memory is a complicated process and experts are still trying to work out what processes are involved. Whenever we think or do anything, an electrical message passes through a brain cell and in the process changes the cell physically.

When the same process is repeated, the cell then changes permanently and new electrical pathways are set up which enable us to remember something or, in other words, to learn it. In this way, a memory literally becomes a physical part of you.

Memory doesn't take place just in one area in the brain; instead different parts of the brain seem to store different memories, That said, the temporal lobe and, the limbic system seem to be particularly concerned with memory and disturbances of these can cause memory disorders. For example, stimulating a particular part of the temporal lobe in patients suffering from temporal lobe epilepsy often evokes the same memory. And, similarly, patients with the illness have uncontrollable flashbacks of past events.

Brain damage is said to occur when nerve cells or tracts have degenerated or died. Damage may be confined to one part of the brain, causing particular problems such as difficulty speaking, or paralysis of one part of the body. Or it may be more widespread, causing mental or physical handicap.

LACK OF OXYGEN
The most common cause of brain damage is lack of oxygen to the brain, medically known as hypoxia. Babies in the womb may be deprived of their oxygen because of narrowing of the blood vessels. in the placenta due to pre-eclamptic toxaemia, which is why such babies are often delivered as early as it is safe to do so.

Other babies sustain brain damage during birth. This type of brain damage can cause cerebral palsy, or spasticity, which results in paralysis, abnormal movements, and sometimes mental retardation and deafness.

In adults, hypoxia may also arise due to cardiac arrest, when the heart stops beating, respiratory arrest, or through the accumulation of toxins in the brain due to the untreated enzyme deficiency, phenylketonuria.

Infections such as encephalitis, or inflammation of the brain, can also cause brain damage, which is one reason why measles in some cases can be such a serious illness.

AFTER-EFFECTS
Brain damage can be due to head injury, stroke, a brain tumour or brain abscess, which leaves the victim with a range of handicaps. These may include problems speaking or moving, mental handicap or even epilepsy. Local damage to the basal ganglia can also occur at birth as a result of haemolytic disease of the newborn.

People who have sustained brain damage, however, do eventually regain some function as other parts of the brain take over from those that were damaged. Younger people, especially children, tend to recover much better than older sufferers. And children, in particular, can make astonishing recoveries from brain damage because their brains are not so rigidly organized as those of adults.

     
     

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