The human brain contains between 10 and 14 thousand million neurons or nerve cells, far more it seems than any of us ever use, and weighs about 1.5 kg (3 lb 5 oz). It generates consciousness and all mental and physical behaviour; rather like a central executive committee, it evaluates stimuli from inside and outside the body and issues orders to the body to adapt in ways which are likely to ensure survival.
The brain, together with its spinal extension, the spinal cord, is known as the central nervous system, as distinct from the peripheral nervous system, which consists of the 31 pairs of nerves which branch off from the spine. Those which leave the spine in the neck region go to the head and arms, those in the thoracic region go to the rib cage and abdomen, those in the lumbar region to the legs, and the lowest of all to the bowels and bladder. The peripheral nerves contain nerve fibres (very elongated nerve cells) which are either sensory or motor in function; if you trap your finger in a door, a sensory nerve fibre shoots a pain signal up to the brain, and the brain sends back the message 'Remove finger from door' via a motor nerve fibre.
Unlike most other cells in the body, nerve cells have only a very limited capacity for self-repair. They are so specialized and have such delicate junctions with other nerve cells that, once damaged, they lose their ability to transmit nerve impulses, or minute voltage electrical signals, and they can be quite easily damaged, by compression, injury, surgery, viral or bacterial infections, or oxygen starvation. The brain is plentifully supplied with arteries - about one eighth of the oxygenated blood leaving the lungs goes to the brain but if any area of it is deprived of blood for more than a few minutes it ceases to function, and the result is a stroke.
Fortunately, every nerve cell in the brain has so many connections that the effects of damage to a small area may be insignificant or not noticed at all; connected areas may take over the function of the damaged area. Obviously the smaller the area devoted to a particular function, the more severe the effects of damage. Injury to the spinal cord may be much more disabling, because sensation and movement in every part of the body supplied by nerves entering and leaving the spine below the point of injury may be lost or severely impaired. However, both the brain and the spinal cord are well protected, the brain by the skull and the spinal cord by the vertebrae; both are also enclosed in three delicate membranes, the meninges, and bathed in shock-absorbing fluid. The nerves themselves are protected and insulated by a fatty substance called myelin; the biggest nerve fibres have the thickest insulation and conduct nerve impulses at about 500 km (300 miles) per hour; the smallest fibres have no myelin covering, and so conduct impulses much more slowly.
In evolutionary terms the cerebellum and brain stem are more primitive than the much folded rind, or cortex, of the brain. The cortex is concerned with higher mental functions such as memory and learning, planning, intellectual effort, and integrating activities with movement, speech, sight, and hearing. Beneath the cortex lies the sub-cortex, containing structures concerned with more fundamental aspects of behaviour. The thalamus, for example, relays sensory information to appropriate areas of the cortex and also enables us to concentrate on one thing at a time; the hypothalamus regulates hunger, thirst, sleep, body temperature, and basic emotions such as fear and anger, as well as controlling the secretions of the pituitary gland beneath it, which in turn controls growth and stimulates other glands in the body to produce hormones.
The cerebellum or hindbrain acts as a non-stop monitor of all movements commanded by the motor area of the cortex, ensuring precision, smoothness, coordination and balance; nerve impulses from the semicircular canals in the inner ear go to the cerebellum for interpretation.
The brain stem is the most primitive part of the brain; from being little more than a terminus for all the ascending and descending nerve tracts in the spinal cord, it has evolved into a very complex relay station, with connections to all parts of the cortex; it controls automatic functions such as heartbeat and breathing, and also consciousness as we know it; without a particular clump of neurons at its core, we would be in a state of perpetual somnolence.
Rather like a walnut, the brain has two obvious halves - these intercommunicate very extensively through a stout bridge of nerve fibres called the corpus callosum - with the right half controlling the left side of the body and vice versa. Much has been made of the division of higher mental labour between the two halves of the brain. In most people the left hemisphere specializes in logical, analytical, and verbal activity, which is why so many people write with their right hand, while the right hemisphere is predominantly 'artistic', better at associative, intuitive, imaginative thought, more concerned with processing spatial information.
Between them the two halves daily impose order and meaning on an avalanche of information coming from specialized nerve endings all over the body - these detect heat, cold, pain, pressure, even texture - and of course from the eyes, ears, nose, and tongue. However, the processing power required to make visual information meaningful is greater than that required to interpret all other forms of sensory information put together; next in terms of sensory importance, but a long way behind the eyes, come the hands and feet, lips and tongue, ears and genitals. When it comes to moving different parts of the body, the hands, feet, mouth, and tongue - all capable of very precise movements - account for the largest areas in the motor cortex. |
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