Monday, September 12, 2005

Brain, the cable operator

Dr. K. Rohiniprasad

Like other animals, we perceive the presence and changes in the environment with out senses. Our entire body is a sensor by virtue of touch. The network of nerves associated with the brain is both elaborate and efficient so that we get a ‘clear picture’ all the time. The central nervous system, consisting of the brain and spinal cord is connected to all the parts of the body by nerve fibres. Human brain is made of approximately 100-billion nerve cells, called neurons. Neurons gather and transmit electrochemical signals like the gates and wires in a computer. Neurons consist of the nerve cell, axons and dendrites. They are similar to other cells, but they can transmit electrochemical messages to each other over several feet. Axons are long, cable-like projections of the cell that carry the messages in the form of nerve impulse or action potential along the length of the cell. Dendrites are thin projections one on or both sides of the nerve cells. The messages travelling across the ‘branches and twigs’ of the network are small electrical pulses generated in the nerve cells. Travelling down the axons these impulses reach the synapses and are converted into chemicals that are released into the gap between the synapse and the gland. Synapses are special structures at the ends of dendrites that make contact with other nerve cells. Some neurons accept information while the others (‘motor’ nerves) carry instructions or simply connect nerves with each other. This is how the extensive ‘cable network’ receives impulses from all over the body and conveys ‘decisions’ made by the brain to muscles and glands to make them work. This information may be heat, pain, smell or sights and sounds and the instructions may include muscle movement, need to release saliva, gastric juice and so forth. Like electrical insulation over cables, some of the axons are covered by fatty ‘myelin’ sheaths manufactured by some of the glial cells in the brain.

High school anatomy teaches us that the nervous system carries out involuntary (very common), voluntary and reflex actions. But some of the automatic impulses are under our control. Our hand recoils from the touch of a hot vessel but we check our impulse to drop it if it is an expensive piece of pottery! It is not surprising that this delicate system of nerves requires proper and adequate blood supply for normal operation. Whatever the initial causes, death is ultimately due to extreme interference with nerve cell functions. Temporary denial of blood supply to legs and other limbs may cause ‘pins and needles’ or other minor discomfort but in the case of the brain it can be deadly, literally. Not only should the respiratory system and circulation work properly but the nutrient composition of the blood should also be good. Cerebral haemorrhage or apoplexy results when a blood vessel carrying supply to the brain bursts. This is more common with older people and in the case of people with arterial diseases.

The brain is of course the single but complex organ that controls our lives, acting as a processor, a library, switchboard and a signal box, all rolled into one super-duper computer. That is why nature encases it in a hard shell of bone. This has not only survived all the rigours of living but also evolved tremendously over the millennia. This will be discussed in the forthcoming articles.


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