The nervous system has a central and peripheral part. The central nervous system comprises the brain and the spinal cord, whereas the peripheral nervous system consists of all nerveand nerve fibres at the periphery.
Apart from this anatomical arrangement, we can also make a functional subdivision. A part of the nervous system functions and controls everything subject to our will. This is called the animal nervous system. A lot of body functions however, take place unconsciously. Heart rate, intestines, glandular function, regulation offlow and so on are all functions regulated by the autonomic nervous system. The autonomic nervous system is located both centrally and peripherally.
The central nervous system is often compared with a telephone exchange; to and from it run the telephone cables: the peripheral nervous system. Sensory impulses, generated at the periphery, travel via the sensory nerves to the spinal cord. In the spinal cord they can be relayed to the brain, where perception of the stimulus takes place. The brain can send effector stimuli down the cord, where effector nerveactivate muscles in response to the sensory stimulus. Sometimes, however, the body needs to react immediately to the stimulus, for instance to withdraw a hand when it is burned. Then the receptor-effector pathway is cut short. In the spinal cord the sensory input is relayed directly to effector nerve cells, resulting in an immediate response. This is called a reflex. The nervous system guarantees us that our body will function in relation to our surroundings. Human existence comprises more than merely an adequate functioning level. A human being needs to be consi-ous, to remember, to have emotions, to think and to plan ahead. The results of decisions need to be evaluated, and so on. All these spiritual functions, essential to human life, come from the nervous system, especially the brain. The human brain compared with animal brains is large and very specialized.
The supporting structure of the nervous system
The neurons are associated with other cells which provide a mechanical framework for, and carry out the maintenance tasks of, the nervous system. Within the central nervous system these cells are collectively termed glial cells. The astrocytes provide some of the supporting framework and are involved in the nutrition of neurons and the repair of injuries. Oligodendrocytes provide a layer of a substance called myelin which coats the axons and dendrites of the neurons. The myelin sheath acts like insulation and has the property of enabling conduction to take place along nerve fibres much more rapidly than would be possible if they were exposed. In the peripheral nervous system the conducting axons are insulated by the cells of Schwann. These also form myelin and roll themselves around the axon. Microglial cells scavenge damaged cells and combat infection.
The brain and the spinal cord are vulnerable structures. The jolting inflicted upon the central nervous system by running and jumping could harm it. Therefore both structures are bathed in cerebrospinal fluid, which is located between the nervous system and the surrounding membranes. This fluid acts as a shock absorber and it also prevents the structures from drying out. Even so, cerebrospinal fluid may fail to cushion the brain against repeated jarring, such as the blows to the head suffered by a boxer. The cerebrospinal fluid is formed in cavities inside the brain. Its composition is likeplasma without proteins. In certain diseases of the central nervous system its composition alters. Withdrawing and analyzing some of the cerebrospinal fluid can tell doctors something about the nature of the illness. This procedure is called a lumbar puncture.