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Organization of the Nervous System

All parts of the nervous system are interrelated. However, for purposes of anatomical discussion, the nervous system can be separated into the following divisions and subdivisions:

The central nervous system includes all the nerves in the brain and spinal cord, and it contains the majority of the body's neurons. Some of the very simplest stimulus-response reflexes are carried out within the spinal cord. The peripheral nervous system consists of the nerves leading from the brain and spinal cord to the other parts of the body. The peripheral nervous system is further subdivided into the somatic system and the autonomic system.

The nerves of the somatic system transmit information about external stimulation from the skin, muscles, and joints to the central nervous system; they make us aware of pain, pressure, and temperature variations. Nerves of the somatic system also carry impulses from the central nervous system back to the body parts, where they initiate action.

The nerves of the autonomic system run to and from the internal organs regulating such processes as respiration, heart rate, and digestion. It derives its name from the fact that many of the activities it controls are autonomous, or self-regulating - such as digestion and circulation, which continue even when a person is asleep or unconscious.

The autonomic nervous system has two divisions -the sympathetic and the parasympathetic — which are often antagonistic in their actions.

The sympathetic division tends to act as a unit. During emotional excitement it simultaneously speeds up the heart, dilates the arteries of the skeletal muscles and heart, and constricts the arteries of the skin and digestive organs; its action also leads to perspiration and to secretion of certain hormones that increase emotional arousal.

Unlike the sympathetic system, the parasympathetic division tends to affect one organ at a time. If the sympathetic system is thought of as dominant during violent and excited activity, the parasympathetic system may be thought of as dominant during quiescence. It participates in digestion and, in general, it maintains the functions that conserve and protect bodily resources.

When both sympathetic and parasympathetic fibers are connected to the same muscle or gland, they usually act in opposite manners. Thus, one speeds the heart rate, the other slows it; one inhibits digestive processes, the other facilitates them, one dilates the pupils of the eyes, the other constricts them. Both systems are usually exerting some influence (that is, both are usually «on»), but one temporarily dominates the other. Their interaction is very complex though, and not fully understood.