The Nervous System and the Brain
M ost of us have heard often enough that the brain, acting as control center for a communication network we call our nervous system, is an incredible computer, weighing a mere three pounds. Its form and functions, however, are often described as being so much more intricate and complex than any existing or imagined computer that thorough knowledge of the brain seems very remote. This is certainly true. But it doesn't prevent us from knowing some general things about the brain and nervous system, or what its most significant parts are and how they work.
Basically, the nervous system has just two functions: first, getting information (impulses, signals, messages) from outside or inside the body to where it can be acted upon, usually in the brain; and secondly, feeding back information (for example, to the muscles) so that the indicated action can be taken. Thus, nerves can be divided by their function into two general types, each following a separate pathway. Those that receive information—for example, from our senses—and pass it along are called sensory , or afferent (inward-traveling). Those that relay information back, with a directive for action, are called motor , or efferent (outward-traveling).
The brain and the spinal cord can be considered as the basic unit of the central nervous system . All sensory and motor information comes or goes from this central core. The spinal cord is the master nerve tract (or nerve trunk) in our body and consists of millions of nerve fibers bundled together, somewhat like many small threads making up a large rope.
Like the spinal cord, all the lesser nerves, shown as single cords in a typical anatomical drawing, are made up of hundreds of thousands of individual fibers. Each fiber is part of a single nerve cell, or neuron . Neurons—there are 12 to 15 billion of them in our brain alone—are the basic structural units of the brain and nervous system, the tubes and transistors and circuits of which our personal computer is built.