The brain, cerebellum, medulla and spinal cord comprise major anatomical parts of the C.N.S. The central nervous system (CNS) is an assemblage of many billion neurons. Individual neurons exhibit great anatomical and neurochemical complexity. On functional basis, the areas of the CNS concerned with one type of activity are linked together.
Thus, it is possible to identify systems concerned with sensory functions, motor activity, and regulation of autonomic functions, control of respiration and memory and association. These systems do not operate independently rather they interact with each other to a considerable degree.
Drug action is seldom, if ever, restricted to the neurons that comprise an anatomical or functional division of the CNS. Many drugs owe their usefulness in therapeutics to the fact that they selectively affect one system over another. Nevertheless, it should not be thought that they exert their actions solely on the neurons that participate in that function.