Section IV. THE NERVOUS SYSTEM
a. The structural and functional unit of the nervous system is the nerve cell, or
neuron, and its processes. A neuron is an elongated cell with dendrites (branches) to
receive and conduct impulses toward the cell body and an axon (an elongation of the
cell body) that conducts impulses away from the cell body. A nerve may be defined as
a collection, or bundle, of nerve fibers.
b. The membrane theory of nerve conduction is explained by electrical and
chemical changes in the neuron. A chain of neurons over which impulses may pass
from a receptor, or sense organ, to a nerve center and back to an effector (muscle or
gland) constitutes a reflex arc. A simple reflex arc (figure 3-16) consists of a sensory
and a motor nerve connected within the spinal cord. Reflex arcs vary greatly in
complexity. They may be quite complicated and involve several centers of the nervous
system, or they may be relatively simple and involve only the lower levels.
Figure 3-16. A simple reflex arc involving a receptor in the skin, a sensory nerve, a
connecting center (ganglion) outside the spinal cord, the spinal cord, a
motor cord, a motor nerve, and an effector.