238. AUTONOMIC NERVOUS SYSTEM
The autonomic nervous system is that part of the nervous system which controls
the activity of cardiac and smooth muscle, sweat and digestive glands, and some of the
endocrine glands. Its control over these reactions is almost wholly involuntary, yet the
behavior of the autonomic system reflects somewhat the activity of the central nervous
system, for the two are connected. The autonomic nervous system is divided into the
sympathetic and the parasympathetic systems.
a. Sympathetic System. A number of ganglia (nerve centers) are the basis of
the sympathetic system. These are located just outside the spine, beside each
vertebra, and extend from the first thoracic to the third lumbar segments. These ganglia
are connected with the spinal nerves by a fiber from a nerve cell in the lateral horn
(sensory) of the spinal cord. From this ganglion comes a nerve fiber that runs with the
motor fibers from the anterior horn to the muscles, organs, and glands (figure 219).
b. Parasympathetic System. The ganglia of the parasympathetic system are
located in the midbrain, the medulla oblongata, and in the sacral region. The fibers in
the midbrain and the medulla send out impulses through cranial nerves (oculomotor,
facial, glossopharyngeal, vagus). The cells comprising the sacral ganglia stem from the
second, third, and fourth sacral nerves.
c. Function of the Autonomic Nervous System.
(1) The function of the sympathetic system is to increase the activity of the
organs of the body, enabling it to meet danger and to undergo strenuous physical
activity. The effects of the parasympathetic system are opposite to those of the
sympathetic system (see Table 21). The opposing functions of these two parts of the
autonomic nervous system balance each other.
(2) During a period of quiet, the effect of the parasympathetic system is
predominant. During a period of danger, emotional crisis, or strenuous exercise, the
sympathetic system is in control. The effects of sympathetic action are apparent in the
individual as the symptoms of anger, of fear, or of exhilaration. It is a useful mechanism
that helps the individual in a period of stress.