b. Spinal Nerves. There are 31 pairs of spinal nerves. They are named for the
region from which they leave the spinal canal through the intravertebral foramina.
Included are 8 pairs of cervical nerves, 12 pairs of thoracic nerves, 5 pairs of lumbar
nerves, 5 pairs of sacral nerves, and 1 pair of coccygeal nerves. Fibers from the spinal
nerves extend to all muscles of the trunk of the body and to the extremities. These
fibers carry impulses between the spinal cord and the skin, muscles, and other
structures. Each spinal nerve is derived from the spinal cord by two roots: a sensory
(dorsal) root and a motor (ventral) root.
c. Autonomic Nervous System (ANS). This system regulates the action of
secretory activity of glands and the involuntary contraction of smooth muscle, as in the
blood vessels, the skin, the heart, and the bronchial tubes. The autonomic nervous
system usually operates without conscious control. Originally, the system was named
automatic because physiologists thought the system functioned completely
automatically --that is, without any control from the central nervous system. Currently, it
is believed that the ANS is structurally and functionally dependent on the central
nervous system. The autonomic nervous system is subdivided into two systems that
counteract each other: the sympathetic system and the parasympathetic system.
(1) Sympathetic system. This system generally increases the activity of the
body. The sympathetic system prepares the body to meet danger or undergo strenuous
physical activity. The system responds to stimuli by releasing adrenalin (epinephrine) or
acetylcholine into the body, thus, making the individual feel that he has extra energy to
flee or fight.
(2) Parasympathetic system. This system acts in opposition to the
sympathetic system and maintains homeostasis (maintenance of a relatively constant
body internal environment). The parasympathetic system exercises nonstress
situational controls on the internal organs of the body allowing the body to rest and
repair. To this end, cholinesterase, a substance which counteracts the effects of
acetylcholine, is replaced.
The two systems, the sympathetic system and the parasympathetic system,
work in opposition to each other to give the body protection.