1-10. PERIPHERAL NERVOUS SYSTEM (PNS)

The peripheral nervous system (PNS) is composed of nerves that connect the central nervous system to remote parts of the body, relaying and receiving messages from these parts of the body. This system is a pathway to the brain for the five senses which help us respond to the world around us. The peripheral nervous system is divided into the cerebrospinal nervous system and the autonomic nervous system (ANS).

a. Cerebrospinal Nervous System. This system is composed of 12 pairs of cranial nerves. The cranial nerves are symmetrically arranged and attached to the brain. Each cranial nerve leaves the skull through a foramen (an opening) at its base. The nerves are numbered in the order in which they emerge from the opening and from front to back. The numbered nerves along with the function of each nerve are given in table 1-1.

NOTE: Neither the olfactory nor the optic nerves are really cranial nerves since these nerves are composed of fibers belonging to the central nervous system.

Table 1-1. 12 pairs of cranial nerves and their functions.

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.

NOTE: The two systems, the sympathetic system and the parasympathetic system, work in opposition to each other to give the body protection.