Peripheral Nervous System
The central nervous system operates through the peripheral nervous system which is the "roadway" that links the central nervous system to the rest of the body. The nerves that carry information to the central nervous system from sensory receptors such as the eye are called sensory nerves or afferent nerves; those that carry impulses away from the central nervous system to effector organs such as the muscles are called motor nerves or efferent nerves. Commonly the fibers of sensory and motor neurons are bundled together to form mixed nerves. There are 12 pairs of cranial nerves that run to or from the brain, such as the optic and vagus nerves. There are 31 pairs of nerves called spinal nerves that originate from the spinal cord, such as the sciatic nerve and ulnar nerve, the nerve that is stimulated when you hit your elbow. Specific areas of the body are served by each of the spinal nerves. All sensory nerves enter the cord through a dorsal root, and all motor nerves exit through a ventral root. If the dorsal section of a root is destroyed, sensation from that area is also destroyed, but the muscles are still able to function. In the opposite situation, damage to the ventral root destroys muscle function, but sensory information is still processed.
There are two main divisions to the peripheral nervous system, the somatic and the autonomic. The somatic system involves the skeletal muscles and is considered voluntary since there is control over movement such as writing or throwing a ball. The cell bodies of the somatic system are in the central nervous system (CNS) with the axons running all the way to the skeletal muscles. The autonomic nervous system (ANS) affects internal organs and is considered involuntary since the processes such as heart beat rate and glandular secretions occur with usually little control on the part of the individual. The autonomic nervous system, in turn, is divided into two divisions, the parasympathetic and sympathetic. The parasympathetic system is most active in normal, restful situations and is dominant during quiet, relaxed periods. It acts to decrease the heartbeat and to stimulate the motility and secretions necessary for digestion. The sympathetic nervous system is most active during times of stress and arousal and is dominant when energy is required, when it increases the rate and strength of the contractions of the heart and inhibits the motility of the intestine. In addition to their effects, the two divisions of the ANS differ anatomically. The nerves of the parasympathetic system originate at the top and bottom of the central nervous system, while those of the sympathetic system emerge from the upper and central spinal cord. At the site of the effector organs, axons in the parasympathetic system release acetylcholine, while those in the sympathetic system release norepinephrine. Together with hormones, the autonomic system maintains homeostasis, the internal balance of the body.
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