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parasympathetic nervous system
part of the autonomic nervous system; its preganglionic fibers leave the central nervous system with cranial nerves III, VII, IX, and X and the first three sacral nerves, and its postganglionic fibers are distributed to the heart, smooth muscles, glands of the head and neck, and viscera in the trunk. A large majority of
the parasympathetic nerve fibers are found in the vagus nerves, which serve the entire thoracic and abdominal regions of the body. The primary secretion of the nerve endings of the parasympathetic
nervous system is acetylcholine, which acts on organs of the body to either excite or inhibit certain activities. It often happens that excitation of the
sympathetic nervous system results in an effect opposite to that of the parasympathetic system; however, most organs are under the almost exclusive
control of either one or the other of these two components of the autonomic nervous system.
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