Principles of Manual Medicine
Parasympathetic Nervous System
Parasympathetic fibers leave the central nervous system through cranial nerves (CN) III, VII, IX, and X. Approximately 80 per cent or more of all parasympathetic nerve fibers are contained in the vagus nerves (CN-X), passing to the heart, the lungs, the esophagus, the stomach, and the small intestine, the proximal half of the colon, the liver, the gallbladder, the pancreas, and the upper portions of the ureters.
Parasympathetic fibers in the third cranial nerve (CN-III) innervate the pupillary sphincters and ciliary muscles of the eye. Fibers from the seventh cranial nerve (CN-VII) innervate the lacrimal, nasal, and submaxillary glands. Fibers from the ninth cranial nerve (CN-IX) innervate the parotid gland. The sacral parasympathetic fibers leave the sacral plexus on each side of the cord and distribute their peripheral fibers to the descending colon, rectum, bladder and lower portions of the ureters. In addition, this sacral group of parasympathetics supplies fibers to the external genitalia to cause various sexual reactions. It is essential for the osteopathic physician to understand and have committed to memory the somatic regions that are likely to be effected by viscero-somatic reflexes resulting from visceral pathology (Click here to open the parasympathetic drill exercise).
The parasympathetic system, like the sympathetic system, has both preganglionic and a postganglionic neurons but, in general, the preganglionic fibers pass uninterrupted to the organ that is to be excited by parasympathetic impulses. In the wall of the organ are located the peripheral ganglia of the parasympathetic system. The preganglionic fibers synapse within the ganglion, and then short postganglionic fibers, 1 millimeter to several centimeters in length, leave the ganglia to travel a short distance to the target organ. This location of the peripheral parasympathetic ganglia in the visceral organ itself is quite different from the arrangement of the sympathetic ganglia, for the cell bodies of the sympathetic postganglionic neurons are always located in the ganglia of the sympathetic chain or in various other discrete ganglia in the abdomen or thorax rather than in the target organ itself. In general, parasympathetic preganglionic neurons are longer than sympathetic postganglionic neurons (Click here for a comparison of preganglionic neurons and postganglionic neurons in the autonomic nervous system).
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