The parasympathetic nervous system is shown in Figure 1, which demonstrates that parasympathetic fibers leave the central nervous system through cranial nerves III, VII, IX, and X; additional parasympathetic fibers leave the lowermost part of the spinal cord through the second and third sacral spinal nerves and occasionally the first and fourth sacral nerves. About 75 percent of all parasympathetic nerve fibers are in the vagus nerves (cranial nerve X), passing to the entire thoracic and abdominal regions of the body. Therefore, a physiologist speaking of the parasympathetic nervous system often thinks mainly of the two vagus nerves. The vagus nerves supply parasympathetic nerves to the heart, lungs, esophagus, stomach, entire small intestine, proximal half of the colon, liver, gallbladder, pancreas, kidneys, and upper portions of the ureters.
Fig1. The parasympathetic nervous system. The blue lines represent preganglionic fibers and the black lines show postganglionic fibers.
Parasympathetic fibers in the third cranial nerve go to the pupillary sphincter and ciliary muscle of the eye. Fibers from the seventh cranial nerve pass to the lacrimal, nasal, and submandibular glands, and fibers from the ninth cranial nerve go to the parotid gland.
The sacral parasympathetic fibers are in the pelvic nerves, which pass through the spinal nerve sacral plexus on each side of the cord at the S2 and S3 levels. These fibers then distribute to the descending colon, rectum, urinary bladder, and lower portions of the ureters. Also, this sacral group of parasympathetics supplies nerve signals to the external genitalia to cause erection.
Preganglionic and Postganglionic Parasympathetic Neurons. The parasympathetic system, like the sympathetic system, has both preganglionic and postganglionic neurons. However, except in the case of a few cranial para sympathetic nerves, the preganglionic fibers pass uninterrupted all the way to the organ that is to be controlled. The postganglionic neurons are located in the wall of the organ. The preganglionic fibers synapse with these neurons, and extremely short postganglionic fibers, a fraction of a millimeter to several centimeters in length, leave the neurons to innervate the tissues of the organ. This location of the parasympathetic postganglionic neurons in the visceral organ is quite different from the arrangement of the sympathetic ganglia because the cell bodies of the sympathetic postganglionic neurons are almost always located in the ganglia of the sympathetic chain or in various other discrete ganglia in the abdomen, rather than in the excited organ.
