The preganglionic fibers of the superior salivatory nucleus enter the facial nerve and, at a point near the geniculate ganglion, form the greater superficial petrosal nerve, through which they reach the sphenopalatine ganglion; postganglionic fibers from the cells of this ganglion innervate the lacrimal gland (see also Figs. 25-2 and 44-3). Other fibers originating in the salivatory nuclei are carried in the facial nerve and traverse the tympanic cavity as the chorda tympani to eventually join the submandibular ganglion. Cells of this ganglion innervate the submandibular and sublingual glands. Axons of the inferior salivatory nerve cells enter the glossopharyngeal nerve and reach the otic ganglion through the tympanic plexus and lesser superficial petrosal nerve; cells of the otic ganglion send fibers to the parotid gland.
Preganglionic fibers, derived from the dorsal motor nucleus of the vagus and adjacent visceral nuclei in the lateral reticular formation (mainly the nucleus ambiguus), enter the vagus nerve and terminate in ganglia situated in the walls of many thoracic and abdominal viscera. The ganglionic cells give rise to short postganglionic fibers that activate smooth muscle and glands of the pharynx, esophagus, and gastrointestinal tract (the vagal innervation of the colon is somewhat uncertain but considered to extend up to the descending colon) and of the heart, pancreas, liver, gallbladder, kidneys, and ureter.
The sacral part of the parasympathetic system originates in the lateral horn cells of the second, third, and fourth sacral segments. Axons of these sacral neurons, constituting the preganglionic fibers, traverse the sacral spinal nerve roots of the cauda equina and synapse in ganglia that lie within the walls of the distal colon, bladder, and other pelvic organs. Thus, the sacral autonomic neurons, like the cranial ones, have long preganglionic and short postganglionic fibers, a feature that permits a circumscribed influence upon the target organ.
In organs containing smooth muscle that is innervated by parasympathetic fibers and therefore not under voluntary control, there is a parallel innervation of adjacent voluntary striated muscle by anterior horn cells. For example, the neurons that activate the external sphincter of the bladder (voluntary muscle) differ from those that supply the smooth muscle of the internal sphincter as discussed further on. In 1900, Onufrowicz (calling himself Onuf) described a discrete group of relatively small cells in the anterior horns of sacral segments 2 to 4. These neurons were originally thought to be autonomic in function, mainly because of their histologic features. There is now evidence that they are somatomotor, innervating the skeletal muscle of the external urethral and anal sphincters (Holstege and Tan). Neurons in sacral cord segments located in a region analogous to the intermediolateral cell column of the sympathetic nervous system (see later), innervate the detrusor and internal sphincter of the bladder wall. In passing, it is worth noting that in motor system disease, in which bladder and bowel functions are usually preserved until late in the disease, the neurons in the Onuf nucleus, in contrast to other somatomotor neurons in the sacral cord, tend not to be involved in the degenerative process (Mannen et al).
There are elaborate connections between supranuclear centers, mainly in the hypothalamus, to the pupillary sphincters, lacrimal and salivary glands that course the brainstem. With regard to the supranuclear innervation of parasympathetic nuclei in the sacral segments, little is known. There appear to be connections to these neurons from the hypothalamus, locus ceruleus, and pontine micturition centers but their course in the human spinal cord has not been identified with certainty.