THE PARASYMPATHETIC SYSTEM

The preganglionic cells of the parasympathetic system are restricted to a number of discrete nuclei within the brainstem and to the lateral column of a short stretch of the spinal cord, generally the second, third, and pos­sibly fourth sacral segments (see Figure 8-73).

The cranial parasympathetic pathways have rather limited anatomical independence. Varying parts of their courses are incorporated within nerves of predomi­nantly somatic composition. Exclusively parasympa­thetic bundles are found only close to the target organs. The chief, grossly visible features of the cranial para­sympathetic outflow have been described with the rele­vant nerves, and it now remains to draw these threads together.

The most rostral parasympathetic nucleus, the para­sympathetic oculomotor nucleus, lies within the mid­brain in association with the motor nucleus of the third cranial nerve. The parasympathetic preganglionic fibers emerge from the main trunk within the orbit to consti­tute the oculomotor (short) root of the ciliary ganglion. Beyond the ganglion, the postganglionic fibers proceed as the short ciliary nerves, which also incorporate sym­pathetic and sensory fibers; these nerves penetrate the sclera to form the ciliary plexus from which the para­sympathetic fibers extend to the ciliary and pupillary sphincter muscles (Figure 8-70/6,10).

The parasympathetic component of the facial nerve originates in the rostral parasympathetic (salivatory) nucleus of the medulla oblongata (Figure 8-70/2). The preganglionic fibers are incorporated within the main facial trunk, run through the somatic geniculate gan­glion without interruption, and later leave in the chorda tympani and the greater petrosal nerve (Figure 8-70/11,13).

The greater petrosal nerve is joined by the deep petrosal (sympathetic) nerve (Figure 8-70/12) to consti­tute the nerve of the pterygoid canal, which leads to the pterygopalatine ganglion (Figure 8-70/7). The postgan­glionic fibers join the lacrimal nerve (after passage through the zygomatic nerve) en route to the lacrimal gland and various other branches of the maxillary nerve en route to glands within the nasal and palatine mucosae.

The parasympathetic component of the glossopha­ryngeal nerve originates from the middle parasympa­thetic nucleus in the medulla oblongata (Figure 8-70/3). The preganglionic fibers pass through the somatic gan­glion of this nerve before joining the tympanic plexus; from this they proceed to the otic ganglion (Figure 8-70/9). The postganglionic fibers are carried via the pterygoid nerve and a communicating branch of the auriculotemporal nerve to the parotid gland.

The parasympathetic component supplies the bulk of the vagus nerve; indeed it is the whole complement distal to the origin of the recurrent laryngeal nerve (Figure 8-73/5,6). The preganglionic fibers proceed to numerous small ganglia scattered along the nerve plex­uses that supply and are often located within the tissues of the target organs. The plexuses include the cardiac and pulmonary plexuses within the chest (Figure 8-73/7) and the gastric, hepatic, mesenteric, gonadal, and renal plexuses formed by the confluence of branches of the vagal trunks with sympathetic nerves within the abdomen (Figure 8-73/10). Broadly, the dorsal vagal trunk supplies hepatic and gastric plexuses, and the larger ventral vagal trunk supplies celiac, mesenteric, renal, and gonadal plexuses.

The fibers of the sacral parasympathetic outflow are initially incorporated in certain sacral ventral rami from which they emerge to constitute the pelvic nerves (Figure 8-73/11). These form a retroperitoneal plexus that is joined by sympathetic fibers delivered by the hypogas­tric nerves that descend from the caudal mesenteric gan­glion. Numerous minute ganglia are found scattered in the plexus, whereas other (terminal) ganglia are embed­ded within the walls of predominantly pelvic viscera: the descending colon, rectum, bladder, uterus, and vagina (in the female); accessory reproductive glands (in the male); and the genital erectile tissue. The parasym­pathetic pathways have their peripheral synapses exclu­sively in the terminal ganglia, whereas some sympathetic peripheral synapses are divided among the plexus and terminal ganglia.