The preganglionic cells of the parasympathetic system are restricted to a number of discrete nuclei within the brainstem and to the lateral gray column of a short stretch of the second, third, and possibly fourth sacral spinal cord segments (see Fig. 8.72).
The aptly designated craniosacral outflow is thus confined to the oculomotor, facial, glossopharyngeal, vagus, and pelvic nerves, in that only these nerves contain parasympathetic fibers.
The cranial parasympathetic pathways generally travel in cranial nerves alongside somatic axons, and exclusively parasympathetic bundles of axons are found only close to the target organs. The grossly visible features of these cranial nerves have previously been described, so the following account focuses specifically on cranial parasympathetic outflow.
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 of the nerve 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 (Fig. 8.71/6 and 10).
The parasympathetic component of the facial nerve originates in the rostral parasympathetic (salivatory) nucleus of the medulla oblongata (Fig. 8.71/2). 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 (Fig. 8.71/11 and 13). The chorda tympani joins with the lingual nerve from which the parasympathetic fibers later emerge to synapse within the mandibular ganglion; the postganglionic fibers supply the mandibular and sublingual salivary glands.
The greater petrosal nerve is joined by the deep petrosal (sympathetic) nerve (Fig.
8.71/12) to constitute the nerve of the pterygoid canal, which leads to the pterygopalatine ganglion (Fig. 8.71/7). The postganglionic parasympathetic 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 and then 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 (Fig. 8.71/3). The preganglionic fibers pass through the somatic ganglion of this nerve before joining the tympanic plexus; from this they proceed to and synapse in the otic ganglion (Fig. 8.71/9). The postganglionic fibers are carried via the pterygoid nerve and a communicating branch of the auriculotemporal nerve to the parotid gland.
The parasympathetic component of the vagus nerve constitutes the bulk of the nerve; indeed, the vagus is composed completely of parasympathetic fibers distal to the origin of the recurrent laryngeal nerve (Fig. 8.72/5 and 6). The preganglionic parasympathetic fibers synapse in 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 (Fig. 8.72/7) and the gastric, hepatic, mesenteric, gonadal, and renal plexuses within the abdomen formed by the convergence of branches of the vagal trunks with sympathetic nerves (Fig. 8.72/10). 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 (Fig. 8.72/11). These nerves form a retroperitoneal plexus, being joined by sympathetic fibers delivered by the hypogastric nerves that descend from the caudal mesenteric ganglion. Numerous small 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 synapse exclusively in the terminal ganglia, whereas some sympathetic peripheral synapses are divided among the plexus and terminal ganglia.