Fig. 11.1 ). Hence, this part of the autonomic nervous system is sometimes called the craniosacral division. The cranial nerve nuclei that contain parasympathetic preganglionic neurons are the Edinger-Westphal nucleus (cranial nerve III), the superior (cranial nerve VII) and inferior (cranial nerve IX) salivatory nuclei, and the dorsal motor nucleus of the vagus and nucleus ambiguus (cranial nerve X). Postganglionic parasympathetic cells are located in cranial ganglia, including the ciliary ganglion (preganglionic input is from the Edinger-Westphal nucleus), the pterygopalatine and submandibular ganglia (input is from the superior salivatory nucleus), and the otic ganglion (input is from the inferior salivatory nucleus). The ciliary ganglion innervates the pupillary sphincter and ciliary muscles in the eye. The pterygopalatine ganglion supplies the lacrimal gland, as well as glands in the nasal and oral pharynx. The submandibular ganglion projects to the submandibular and sublingual salivary glands and to glands in the oral cavity. The otic ganglion innervates the parotid salivary gland and glands in the mouth.
Other parasympathetic postganglionic neurons are located near or in the walls of visceral organs in the thoracic, abdominal, and pelvic cavities. Neurons of the enteric plexus include cells that can also be considered parasympathetic postganglionic neurons. All of these cells receive input from the vagus or pelvic nerves. The vagus nerves innervate the heart, lungs, bronchi, liver, pancreas, and gastrointestinal tract from the esophagus to the splenic flexure of the colon. The remainder of the colon and rectum, as well as the urinary bladder and reproductive organs, is supplied by sacral parasympathetic preganglionic neurons that travel through the pelvic nerves to postganglionic neurons in the pelvic ganglia.
The parasympathetic preganglionic neurons that project to the viscera of the thorax and part of the abdomen are located in the dorsal motor nucleus of the vagus (see
Fig.
4.6E ) and the nucleus ambiguus. The dorsal motor nucleus is largely secretomotor (it activates glands), whereas the nucleus ambiguus is visceromotor (it modifies the activity of cardiac muscle). The dorsal motor nucleus supplies visceral organs in the neck (pharynx, larynx), thoracic cavity (trachea, bronchi, lungs, heart, and esophagus), and abdominal cavity (including much of the gastrointestinal tract, liver, and pancreas). Electrical stimulation of the dorsal motor nucleus results in gastric acid secretion, as well as secretion of insulin and glucagon by the pancreas. Although projections to the heart have been described, their function is uncertain. The nucleus ambiguus contains two groups of neurons: (1) a dorsal group (branchiomotor) that activates striated muscle in the soft palate, pharynx, larynx, and esophagus and (2) a ventrolateral group that innervates and slows the heart (see also
The visceral motor fibers in the autonomic nerves are accompanied by visceral afferent fibers. Most of these afferent fibers supply information that originates from sensory receptors in the viscera. The activity of these sensory receptors only rarely reaches the level of consciousness; however, these receptors initiate the afferent limb of reflex arcs. Both viscerovisceral and viscerosomatic reflexes are elicited by these afferent fibers. Even though these visceral reflexes generally operate at a subconscious level, they are very important for homeostatic regulation and adjustment to external stimuli.