The cranial segment of the parasympathetic nervous system consists of presynaptic cell bodies in the brainstem and parasympathetic fibers exiting the brainstem in cranial nerves III (oculomotor), VII (facial), IX (glossopharyngeal), and X (vagus). Cranial nerves III, VII, and IX provide parasympathetic innervation to the lacrimal, nasal, palatine and pharyngeal glands; the ciliary muscle of the lens and the sphincter pupillae muscle of the iris; and the parotid, sublingual, and submandibular glands. In general, the parasympathetic nervous system promotes secretion from these glands, causes the pupil of the eye to constrict, and the lens to thicken to accomodate for near vision. The presynaptic parasympathetic fibers from these three cranial nerves synapse in four parasympathetic ganglia. Parasympathetic fibers from cranial nerve III synapse in the ciliary ganglion. Parasympathetic fibers from cranial nerve VII synapse in the pterygopalatine and submandibular ganglia, and parasympathetic fibers from cranial nerve IX synapse in the otic ganglion. You will learn more about these four ganglia in other modules, but remember that they are parasympathetic ganglia!

Cranial nerve X, the vagus nerve, plays a very important role in the parasympathetic nervous system. It provides parasympathetic innervation to all the thoracic viscera via cardiac branches, and to the GI tract from the esophagus to the left colic flexure of the colon, including the pancreas, spleen, liver and gallbladder. In general, parasympathetic innervation to the digestive tract stimulates peristalsis and the secretion of digestive juices. Parasympathetic innervation to the gallbladder causes it to empty. Presynaptic parasympathetic fibers from the vagus nerve synapse in individual cell bodies located on or in the wall of the effector organ, and reach organs in the abdomen via combined autonomic perivascular plexuses. You will learn more about the vagus nerve in other modules.