CN IX. Glossopharyngeal Nerve
The glossopharyngeal nerve as its name suggests is related to the tongue and the pharynx. The ninth cranial nerve exits the brain stem as a the most rostral of a series of nerve rootlets that protrude between the olive and inferior cerebellar peduncle. These nerve rootlets come together to form the ninth cranial nerve and leave the skull through the jugular foramen. The tympanic nerve is a branch that is occurs prior to exit the skull. The visceromotor or parasympathetic part of the ninth nerve originate in the inferior salivatory nucleus. Nerve fibers from this nucleus join the other components of the ninth nerve during their exit from the brain stem. They branch in the cranium as the tympanic nerve. The tympanic nerve exits the jugular foramen and passes by the inferior glossopharyngeal ganglion. It re-enters the skull through the inferior tympanic canaliculus and reaches the tympanic cavity where it forms a plexus in the middle ear cavity. The nerve travels from this plexus through a canal and out into the middle cranial fossa adjacent to the exit of the greater petrosal nerve. It is here the nerve becomes the lesser petrosal nerve. The lesser petrosal nerve exits the cranium via the foramen ovali and synapses in the otic ganglion. The otic ganglion provides nerve fibers that innervate and control the parotid gland, an important salivary gland. The branchial motor component supplies the stylopharyngeas muscle which elevates the pharynx during swallowing and talking. In the jugular foramen are two sensory ganglion connected to the ninth cranial nerve: the superior and inferior glossopharyngeal ganglia. General sensory components from the skin of the external ear, inner surface of the tympanic membrane, posterior one-third of the tongue and the upper pharynx join either the superior or inferior glossopharyngeal ganglia. The ganglia send central processes into the brain stem which terminate in the caudal part of the spinal trigeminal nucleus. Visceral sensory nerve fibers originate from the carotid body (oxygen tension measurement) and carotid sinus (blood pressure changes). The visceral sensory nerve components connect to the inferior glossopharngeal ganglion. The central process extend from the ganglion and enter the brain stem to terminate in the nucleus solitarius. Taste from the posterior one-third of the tongue travels via nerve fibers that enter the inferior glossopharnygeal ganglion. The central process that carry this special sense travel through the jugular foramen and enter the brain stem. They terminate in the rostral part of the nucleus solitarius (gustatory nucleus).