glossopharyngeal nerve (CN IX)
The glossopharyngeal nerve is the ninth (IX) of the twelve pairs of cranial nerves. It exits the brainstem out from the sides of the upper medulla, just rostral to the vagus nerve and has sensory, motor, and autonomic components.
- The sensory ganglion cells lie in the superior and inferior ganglia of the nerve, whos central processes passing to two nuclei:
- nucleus of the tractus solitarius, located in the dorsolateral medulla oblongata and lower pons, which conveys taste sensation from the posterior 3rd of the tongue and vallate papillae
- sensory nucleus of the trigeminal nerve, located in the caudal aspect of the pons , which conveys somatic sensation
- The motor fibers originate in the upper part of the nucleus ambiguus in the rostral pons which receives bilateral supranuclear innervation from the corticobulbar fibers. This nucleus supplies the stylopharyngeus muscle.
- The autonomic parasympathetic fibers arise in the inferior salivary nucleus located in the dorsal pons. These fibers are carried in the lesser petrosal nerve via the tympanic branch to the otic ganglion. The postganglionic fibers are distributed to the parotid gland via the auriculotemporal nerve.
It exits the medulla oblongata from the postolivary sulcus, the glossopharyngeal nerve passes laterally across the flocculus, and leaves the skull through the pars nervosa of the jugular foramen in a separate sheath of the dura mater. It then passes between the internal jugular vein and internal carotid artery. It descends in front of the latter vessel, and beneath the styloid process and the muscles connected with it, to the lower border of the stylopharyngeus. It then curves forward, forming an arch on the side of the neck and lying upon the stylopharyngeus and middle pharyngeal constrictor muscle. From there it passes under a cover of the hyoglossus muscle, and is finally distributed to the palatine tonsil, the mucous membrane of the fauces and base of the tongue, and the mucous glands of the mouth.
In passing through the jugular foramen, the nerve presents two ganglia, the superior and the petrous:
- superior ganglion (jugular ganglion): situated in the upper part of the groove in which the nerve is lodged during its passage through the jugular foramen; it is very small and is usually regarded as a detached portion of the petrous ganglion
- inferior ganglion (petrous ganglion): larger than the superior and is situated in a depression in the lower border of the petrous portion of the temporal bone called the petrosal fossula
- tympanic nerve (nerve of Jacobson): 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 (the tympanic 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; here the nerve becomes the lesser petrosal nerve; the lesser petrosal nerve exits the cranium via the foramen ovale and synapses in the otic ganglion. It supplies the inner surface of the tympanic membrane (the external surface is supplied by the auriculotemporal nerve and the greater auricular nerve).
- muscular branch: innervates the stylopharyngeus muscle
- carotid branches: (superior and inferior caroticotympanic nerves) descend along the trunk of the internal carotid artery as far as its origin, communicating with the pharyngeal branch of the vagus, and with branches of the sympathetic nerves
- pharyngeal branches: are three or four filaments which unite, opposite the middle pharyngeal constrictor with the pharyngeal branches of the vagus and sympathetic nerves, to form the pharyngeal plexus: branches from this plexus perforate the muscular coat of the pharynx and supply its muscles and mucous membrane
- tonsillar branches: supply the palatine tonsil, forming around it a plexus from which filaments are distributed to the soft palate and fauces, where they communicate with the greater and lesser palatine nerves
- lingual branches: are two in number; one supplies the vallate papillae and the mucous membrane covering the base of the tongue; the other supplies the mucous membrane and follicular glands of the posterior part of the tongue, and communicates with the lingual nerve
- sinus nerve, or "Hering's nerve": the branch of the glossopharyngeal nerve to the carotid sinus is the nerve that runs downwards anterior to the internal carotid artery communicates with the vagus and sympathetic then divides in the angle of bifurcation of the common carotid artery to supply the carotid body and carotid sinus; it carries impulses from the baroreceptors in the carotid sinus (to help maintain more consistent blood pressure) and from chemoreceptors in the carotid body
There are a number of functions of the glossopharyngeal nerve:
- receives general sensory fibers from the tonsils, the pharynx, the middle ear and the posterior one-third of the tongue
- receives special sensory fibers (taste) from the posterior one-third of the tongue
- receives visceral sensory fibers from the carotid bodies
- supplies parasympathetic fibers to the parotid gland via the otic ganglion
- supplies motor fibers to stylopharyngeus muscle, the only motor component of this cranial nerve
- contributes to the pharyngeal plexus
- absent gag reflex in patients with damage to the glossopharyngeal nerve: it is responsible for the afferent limb of the reflex
- glossopharyngeal neuralgia