Glossopharyngeal nerve

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 thevagus nerveand and has sensory, motor, and autonomic components.

Gross anatomy

Origin

The sensory ganglion cells lie in the superior and inferior ganglia of the nerve. The central processes pass to the nucleus of the tractus solitarius, conveying taste sensation, and to the nucleus of the spinal tract of the Vth nervetrigeminal nerve conveying somatic sensation. 

The motor nucleus lies in the upper part of the nucleus ambiguous which receives bilateral supranuclear innervation from the corticobulbar fibres.This nucleus supplies the the stylopharyngeus muscle. 

The autonomic parasympathetic fibres arise in the inferior salivary nucleus. These fibres are carried in the the lesser petrosal nerve via the tympanic branch to the otic ganglion. The postganglionic fibres are distributed to the parotid gland via the auriculotemporal nerve.

Course

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.

Glossopharyngeal ganglia  

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

Branches

• 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
• muscular branch: distributed to the stylopharyngeus muscle
• carotid branches: (superior caroticotympanic nerve and inferior caroticotympanic nerve) 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 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 a more consistent blood pressure) and from chemoreceptors in the carotid body

Supply

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 fibres to the parotid gland via the otic ganglion
• supplies motor fibres to stylopharyngeus muscle, the only motor component of this cranial nerve
• contributes to the pharyngeal plexus

 Related pathology

• absent gag reflex in patients with damage to the glossopharyngeal nerve: it is responsible for the afferent limb of the reflex
• glossopharyngeal neuralgia