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Glossopharyngeal nerve (CN IX)
The glossopharyngeal nerve (ninth cranial nerve, CN IX, Latin: nervus glossopharyngeus) is a mixed cranial nerve. The glossopharyngeal nerve provides motor innervation to the stylopharyngeus muscle and the superior constrictor pharyngeal muscle. With sensory fibers the nerve supplies the root of the tongue (including the vallate papillae), as well as the mucosa of the tympanic cavity, the auditory tube, and the mastoid cells. The glossopharyngeal nerve also controls the secretion of the parotid gland via its preganglionic parasympathetic fibers that synapse in the otic ganglion.
The fibers of the glossopharyngeal nerve arise from three nuclei located in the brainstem:
- nucleus ambiguus,
- solitary tract nucleus,
- inferior salivatory nucleus.
The nucleus ambiguus is a cluster of motor neurons supplying the ninth cranial nerve with special visceral efferent fibers that innervate muscles of branchiomeric origin (specifically, originating from the third branchial arch), which are the stylopharyngeus and superior constrictor pharyngeal muscles.
The solitary tract nucleus is a sensory nucleus that belongs to the solitary tract nuclear complex where sensory fibers synapse with the central neurons. There are two types of fibers reaching the nucleus of the solitary tract via the glossopharyngeal nerve: the special visceral afferent fibers (with cell bodies located in the superior ganglion) transmit information of taste from the tongue, specifically, the vallate papillae, while the general visceral afferent fibers (with cell bodies also in the superior ganglion) transmit visceral sensations to the solitary tract nucleus from the middle ear, pharynx, tongue, and carotid sinus.
The inferior salivatory nucleus contains bodies of preganglionic parasympathetic neurons supplying the glossopharyngeal nerve with general visceral efferent fibers that synapse in the otic ganglion, and thus are involved in controlling the secretion of the parotid gland.
The glossopharyngeal nerve also carries general somatic afferent fibers that transmit sensory information from mucosa of the posterior one third of the tongue, soft palate, upper pharynx, and auditory tube. These fibers synapse with central neurons in the spinal trigeminal nucleus.
The ninth cranial nerve emerges from the medulla oblongata in the retro-olivary groove together with the vagus nerve (CN X) and accessory nerve (CN XI). The glossopharyngeal nerve along the other two cranial nerves leaves the skull through the jugular foramen. The glossopharyngeal nerve forms two enlargements in the area of the jugular foramen: the superior ganglion in the jugular foramen, and the inferior ganglion of the glossopharyngeal nerve below the jugular foramen. Both ganglia contain cell bodies of sensory neurons - pseudounipolar neurons - with dendrites receiving information in the periphery and travelling to the center via the CN IX, and axons travelling to the sensory nuclei in the brainstem.
After emerging on the external surface of the skull, the glossopharyngeal nerve descends along the stylopharyngeus muscle, at first, behind the internal carotid artery and then between the artery and the internal jugular vein. Then the glossopharyngeal nerve passes between the superior and middle constrictor pharyngeal muscles, enters the oral part of the pharynx reaching the root of the tongue, where it diverges into lingual branches.
The lingual branches of the glossopharyngeal nerve innervate the mucosa of the posterior one third of the tongue with general visceral afferent fibers, the vallate papillae with special visceral afferent (taste) fibers, and the lingual glands with general visceral efferent (parasympathetic) fibers.
The glossopharyngeal nerve gives several side branches on its way:
- branch to stylopharyngeus muscle (motor),
- pharyngeal branches (sensory),
- tonsillar branches (sensory),
- carotid branch (sensory),
- tympanic nerve (sensory and parasympathetic).
The branch to the stylopharyngeus muscle is a small motor branch of the glossopharyngeal nerve supplying the stylopharyngeus muscle.
The pharyngeal branches of the glossopharyngeal nerve are 2-3 side branches that mainly carry sensory fibers. The pharyngeal branches participate in forming the pharyngeal nerve plexus and eventually innervate the mucosa lining the upper part of the pharynx.
The tonsillar branches of the glossopharyngeal nerve are 2-3 side branches carrying sensory fibers that supply the mucosal lining of the palatoglossal arch and the palatopharyngeal arch, as well as the palatine tonsils.
The carotid branch of the glossopharyngeal nerve is a side branch of the ninth cranial nerve that descends to the neck region, runs anterior to the internal carotid artery and reaches the bifurcation of the common carotid artery, where the nerve divides and innervates the carotid sinus and the carotid body. The general visceral afferent fibers carried by the carotid branch of the glossopharyngeal nerve transmit information from the baroreceptors (in the carotid sinus) and chemoreceptors (in the carotid body), thus participating in the reflectory regulation of blood pressure and forming the afferent pathway of the cardiac reflexes.
The tympanic nerve is a branch of the glossopharyngeal nerve that carries sensory and preganglionic parasympathetic fibers to the tympanic cavity, where it forms the tympanic plexus providing sensory innervation for the mucosa of the tympanic cavity, the auditory tube, and the mastoid cells. The parasympathetic fibers separate from the nerve and leave the middle ear as the lesser petrosal nerve.
The tympanic nerve branches from the glossopharyngeal nerve within or immediately below the jugular foramen. The branch re-enters the temporal bone via the tympanic canaliculus, enters the middle ear cavity, and participates in the formation of the tympanic plexus.
The sensory neuronal cell bodies are located in the inferior ganglion of the glossopharyngeal nerve and provide the tympanic nerve with general somatic afferent fibers, which transmit general sensory information from the mucosal membrane lining the tympanic cavity, the auditory tube, and the mastoid cells. The neurons of the inferior ganglion project their axons to the solitary tract nucleus.
The cell bodies of the preganglionic parasympathetic neurons of the tympanic nerve are located in the inferior salivatory nucleus and their axons serve as general visceral efferent fibers, which travel via the tympanic nerve to the tympanic cavity, where they separate and leave the cavity as the lesser petrosal nerve. This nerve carries the preganglionic parasympathetic fibers to the otic ganglion, where they synapse with the postganglionic parasympathetic neurons there to provide the parasympathetic secretomotor innervation for the parotid gland for salivation.