The glossopharyngeal nerve is the ninth (IX) oftwelve pairs of cranial nerves (24 nerves total). Itexits the brainstem out from the sides of theupper medulla, just rostral (closer to the nose) tothe vagus nerve.The motor division of theglossopharyngeal nerve is derived from the basalplate of the embryonic medulla oblongata, whilethe sensory division originates from the cranialneural crest.
There are a number of functions of the glossopharyngeal nerve: It receives general sensory fibers (ventral trigeminothalamic tract) fromthe tonsils, the pharynx, the middle ear and the posterior 1/3 of thetongue. It receives special sensory fibers (taste) from the posterior one-third ofthe tongue. It receives visceral sensory fibers from the carotid bodies, carotid sinus. It supplies parasympathetic fibers to the parotid gland via the oticganglion. (From: inferior salivary nucleus - through jugular foramen - tympanicn.(of Jacobson)- lesser petrosal n. - through foramen ovale - Oticganglion (Pre-Ganglionic Parasympathetic fibers synapse, to start Post-Ganglionic Parasympathetic fibers) - Auriculotemporaln.(Parasympathetics hitchhikes to reach Parotid gland) It supplies motor fibers to stylopharyngeus muscle, the only motorcomponent of this cranial nerve. It contributes to the pharyngeal plexus.
The glossopharyngeal nerve consists of five componentswith distinct functions: Branchial motor (special visceralefferent) - supplies the stylopharyngeus muscle.Visceralmotor (general visceral efferent) providesparasympathetic innervation of the parotid gland.Visceralsensory (general visceral afferent) carries visceral sensoryinformation from the carotid sinus and body. Generalsensory (general somatic afferent) provides generalsensory information from the skin of the external ear,internal surface of the tympanic membrane, upperpharynx, and the posterior one-third of the tongue.Special sensory (special afferent) provides taste sensationfrom the posterior one-third of the tongue.
The glossopharyngeal as noted above is a mixednerve consisting of both sensory and motornerve fibers.The sensory fibers origin includethe pharynx, middle and outer ear, posteriorone-third of the tongue (including taste buds);and the internal carotid artery.These fibersterminate at the medulla oblongata.The motorfibers origin is the medulla oblongata, andterminate at the Parotid salivary gland, glands ofthe posterior tongue, and the stylopharyngealmuscle (which dilates pharynx duringswallowing).
The Glossopharyngeal nerve if damaged canhave several effects on the human body.Theseeffects include loss of bitter and sour taste, andimpaired swallowing.The clinical tests to see ifthe Glossopharyngeal nerve has been damagedincludes testing the gag reflex of the mouth. Askthe patient to swallow or cough, and Other signsinclude speech impediments. Finally, test theposterior one-third of the tongue with bitter andsour substances.
The branchial motor component of CN IX provides voluntary controlof the stylopharyngeus muscle, which elevates the pharynx duringswallowing and speech. Origin and central courseThe branchial motor component originates from the nucleusambiguus in the reticular formation of the medulla Rostral medulla.Fibers leaving the nucleus ambiguus travel anteriorly and laterally toexit the medulla, along with the other components of CN IX,between the olive and the inferior cerebellar peduncle. Intracranial courseUpon emerging from the lateral aspect of the medulla the branchialmotor component joins the other components of CN IX to exit theskull via the jugular foramen.The glossopharyngeal fibers travel justanterior to the cranial nerves X and XI, which also exit the skull viathe jugular foramen.
Extra-cranial course and final innervationUpon exiting the skull the branchial motor fibers descenddeep to the styloid process and wrap around the posteriorborder of the stylopharyngeus muscle before innervating it. Voluntary control of the stylopharyngeusmuscleSignals for the voluntary movement of stylopharyngeus muscle originatein the pre-motor and motor cortex (in association with other corticalareas) and pass via the corticobulbar tract in the posterior limb of theinternal capsule to synapse bilaterally on the ambiguus nuclei in themedulla.
Parasympathetic component of the glossopharyngeal nervethat innervates the ipsilateral parotid gland. Origin and central courseThe preganglionic nerve fibers originate in the inferior salivatorynucleus of the rostral medulla and travel anteriorly and laterally to exitthe brainstem between the medullary olive and the inferior cerebellarpeduncle with the other components of CN IX. Note:These neurons donot form a distinct nucleus visible on cross-section of the brainstem.Theposition indicated on the diagram is representative of the location of thecell bodies of these fibers.
Intracranial courseUpon emerging from the lateral aspect of the medulla, the visceralmotor fibers join the other components of CN IX to enter the jugularforamen.Within the jugular foramen, there are two glossopharyngealganglia that contain nerve cell bodies that mediate general, visceral, andspecial sensation.The visceral motor fibers pass through both gangliawithout synapsing and exit the inferior ganglion with CN IX generalsensory fibers as the tympanic nerve. Before exiting the jugular foramen,the tympanic nerve enters the petrous portion of the temporal bone andascends via the inferior tympanic canaliculus to the tympanic cavity.Within the tympanic cavity the tympanic nerve forms a plexus on thesurface of the promontory of the middle ear to provide generalsensation.The visceral motor fibers pass through this plexus and mergeto become the lesser petrosal nerve.The lesser petrosal nerve re-entersand travels through the temporal bone to emerge in the middle cranialfossa just lateral to the greater petrosal nerve. It then proceeds anteriorlyto exit the skull via the foramen ovale along with the mandibularnerve component of CNV (V3).
Extra-cranial course and final innervationsUpon exiting the skull, the lesser petrosal nerve synapses in the oticganglion, which is suspended from the mandibular nerve immediatelybelow the foramen ovale. Postganglionic fibers from the otic gangliontravel with the auriculotemporal branch of CNV3 to enter the substanceof the parotid gland. Hypothalamic InfluenceFibers from the hypothalamus and olfactory system project via the dorsallongitudinal fasciculus to influence the output of the inferior salivatorynucleus. Examples include: 1) dry mouth in response to fear (mediated bythe hypothalamus); 2) salivation in response to smelling food (mediatedby the olfactory system)
This component of CN IX innervates the baroreceptors of the carotidsinus and chemoreceptors of the carotid body. Peripheral and intracranial course.Sensory fibers arise from the carotidsinus and carotid body at the common carotid artery bifurcation, ascendin the sinus nerve, and join the other components of CN IX at the inferiorhypoglossal ganglion.The cell bodies of these neurons reside in theinferior ganglion.The central processes of these neurons enter the skullvia the jugular foramen.Central course - visceral sensorycomponentOnce inside the skull, the visceral sensory fibers enter thelateral medulla between the olive and the inferior cerebellarpeduncle and descend in the tractus solitarius to synapse in thecaudalnucleus solitarius. From the nucleus solitarius, connections aremade with several areas in the reticular formation and hypothalamus tomediate cardiovascular and respiratory reflex responses to changes inblood pressure, and serum concentrations of CO2 and O2.
This component of CN IX carries general sensory information (pain,temperature, and touch) from the skin of the external ear, internalsurface of the tympanic membrane, the walls of the upper pharynx, andthe posterior one-third of the tongue. Peripheral courseSensory fibers from the skin of the external ear initiallytravel with the auricular branch of CN X, while those from the middle eartravel in the tympanic nerve as discussed above (CN IX visceral motorsection).General sensory information from the upper pharynx andposterior one-third of the tongue travel via the pharyngeal branches ofCN IX.These peripheral processes have cell their cell body in either thesuperior or inferior glossopharyngeal ganglion.Central course - generalsensory component.The central processes of the general sensoryneurons exit the glossopharyngeal ganglia and pass through the jugularforamen to enter the brainstem at the level of the medulla. Uponentering the medulla these fibers descend in the spinal trigeminal tractand synapse in the caudal spinal nucleus of the trigeminal.
Central course - general sensory componentAscending secondaryneurons originating from the spinal nucleus of CNV project to thecontralateral ventral posteromedial (VPM) nucleus of the thalamus viathe anterolateral system (ventral trigeminothalamic tract).Tertiaryneurons from the thalamus project via the posterior limb of the internalcapsule to the sensory cortex of the post-central gyrus.Clinicalcorrelation.The general sensory fibers of CN IX mediate the afferent limbof the pharyngeal reflex in which touching the back of the pharynxstimulates the patient to gag (i.e., the gag reflex).The efferent signal tothe musculature of the pharynx is carried by the branchial motor fibers ofthe vagus nerve.
The special sensory component of CN IX provides taste sensation fromthe posterior one-third of the tongue. Peripheral courseSpecial sensory fibers from the posterior one-third ofthe tongue travel via the pharyngeal branches of CN IX to the inferiorglossopharyngeal ganglion where their cell bodies reside.Central course -special sensory componentThe central processes of these neurons exitthe inferior ganglion and pass through the jugular foramen to enter thebrainstem at the level of the rostral medulla between the olive andinferior cerebellar peduncle. Upon entering the medulla, these fibersascend in the tractus solitarius and synapse in the caudal nucleussolitarius.Taste fibers from CNVII and X also ascend and synapse here.Ascending secondary neurons originating in nucleus solitarius projectbilaterally to the ventral posteromedial (VPM) nuclei of the thalamus viathe central tegmental tract.Tertiary neurons from the thalamus projectvia the posterior limb of the internal capsule to the inferior one-third ofthe primary sensory cortex (the gustatory cortex of the parietal lobe).
The glossopharyngeal nerve is mostly sensory.Theglossopharyngeal nerve also aids in tasting, swallowing andsalivary secretions. Its superior and inferior (petrous) gangliacontain the cell bodies of pain fibers. It also projects into manydifferent structures in the brainstem: Solitary nucleus:Taste from the posterior one-third of the tongueand information from carotid baroreceptors and carotid bodychemoreceptors Spinal nucleus of the trigeminal nerve: Somatic sensory fibersfrom the middle ear Lateral Nucleus of Ala Cinerea:Visceral pain Nucleus ambiguus:The lower motor neurons forthe stylopharyngeus muscle Inferior salivatory nucleus: Parasympathetic input tothe parotid and mucous glands.
From the anterior portion of the medulla oblongata, theglossopharyngeal nerve passes laterally across or below the flocculus,and leaves the skull through the central part of the jugular foramen.From the superior and inferior ganglia in jugular foramen it has its ownsheath of dura mater.The inferior ganglion on the inferior surface ofpetrous part of temporal is related with a triangular depression intowhich the aqueduct of cochlea opens.On the inferior side, theglossopharyngeal nerve is lateral and anterior to the vagusnerve and accessory nerve. In its passage through the jugular foramen (with X and XI), it passesbetween the internal jugular vein and internal carotid artery. It descendsin front of the latter vessel, and beneath the styloid process and themuscles connected with it, to the lower border of the stylopharyngeus. Itthen curves forward, forming an arch on the side of the neck and lyingupon the stylopharyngeus andmiddle pharyngeal constrictor muscle.From there, it passes under cover of the hyoglossus muscle, and is finallydistributed to the palatine tonsil, the mucous membrane ofthe fauces and base of the tongue, and the mucous glands of the mouth
Tympanic Stylopharyngeal Tonsillar Nerve to carotid sinus Branches to the posterior third of tongue Lingual branches A communicating branch to theVagus nerve Note:The glossopharyneal nerve contributes inthe formation of the pharyngeal plexus alongwith the vagus nerve.
The integrity of the glossopharyngeal nervemay be evaluated by testing the patientsgeneral sensation and that of taste on theposterior third of the tongue.The gag reflexcan also be used to evaluate theglossphyaryngeal nerve, but also tests thevagus nerve, as only the afferent fibresinvolved in the reflex are carried by theglossopharyngeal nerve.