9th Cranial Nerve (CN IX)/The glossopharyngeal nerve is the 9th cranial nerve (CN IX). It is one of the four cranial nerves that has sensory, motor, and parasympathetic functions. It originates from the medulla oblongata and terminates in the pharynx. This nerve is most clinically relevant in the setting of glossopharyngeal neuralgia, but an injury to it can also be a complication of carotid endarterectomy.
The glossopharyngeal nerve known as the ninth cranial nerve (CN IX), is a mixed nerve that carries afferent sensory and efferent motor information. It exits the brainstem out from the sides of the upper medulla, just anterior (closer to the nose) to the vagus nerve. The motor division of the glossopharyngeal nerve is derived from the basal plate of the embryonic medulla oblongata, while the sensory division originates from the cranial neural crest.
Structure of Glossopharyngeal Nerve
The glossopharyngeal nerve carries sensory, efferent motor, and parasympathetic fibers. Its branches consist of tympanic, tonsillar, stylopharyngeal, carotid sinus nerve, branches to the tongue, lingual branches, and a communicating branch to cranial nerve X (vagus nerve).
Special visceral efferent fibers (branchial motor) are the main motor fibers of the glossopharyngeal nerve and supply motor innervation to the stylopharyngeus muscle. This muscle is responsible for elevating the larynx and pharynx, especially during speaking and swallowing.
General visceral efferent fibers (visceral motor) provide parasympathetic innervation to the parotid glands. The fibers originate in the inferior salivary nucleus then travel with the tympanic nerve through the foramen ovale, and synapse at the otic ganglion.
General visceral afferent fibers (visceral sensory) carry sensory information from the carotid sinus and carotid body.
General somatic afferent fibers (general sensory) provides sensory innervation to the upper pharynx, the inner surface of the tympanic membrane, and the posterior third of the tongue. Special sensory fibers provide taste afferents from this portion of the tongue as well.[rx]
Overview of the branchial motor component
The branchial motor component of CN IX provides voluntary control of the stylopharyngeus muscle, which elevates the pharynx during swallowing and speech.
- Origin and central course – The branchial motor component originates from the nucleus ambiguous in the reticular formation of the medulla Rostral medulla. Fibers leaving the nucleus ambiguous travel anteriorly and laterally to exit the medulla, along with the other components of CN IX, between the olive and the inferior cerebellar peduncle.
- Intracranial course – Upon emerging from the lateral aspect of the medulla the branchial motor component joins the other components of CN IX to exit the skull via the jugular foramen. The glossopharyngeal fibers travel just anterior to the cranial nerves X and XI, which also exit the skull via the jugular foramen.
- Extra-cranial course and final innervation – Upon exiting the skull the branchial motor fibers descend deep to the temporal styloid process and wrap around the posterior border of the stylopharyngeus muscle before innervating it.
- Voluntary control of the stylopharyngeus muscle – Signals for the voluntary movement of the stylopharyngeus muscle originate in the pre-motor and motor cortex (in association with other cortical areas) and pass via the corticobulbar tract in the genu of the internal capsule to synapse bilaterally on the ambiguous nuclei in the medulla.
Overview of the visceral motor component
Parasympathetic component of the glossopharyngeal nerve that innervates the ipsilateral parotid gland.
- Origin and central course – The preganglionic nerve fibers originate in the inferior salivatory nucleus of the rostral medulla and travel anteriorly and laterally to exit the brainstem between the medullary olive and the inferior cerebellar peduncle with the other components of CN IX. Note: These neurons do not form a distinct nucleus visible on cross-section of the brainstem. The position indicated on the diagram is representative of the location of the cell bodies of these fibers.
- Intracranial course – Upon emerging from the lateral aspect of the medulla, the visceral motor fibers join the other components of CN IX to enter the jugular foramen. Within the jugular foramen, there are two glossopharyngeal ganglia that contain nerve cell bodies that mediate general, visceral, and special sensation. The visceral motor fibers pass through both ganglia without synapsing and exit the inferior ganglion with CN IX general sensory fibers as the tympanic nerve. Before exiting the jugular foramen, the tympanic nerve enters the petrous portion of the temporal bone and ascends via the inferior tympanic canaliculus to the tympanic cavity.
- Extra-cranial course and final innervations – Upon exiting the skull, the lesser petrosal nerve synapses in the otic ganglion, which is suspended from the mandibular nerve immediately below the foramen ovale. Postganglionic fibers from the otic ganglion travel with the auriculotemporal branch of CN V3 to enter the substance of the parotid gland.
- Hypothalamic Influence – Fibers from the hypothalamus and olfactory system project via the dorsal longitudinal fasciculus to influence the output of the inferior salivatory nucleus. Examples include: 1) dry mouth in response to fear (mediated by the hypothalamus); 2) salivation in response to smelling food (mediated by the olfactory system)
Overview of the visceral sensory component
This component of CN IX innervates the baroreceptors of the carotid sinus and chemoreceptors of the carotid body.
- Peripheral and intracranial course – Sensory fibers arise from the carotid sinus and carotid body at the common carotid artery bifurcation, ascend in the carotid sinus nerve, and join the other components of CN IX at the inferior glossopharyngeal ganglion. The cell bodies of these neurons reside in the inferior glossopharyngeal ganglion. The central processes of these neurons enter the skull via the jugular foramen.
- Central course – visceral sensory component once inside the skull, the visceral sensory fibers enter the lateral medulla between the olive and the inferior cerebellar peduncle and descend in the solitary tract to synapse in the caudal solitary nucleus. From the solitary nucleus, connections are made with several areas in the reticular formation and hypothalamus to mediate cardiovascular and respiratory reflex responses to changes in blood pressure and serum concentrations of CO2 and O2.
- Clinical correlation – The visceral sensory fibers of CN IX mediate the afferent limb of the pharyngeal reflex in which touching the back of the pharynx stimulates the patient to gag (i.e., the gag reflex). The efferent signal to the musculature of the pharynx is carried by the branchial motor fibers of the vagus nerve.
Overview of the somatic sensory component
This component of CN IX carries general sensory information (pain, temperature, and touch) from the skin of the external ear, the internal surface of the tympanic membrane, the walls of the upper pharynx, and the posterior one-third of the tongue, anterior surface of the epiglottis, vallecula.
- Peripheral course – Sensory fibers from the skin of the external ear initially travel with the auricular branch of CN X, while those from the middle ear travel in the tympanic nerve as discussed above (CN IX visceral motor section). General sensory information from the upper pharynx and posterior one-third of the tongue travel via the pharyngeal branches of CN IX. These peripheral processes have their cell body in either the superior or inferior glossopharyngeal ganglion.
- Central course – The central processes of the general sensory neurons exit the glossopharyngeal ganglia and pass through the jugular foramen to enter the brainstem at the level of the medulla. Upon entering the medulla these fibers descend in the spinal trigeminal tract and synapse in the caudal spinal nucleus of the trigeminal.
Overview of the special sensory component
The special sensory component of CN IX provides taste sensation from the posterior one-third of the tongue.
- Peripheral course – Special sensory fibers from the posterior one-third of the tongue travel via the pharyngeal branches of CN IX to the inferior glossopharyngeal ganglion where their cell bodies reside.
- Central course – special sensory component. The central processes of these neurons exit the inferior ganglion and pass through the jugular foramen to enter the brainstem at the level of the rostral medulla between the olive and inferior cerebellar peduncle. Upon entering the medulla, these fibers ascend in the tractus solitarius and synapse in the gustatory part of nucleus solitarius. Taste fibers from CN VII and X also ascend and synapse here. Ascending secondary neurons originating in nucleus solitarius project bilaterally to the ventral posteromedial (VPM) nuclei of the thalamus via the central tegmental tract.
- Tertiary neurons from the thalamus project via the posterior limb of the internal capsule to the inferior one-third of the primary sensory cortex (the gustatory cortex of the parietal lobe).
- Branchial motor (special visceral efferent) – supplies the stylopharyngeus muscle.
- Visceral motor (general visceral efferent) – provides parasympathetic innervation of the parotid gland via the otic ganglion
- Visceral sensory (general visceral afferent) – carries visceral sensory information from the carotid sinus and carotid body.
- General sensory (general somatic afferent) – provides general sensory information from inner surface of the tympanic membrane, upper pharynx (GVA), and the posterior one-third of the tongue.
- Visceral afferent (special visceral afferent) – provides taste sensation from the posterior one-third of the tongue, including circumvallate papillae.
- Special visceral efferent – this component of the glossopharyngeal nerve transmits information to the stylopharyngeus muscle, a muscle originating from the third pharyngeal arch. This muscle is responsible for raising the larynx and pharynx, and functions during swallowing
- General visceral efferent – this component of the glossopharyngeal nerve transmits the parasympathetic information to the parotid salivary gland. Although the facial nerve goes through the parotid gland, it is not that nerve that innervates it, but the glossopharyngeal nerve.
- General visceral afferent – this component of the glossopharyngeal nerve transmits sensory information from the carotid body and sinus, middle ear, posterior one-third of the tongue and vallate papillae, mucous membranes of the soft palate and palatine tonsils and the oropharynx.
- Special visceral afferent – this component of the glossopharyngeal nerve transmits information related to taste from the posterior one-third of the tongue. Remember that taste fibers from the front two-thirds of the tongue are carried in the facial nerve.
- General somatic afferent – this component of the glossopharyngeal nerve transmits general sensory information from inside of the tympanic membrane, skin of the external ear, upper portion of the pharynx and general sensation from the posterior one-third of the tongue.
Nerves
The glossopharyngeal nerve has its origin in the medulla oblongata and exits the skull via the jugular foramen, which is where the tympanic nerve branches off to give parasympathetic innervation to the parotid gland. After the jugular foramen are the superior and inferior ganglia, which house the cell bodies of the sensory fibers, and then the nerve descends the neck, where it provides innervation to the stylopharyngeus and sensation to the carotid sinus and body. It terminates in the pharynx between the superior and middle pharyngeal constrictors, splitting into its other branches – lingual, pharyngeal, and tonsillar.[rx]
Branches
- Tympanic
- Stylopharyngeal
- Tonsillar
- carotid sinus nerve
- Branches to the posterior third of tongue
- Lingual branches
- A communicating branch to the Vagus nerve
The branches of the glossopharyngeal nerve are
- Tympanic nerve (AKA nerve of Jacobson) – carries parasympathetic fibers and eventually becomes the lesser petrosal nerve, exiting the skull via the foramen ovale and synapses in the otic ganglion
- Stylopharyngeal nerve – provides motor innervation to the stylopharyngeus muscle
- The nerve to carotid sinus – communicates with the vagus nerve to carry signals from the baroreceptors in the carotid sinus and chemoreceptors in the carotid body – this helps in regulating blood pressure (carotid sinus) and monitoring blood oxygen and CO2 levels (carotid body)
- Pharyngeal branches – join with the pharyngeal branches of the vagus nerve and sympathetic nerves to form the pharyngeal plexus, which innervates the muscles of the pharynx
- Tonsillar branches – provides sensory innervation to the palatine tonsil
- Lingual branches – supply the vallate papillae, mucous membrane, and follicular glands of the posterior tongue.
Nucleus
This nerve has altogether 4 nuclei that are located in the medulla oblongata:
- The ambiguous nucleus
- The inferior salivary nucleus
- The spinal nucleus of the trigeminal nerve
- The solitary nucleus
The ambiguous nucleus
This is a group of motor neurons located deep in the medullary reticular formation. Besides the glossopharyngeal nerve, this nucleus is common for the vagus nerve and the accessory nerve. This nucleus contains neurons that provide motor innervation to several muscles of the soft palate, the pharynx, the larynx and the upper part of the esophagus. The axons of these neurons are distributed to the cranial nerves IX, X and XI. The axons that belong to the glossopharyngeal nerve provide motor innervation to the stylopharyngeus muscle and the superior pharyngeal constrictor muscle.
The inferior salivary nucleus
This nucleus contains neurons whose fibers innervate the parotid gland. These fibers are actually preganglionic parasympathetic fibers that project to the otic ganglion. Then, the postganglionic fibers of this ganglion project to the salivary parotid gland on which they have a secretory effect.
This nucleus receives fibers from the other nuclei of the cranial nerves, such are solitary nucleus and the sensory nuclei of the trigeminal nerve, and also from the hippocampus and the olfactory system. In this way, different stimuli control these nuclei and therefore cause the excretion of the saliva on various occasions.
The spinal nucleus of the trigeminal nerve
This is a sensory nucleus where the fibers from the posterior one-third of the tongue, palatine tonsils, oropharynx, mucosa of the middle ear, pharyngotympanic tube, and mastoid cells arrive. Besides the glossopharyngeal nerve, this nucleus participates in the forming of the trigeminal nerve, facial nerve, and vagus nerve. The part of the nucleus that belongs to the glossopharyngeal nerve is the caudal portion of the nucleus.
Solitary nucleus
This is a sensory nucleus that receives the fibers from the glossopharyngeal nerve, but also from the facial and vagus nerves. The caudal portion of the nucleus receives sensory information from the baroreceptors of the carotid sinus, while the rostral portion receives gustatory (taste) fibers from the posterior one-third of the tongue. The rostral part is because of its function referred to as the gustatory nucleus.
In general, the field of innervation that belongs to the glossopharyngeal nerve is tightly connected to the branches of the vagus nerve and the accessory nerve. All of these three cranial nerves are involved in the complex and important functions of the body, such as blood pressure regulation, heart rate, breathing, swallowing and vomiting.
Muscles and 9th Cranial Nerve (CN IX)
As stated above, the glossopharyngeal nerve provides motor innervation to the stylopharyngeus muscle, which is responsible for elevating the pharynx and larynx.
Functions of 9th Cranial Nerve (CN IX)
- It receives general somatic sensory fibers (ventral trigeminothalamic tract) from the tonsils, the pharynx, the middle ear, and the posterior 1/3 of the tongue.
- It receives special visceral sensory fibers (taste) from the posterior 1/3 of the tongue.
- It receives visceral sensory fibers from the carotid bodies, carotid sinus.[3]
- It supplies parasympathetic fibers to the parotid gland via the otic ganglion.
- It supplies motor fibers to the stylopharyngeus muscle, the only motor component of this cranial nerve.
- It contributes to the pharyngeal plexus.
Sensory Functions
The glossopharyngeal nerve provides sensory innervation a variety of structures in the head and neck.
The tympanic nerve arises as the nerve traverses the jugular foramen. It penetrates the temporal bone and enters the cavity of the middle ear. Here, it forms the tympanic plexus – a network of nerves that provide sensory innervation to the middle ear, internal surface of the tympanic membrane and Eustachian tube.
At the level of the stylopharyngeus, the carotid sinus nerve arises. It descends down the neck to innervate both the carotid sinus and carotid body, which provide information about blood pressure and oxygen saturation respectively.
The glossopharyngeal nerve terminates by splitting into several sensory branches:
- Pharyngeal branch – combines with fibres of the vagus nerve to form the pharyngeal plexus. It innervates the mucosa of the oropharynx.
- Lingual branch – provides the posterior 1/3 of the tongue with general and taste sensation
- Tonsillar branch – forms a network of nerves, known as the tonsillar plexus, which innervates the palatine tonsils.
Special Sensory
The glossopharyngeal nerve provides taste sensation to the posterior 1/3 of the tongue, via its lingual branch (Note: not to be confused with the lingual nerve).
Motor Functions
The stylopharyngeus muscle of the pharynx is innervated by the glossopharyngeal nerve. This muscle acts to shorten and widen the pharynx and elevate the larynx during swallowing.
Parasympathetic Functions
The glossopharyngeal nerve provides parasympathetic innervation to the parotid gland. These fibres originate in the inferior salivatory nucleus of CN IX. These fibres travel with the tympanic nerve to the middle ear. From the ear, the fibres continue as the lesser petrosal nerve, before synapsing at the otic ganglion.
The fibres then hitchhike on the auriculotemporal nerve to the parotid gland, where they have a secretomotor effect.
References
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