CN IX: Glossopharyngeal nerve

In this note, we are going to discuss the anatomy and function of the glossopharyngeal nerve (n. glossopharyngeus), which is the CN IX.

This nerve carries out several functions:

Provides the somatic sensory input from the pharynx, soft palate and posterior one-third of the tongue
Provides the gustatory sensory input from the posterior one-third of the tongue
Carries the sensory information from the carotid sinus and carotid body or glomus (to regulate blood pressure)
Innervates the salivary parotid gland, provides speech and swallowing

According to the generally accepted classification of functional components, it contains the following types of fibers:

general somatic afferent fibers (GSA)
general visceral afferent fibers (GVA)
special visceral afferent fibers (SVA)
general visceral efferent fibers (GVE)
special visceral efferent fibers (SVE)

That means that it is a mixed nerve.

The motor fibers arise from the nucleus ambiguus (nucl. ambiguus), which is located deep in the medullary reticular formation.

The sensory fibers arise from the superior and inferior sensory ganglia (ganglion superius et inferius), which are located in the region of the jugular foramen.

Inferior ganglion (ganglion inferius / caudalis)

The peripheral processes of the sensory neurons pass to the innervation areas, while the central processes enter the brain and end on the neurons of the solitary nucleus (nucl. tractus solitarii),

Solitary nucleus (nucleus tractus solitarii / nucleus solitarius)

which is located in the dorsal part of the medulla oblongata, as well as the spinal nucleus of trigeminal nerve (nucleus spinalis nervi trigemini).

The inferior salivatory nucleus (nucl. salivatorius inferior) is the source of the preganglionic parasympathetic fibers. This nucleus is also located in the dorsal part of the medulla oblongata.

Inferior salivatory nucleus (nucleus salivatorius inferior)

The glossopharyngeal nerve leaves the brainstem from the retroolivary groove in a form of five to six rootlets. Soon after the exit, the rootlets merge to form the trunk of the glossopharyngeal nerve.

It leaves the cranium through the jugular foramen (foramen jugulare).

First, the nerve passes as part of the carotid sheath, and then, circumflexing the stylopharyngeus muscle, it turns anteriorly and lies between the superior and middle pharyngeal constrictors.

Glossopharyngeal nerve (n. glossopharyngeus) after exiting the jugular foramen

Glossopharyngeal nerve (n. glossopharyngeus) and its branches

Along its course, the nerve gives off several branches:

1. Preganglionic parasympathetic fibers pass through the superior and inferior ganglia of the glossopharyngeal nerve, but they do not synapse with the cells of these ganglia. The tympanic nerve (n. tympanicus) separates from the glossopharyngeal nerve directly under the jugular foramen. It contains sensory and parasympathetic fibers.

Through the tympanic canal, the nerve enters the tympanic cavity, where it gives off branches that form the tympanic plexus (plexus tympanicus). It sends branches that innervate the mucosa of the middle ear.

Preganglionic parasympathetic fibers exit the tympanic cavity in the form of the lesser petrosal nerve (n. petrosus minor).

It passes through the canal, hiatus (hiatus canalis nervi petrosi minoris),

Hiatus for lesser petrosal nerve (hiatus canalis nervi petrosi minoris)

and sulcus of the lesser petrosal nerve (sulcus nervi petrosi minoris),

exits the cranium through the sphenopetrosal fissure

Sphenopetrosal fissure (fissura sphenopetrosa)

and ends in the otic ganglion (ganglion oticum).

As part of the auriculotemporal nerve (n. auriculotemporalis), postganglionic fibers reach the parotid salivary gland, providing its secretory innervation.

Parotid salivary gland (glandula parotidea)

2. General visceral afferent fibers (GVA) are represented by the carotid branch (r. sinus carotici). It innervates the carotid body and carotid sinus. The carotid sinus (sinus caroticus) is a dilated area of the terminal part of the common carotid artery or the initial part of the internal carotid artery. The carotid sinus is the reflex area consisting of baroreceptors which monitor blood pressure.

Stimulation of these receptors causes reflex slowing of the heart rate and dilation of arterioles.

The carotid body is a small structure located behind the bifurcation of the common carotid artery, which contains chemoreceptors that respond to increased concentration of carbon dioxide and decreased oxygen concentration. When these receptors are stimulated, a reflex increase in blood pressure, respiratory rate and heart rate occurs.

3. The stylopharyngeal branch (r. m. stylopharyngei) gives off branchial motor fibers to the stylopharyngeus muscle. This type of fiber innervates structures, originated from the pharyngeal arches. In this case, the stylopharyngeus muscle is a derivative of the third pharyngeal arch. During swallowing or speech, this muscle lifts the pharynx.

Stylopharyngeus muscle (m. stylopharyngeus)

4. Pharyngeal branches (rr. pharyngei) provide sensory innervation to the mucous membrane of the pharynx, tonsils, and soft palate. These branches create the anastomosis with the branches of the vagus nerve as well as with the branches of the sympathetic trunk to form the pharyngeal plexus.

5. Tonsillar branches (rr. tonsillares) provide sensory innervation to the palatine tonsils, palatoglossal and palatopharyngeal arches.

6. Lingual branches (rr. linguales), which are the terminal branches of the glossopharyngeal nerve, enter the tongue below the styloglossus muscle and innervate the mucous membrane of the posterior one-third of the tongue.

They provide general somatic and special gustatory or taste sensation from the posterior one-third of the tongue.

Gustatory innervation of the posterior one-third of tongue

As mentioned previously, the glossopharyngeal nerve has two sensory nuclei and two sensory ganglia.

So, the fibers of general (or somatic) sensation synapse with the cells of the superior ganglion (ganglion superius) and travel to the spinal nucleus of the trigeminal nerve (nucleus spinalis nervi trigemini).

Spinal nucleus of the trigeminal nerve (nucleus spinalis nervi trigemini)

And the fibers of visceral and special sensation synapse with the cells of the inferior ganglion (ganglion inferius) and travel to the solitary nucleus (nucleus tractus solitarii).

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