Glossopharyngeal nerve (Cranial nerve IX)
Just like there is the Club 27 in the world of rock ‘n’ roll, there is sort of the Club 12 when it comes to the anatomy of head and neck. Of course, we are not speaking about the legends of the rock ‘n’ roll, but about the 12 cranial nerves.
Explaining all of them at once would mean too much information at one moment, so in this particular article, we will be focused on the anatomy and the clinical importance of the ninth cranial nerve called the glossopharyngeal nerve (or cranial nerve IX or simply CN IX).
|Field of innervation||
Afferent: posterior one-third of tongue, palatine tonsils, oropharynx, mucosa of the middle ear, pharyngotympanic tube and the mastoid air cells, carotid body, taste
- Glossopharyngeal nerve function
- Course and relations
- Lateral branches
- Terminal branches
- Clinical aspects
- 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 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 forming of the trigeminal nerve, facial nerve, and the vagus nerve. The part of the nucleus that belongs to the glossopharyngeal nerve is the caudal portion of the nucleus.
This is a sensory nucleus which 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.
Glossopharyngeal nerve function
The glossopharyngeal nerve is a mixed nerve that contains both motor and sensory fibers. It originates from the medulla oblongata, where the four of its nuclei are located. Those nuclei give rise to the functionally diverse neuronal fibers that are involved in the motor and sensory functions of this nerve. These fibers are:
- Somatic sensory, which provide the sensory input from the posterior one-third of the tongue, palatine tonsils, oropharynx, mucosa of the middle ear, pharyngotympanic tube and the mastoid air cells
- Visceral sensory, that carry the sensory information from the carotid body
- Special sensory for the sense of taste from the posterior one-third of the tongue
- Parasympathetic, that innervate the salivary parotid gland
Motor, which innervate the muscles derived from the third pharyngeal arch, which are:
- The stylopharyngeus muscle
- The superior pharyngeal constrictor muscle.
Course and relations
The glossopharyngeal nerve leaves the brainstem in a form of the several rootlets from the posterior lateral sulcus of the medulla oblongata, superiorly to the place of the exit of the vagus nerve. Soon after the exit, the rootlets merge to form the body of the glossopharyngeal nerve, which courses forward and laterally, together with the vagus nerve and the accessory nerve. It crosses over the jugular tuberculum and finally leaves the cranium through the anterior, narrow part of the jugular foramen. Immediately outside the jugular foramen are placed two sensory ganglia that belong to the glossopharyngeal nerve, the superior and the inferior ganglia.
After passing through the jugular foramen, the glossopharyngeal nerve descends forward and inferiorly, aiming to the root of the tongue. In that phase of its course, the nerve forms an arch that is placed superiorly and parallel to the arch of the hypoglossal nerve.
After this, the nerve passes between the stylopharyngeus muscle and the styloglossus muscle, enters the prestyloid space and then divides under the mucosa of the posterior one-third of the tongue, to its many branches.
The glossopharyngeal nerve, besides the anastomotic branches with the facial nerve, vagus nerve, and the sympathetic fibers, also gives off 5 lateral branches:
- The tympanic nerve - Jacobson
- The carotid sinus nerve
- The pharyngeal nerves
- Stylopharyngeus muscle nerve
- Tonsillar nerves
This is the parasympathetic branch of the glossopharyngeal nerve that innervates the parotid gland. It separates from the glossopharyngeal nerve directly under the jugular foramen, and then it courses forward and laterally across the inferior side of the temporal pyramid, where it enters the tympanic canal.
The nerve ascends upwards through this canal and enters the tympanic cavity. Inside the cavity, the tympanic nerve extends upwards by crossing through the promontory sulcus where it forms the neural tympanic plexus. Sympathetic caroticotympanic nerves that extend from the internal carotid plexus also participate in the making of the tympanic plexus, along with the branch of the facial nerve that is called the communicant branch with the tympanic plexus.
The tympanic plexus sends branches that innervate the mucosa of the middle ear. From the superior part of the plexus, the lesser petrosal nerve rises. It then extends upwards, superiorly to the promontory, and after passing through the canal of the lesser petrosal nerve, it reaches the anterior side of the temporal pyramid. The lesser petrosal nerve then extends across this side of the temporal pyramid coursing forward and medially, all along the way being placed inside its groove called the sulcus of the lesser petrosal nerve. The nerve leaves the cranium as it passes through the sphenopetrosal fissure, and it finally ends in the otic ganglion.
This is important because the lesser petrosal nerve carries the secretory parasympathetic fibers (that originate from the glossopharyngeal nerve) that synapse with the cells of the otic ganglion. In that way, the postganglionic fibers in a form of the auriculotemporal nerve reach the parotid gland and supply it with the parasympathetic innervation that is necessary for the proper stimulation of the gland.
Carotid sinus nerve
This nerve consists of the general visceral afferent fibers that serve for the reflex lowering of the blood pressure when it is increased. The carotid sinus nerve emerges from the glossopharyngeal nerve from about 0.4 inches (1 cm) beneath the basis of the cranium. It then descends along the body of the internal carotid artery aiming towards the bifurcation of the common carotid artery. This nerve ends by sending terminal branches to the carotid sinus and the carotid body.
There are usually three of these nerves. They course medially, towards the pharynx. These branches create the anastomosis with the branches of the vagus nerve and as well as with the laryngopharyngeal nerves that originate from the cervical sympathetic plexus.
With these anastomoses, these nerves build a neural network that is called pharyngeal plexus. The nerves of this plexus innervate many of the pharyngeal muscles, soft palate muscles and the mucosa of the pharynx.
Stylopharyngeus muscle nerve
This is the motor branch that provides the motor innervation for the stylopharyngeus muscle. It reaches the muscle from its lateral surface, in the part of the glossopharyngeal nerve course between the stylopharyngeus and the styloglossus muscle.
These nerves provide sensory innervation to the palatine tonsils and to the mucosa of the isthmus of the fauces.
The glossopharyngeal nerve ends its course beneath the mucosa of the posterior one-third of the tongue, medially to the styloglossus muscle. On this place, the nerve divides into its terminal lingual branches. These nerves are sensory and they innervate the mucosa of the posterior one-third of the tongue, starting from the terminal sulcus of the tongue, and up to the epiglottis. Besides this, these branches carry the afferent gustatory fibers from the lingual papillae and therefore are in charge of the reception impulses about the taste.
It’s clear now that the glossopharyngeal nerve has many functions, so the damages of the nerve can cause different symptoms.
One of the ways to test the functionality of the glossopharyngeal nerve is to test the patient’s gag reflex. Since the glossopharyngeal nerve innervates the mucosa of the pharynx, and at the same time, it creates numerous anastomosis with the vagus nerve (dominant for the gastrointestinal functions), the slight stimulation of the posterior part of the tongue and throat should cause the patient to gag or wrench. The complete absence of this reflex is a pathological finding, and it suggests that the patient suffers bilateral lesion of the glossopharyngeal nerve, whereas the pathological result only on the one side of the throat suggests that there is a lesion of the ipsilateral nerve.
The glossopharyngeal nerve may be injured, and in that case, it manifests as a neuralgia. It often mimics the trigeminal neuralgia, by presenting with the facial and jaw pain. But compared to the trigeminal neuralgia, the glossopharyngeal neuralgia is relatively rare. A diagnostic interventional glossopharyngeal block is a way of differentiation the glossopharyngeal and the trigeminal neuralgia and maybe the only available mechanism for the proper pain management therapy for the patient.
The glossopharyngeal nerve has a very wide field of innervation and it is important for many of the physiological functions of the human organism. Because of that, the damages of the nerve may cause different states varying from uncomfortable to the life-threatening. For that reason, it is very important for the students to understand the anatomy and the function of the glossopharyngeal nerve, to make their tomorrows life at the clinic a lot easier.