Anatomy of the glossopharyngeal nerve (CN IX)

The glossopharyngeal nerve is the ninth cranial nerve and it has several roles. From helping us taste a freshly picked strawberry on the back of the tongue through its special sensory role, to keeping the carotid sinus in the loop about blood pressure variations through its visceral sensory component, as well as providing branchial motor innervation to a muscle in the pharynx and parasympathetic innervation to the parotid gland.

It may also receive somatic sensory information from a tiny part of skin on the external ear, but this isn’t important to remember.

This nerve is working four - sometimes five - jobs, and doing them all really well! But now, where did the glossopharyngeal nerve come from?

The glossopharyngeal nerve actually originates from a structure called the third pharyngeal arch that forms during intrauterine life, when the embryo is roughly the size of a poppy seed.

Remember that the pharyngeal arches are 6 embryological structures, of which only 5 eventually develop into the muscles, arteries, bones and cartilage of the head and neck.

Now, the third pharyngeal arch is less complex than the first two arches and gives rise to only a few structures such as the common carotid artery, the proximal portion of the internal carotid artery, the lower body and the greater horn of the hyoid bone and the stylopharyngeus muscle.All of these structures are innervated by the nerve of the third pharyngeal arch, which is the glossopharyngeal nerve.

So let’s take a closer look at the roles of this nerve. First up, there’s the special sensory component, that picks up taste information from the taste buds located in the posterior one-third of the tongue.

This pathway starts with special sensory fibers in the tongue, that travel via lingual branches of CN IX to the inferior glossopharyngeal, or petrosal, ganglion which contains the cell bodies of these first order neurons, in the jugular foramen.

Then, the fibers enter the cranial cavity and reach the brainstem at the level of the rostral medulla. From here, they travel in the solitary tract and synapse in the gustatory part of the nucleus of the solitary tract, on a second order neuron, to let us know that strawberry is, indeed, yummy!

Next up, is the visceral sensory component, which receives sensory information from the baroreceptors of the carotid sinus and the chemoreceptors of the carotid body, as well as the mucosa of the oropharynx and the posterior one-third of the tongue.

Fibers from the carotid body and sinus join with the sensory fibers from the tongue and the pharyngeal plexus. Somatic sensory fibers typically travel to the superior glossopharyngeal ganglion and synapse in the spinal trigeminal nucleus.

Meanwhile, visceral and special sensory fibers typically travel to the inferior glossopharyngeal ganglion and synapse in the nucleus of the solitary tract.

Next, there’s the branchial motor component of the glossopharyngeal nerve, which innervates the stylopharyngeus muscle. This is a muscle in the pharynx that elevates it during swallowing and speaking.

The branchial motor pathway starts from the motor cortex and its axons travel to the nucleus ambiguus located in the medulla, containing cell bodies of the lower motor neurons.

After they emerge from the medulla, they join the other components of CN IX to exit the skull via the jugular foramen. Finally, they descend deep to the styloid process and wrap around the posterior border of the stylopharyngeus muscle to innervate it.

Now, let’s take a look at the gag reflex which helps prevent objects in the oral cavity from entering the throat and also helps prevent choking.

This reflex has an afferent limb supplied by the glossopharyngeal nerve, and an efferent limb supplied by the vagus nerve.

Now, this pathway starts with stimulation of the posterior pharyngeal wall, the tonsillar area, or the base of the tongue which sends signals to the glossopharyngeal nerve that carries this information to the superior glossopharyngeal ganglion located in the jugular foramen.

Then, these neurons send fibers to the nucleus ambiguus and from here efferent nerve fibers are sent through the vagus nerve to the pharyngeal muscles which results in bilateral contraction of the posterior pharyngeal muscles.

Finally, the parasympathetic component of CN IX provides innervation to the parotid gland. This pathway starts with preganglionic fibers that originate from the inferior salivatory nucleus located in the upper medulla.

These fibers exit the brainstem along with the other parts of CN IX, and enter the jugular foramen. Then, they travel through both the superior and inferior glossopharyngeal ganglia without synapsing and exit the inferior ganglion as the tympanic nerve.

The tympanic nerve then enters the petrous portion of the temporal bone and ascends to the tympanic cavity where it contributes to the tympanic plexus.

The parasympathetic fibers pass through this plexus and join to become the lesser petrosal nerve which emerges in the middle cranial fossa and then exits the skull through the foramen ovale.

After exiting the skull, the lesser petrosal nerve synapses in the otic ganglion, and postganglionic fibers from the otic ganglion travel with the auriculotemporal branch of CN V3 to the parotid gland.

Alright, as a quick recap. The glossopharyngeal nerve is the ninth cranial nerve and its functions include somatic sensory, visceral sensory, special sensory, branchial motor, and parasympathetic innervation.

It provides branchial motor innervation to the stylopharyngeus muscle, special sensory innervation to the posterior third of the tongue, visceral sensory innervation for the carotid sinus and the carotid body, the mucosa of the oropharynx, the palatine tonsil and the soft palate, and parasympathetic innervation to the parotid gland.

The glossopharyngeal nerve also plays an important role in the gag reflex.