The Glossopharyngeal Nerve (CN IX)
The glossopharyngeal nerve, CN IX, is the ninth paired cranial nerve. In this article, we shall look at the anatomical course of the nerve, and the motor, sensory and parasympathetic functions of its terminal branches.
Embryologically, the glossopharyngeal nerve is associated with the derivatives of the third pharyngeal arch.
Sensory: Innervates the oropharynx, carotid body and sinus, posterior 1/3 of the tongue, middle ear cavity and Eustachian tube.
Special Sensory: Provides taste sensation to the posterior 1/3 of the tongue.
Parasympathetic: Provides parasympathetic innervation to the parotid gland.
Motor: Innervates the stylopharyngeus muscle of the pharynx.
The glossopharyngeal nerve originates in the medulla oblongata of the brain. It emerges from the anterior aspect of the medulla, moving laterally in the posterior cranial fossa. The nerve leaves the cranium via the jugular foramen. At this point, the tympanic nerve arises. It has a mixed sensory and parasympathetic composition.
Immediately outside the jugular foramen lie two ganglia (collections of nerve cell bodies). They are known as the superior and inferior (or petrous) ganglia – they contain the cell bodies of the sensory fibres in the glossopharyngeal nerve.
Now extracranial, the glossopharyngeal nerve descends down the neck, anterolateral to the internal carotid artery. At the inferior margin of the stylopharyngeus, several branches arise to provide motor innervation to the muscle. It also gives rise to the carotid sinus nerve, which provides sensation to the carotid sinus and body.
The nerve enters the pharynx by passing between the superior and middle pharyngeal constrictors. Within the pharynx, it terminates by dividing into several branches – lingual, tonsil and pharyngeal.
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.
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).
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.
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.
Remember – although the facial nerve splits into its five terminal branches in the parotid gland, it is the glossopharyngeal nerve that actually supplies the gland.
Clinical Relevance – Gag Reflex
The glossopharyngeal nerve supplies sensory innervation to the oropharynx, and thus carries the afferent information for the gag reflex. When a foreign object touches the back of the mouth, this stimulates CNIX, beginning the reflex. The efferent nerve in this process is the vagus nerve, CNX.
An absent gag reflex signifies damage to the glossopharyngeal nerve.