The Five Senses and Beyond: The Encyclopedia of Perception - Jennifer L. Hellier 2017

Glossopharyngeal Nerve

The glossopharyngeal nerve is a pair of cranial nerves that contain both motor and sensory fibers. It is the ninth of the 12 paired cranial nerves and therefore is also known as cranial nerve IX. As its name implies, it serves both the tongue and the pharynx (throat). The sensory fibers originate at many different sites including the pharynx, middle and outer ear, internal carotid artery, and the posterior one-third of the tongue, which contains taste buds. These nerve fibers terminate in the brainstem, specifically at the medulla oblongata (generally called the medulla). The motor fibers originate at the medulla and end at the parotid salivary gland in the cheek just in front of the ear, posterior tongue glands, and the stylopharyngeal muscle, which moves the pharynx up and down.

Anatomy

The glossopharyngeal nerve has six distinct branches: tympanic, stylopharyngeal, tonsillar, carotid sinus nerve, lingual branches, and branch to the vagus tympanic. The tympanic nerve or nerve of Jacobson is found near the ear. The stylopharyngeal branch goes to the stylopharyngeus muscle. The tonsillar branches supply the palatine tonsils. The branch to the carotid sinus runs down to the internal carotid artery. The lingual branches have two nerves: one supplies the base of the tongue and the other supplies part of the tongue, which also meets with the lingual nerve. And finally, the pharyngeal branches have three or four filaments that meet with the pharyngeal branches of the vagus nerve.

The glossopharyngeal nerve consists of five components: branchial motor, visceral motor, visceral sensory, general sensory, and special sensory. This entry will focus on the sensory portions of the nerve.

The visceral sensory component of the glossopharyngeal nerve originates in the carotid sinus where the common carotid artery splits into two. It travels up in the sinus nerve and joins the other components of the glossopharyngeal nerve at the inferior hypoglossal ganglion. From here they enter the skull via the jugular foramen, which is an opening through which multiple arteries and veins travel. The visceral sensory fibers enter the brainstem at the level of the medulla between distinct bumps called the olives and the inferior cerebellar peduncles. The fibers then travel downward within the tractus solitarius to communicate with neurons located in the nucleus solitarius. From here, the fibers continue and synapse in the reticular formation of the brainstem and hypothalamus.

The general sensory component of the glossopharyngeal nerve originates in several different places. General sensory fibers from the skin of the external ear and middle ear, as well as general sensory fibers from the upper pharynx and posterior one-third of the tongue meet at the superior or inferior glossopharyngeal ganglion. Next, the fibers of the general sensory neurons need to pass through the skull via the jugular foramen. Just like the visceral sensory fibers, the general sensory fibers enter the medulla and then connect to neurons in the caudal spinal nucleus of the trigeminal.

The special sensory component of the glossopharyngeal nerve originates from the back one-third of the tongue and travels through the pharyngeal branches of the glossopharyngeal nerve to the inferior glossopharyngeal ganglion. Again these fibers enter the brainstem at the level of the medulla but on the rostral end and follow the tractus solitarius to synapse in the caudal portion of the nucleus solitarius. In addition, taste fibers from cranial nerves VII and X (facial and vagus nerves, respectively) synapse here. From the nucleus solitarius, the nerves travel bilaterally to the ventral posteromedial nuclei of the thalamus. Finally, they reach the lower one-third of the primary sensory cortex, which is the gustatory cortex of the parietal lobe and where the sense of taste is processed in the brain.

Physiology

The visceral sensory component of the glossopharyngeal nerve receives important information about blood pressure from the carotid sinus and helps the autonomic nervous system during a “fight-or-flight” response. Specifically, the visceral sensory component detects the amount of stretch within the blood vessel walls (determines blood pressure) and sends the signal to the hypothalamus in response to the stretch. This nerve also transmits information from the carotid body about oxygen and carbon dioxide levels in the blood.

The general sensory component of the glossopharyngeal nerve carries general sensory information 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 to the medulla. This information includes pain, temperature, and touch signals.

The special sensory component of the glossopharyngeal nerve provides taste and touch signals from the posterior one-third of the tongue to the taste centers of the brain.

Diseases

The glossopharyngeal nerve, if damaged, can have several effects on the human body. Because this nerve is involved in the functions of tasting, swallowing, and maintenance of blood pressure, any damage or injury affects these functions. These effects include loss of bitter and sour taste as well as impaired swallowing. In one-sided damage to the sensory component of the glossopharyngeal nerve, there is no gag response when touching the back wall of the mouth on the same side of the damaged nerve. Other signs include speech impediments and loss of taste on the posterior one-third of the tongue.

The most common disorder of this cranial nerve is glossopharyngeal neuralgia. Glossopharyngeal neuralgia is a condition in which there is severe pain in the tongue, throat, ears, and tonsils. This pain can last from a few seconds to a few minutes. These pain symptoms can be triggered by swallowing, drinking cold liquids, sneezing, coughing, talking, clearing the throat, or even touching the gums or inside of the mouth. It is considered to be a rare disorder and usually begins after the age of 40, occurring more often in men. There are several causes of glossopharyngeal neuralgia, but generally it is caused by compression of the glossopharyngeal nerve from a blood vessel or tumor. Glossopharyngeal neuralgia may also be associated with multiple sclerosis.

Mario J. Perez

See also: Cranial Nerves; Taste System

Further Reading

Liang, Barbara. (2012). The 12 cranial nerves. Retrieved from https://www.wisc-online.com/learn/general-education/anatomy-and-physiology1/ap11504/the-12-cranial-nerves

Yale University School of Medicine. (1998). Cranial nerves. Retrieved from http://www.yale.edu/cnerves/