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Cranial Nerve IX – Glossopharyngeal Nerve
Figure 26. Cranial Nerve IX – Glossopharyngeal Nerve
The glossopharyngeal nerve contains, like the facial and along with the vagus nerve which we have not studied yet, all four types of nerve fibers. Unlike those other two nerves where motor fibers predominate the glossopharyngeal nerve is mainly a sensory nerve. The somatic motor fibers innervate only the stylopharyngeus muscle. The visceral motor fibers travel to the otic ganglion where they synapse and continue on mandibular nerve branches to the parotid gland. The special sensory fibers carry fibers that originate in the taste buds in the posterior one-third of the tongue. The general sensory fibers convey sensations from both internal receptors, the baroreceptors of the carotid sinus and the chemoreceptors found in the carotid body which is in the area where the carotid artery splits into the internal and external carotids. The carotid body importantly monitors blood gases. There are also more conventional receptors it carries signals from. There are general sensory ones in the posterior one-third of the tongue and the adjacent tissues covering the epiglottis, the upper pharyngeal walls and the skin of the ear and the deep surface of the tympanic membrane. This is nerve that is the main carrier of pain when you have a sore throat.
Having made a long introduction let us follow the path of the nerve. Like the other mixed nerves we have discussed we will start with the cranial exit but one should recognize that the sensory nerves are not leaving there but are entering the cranium. The superior and inferior ganglia are sensory in function. Only the otic ganglion is associated with parasympathetic motor fibers.
Figure 27. Cranial Nerve IX – Glossopharyngeal Nerve
The cranial exit/entrance of the glossopharyngeal nerve is the jugular foramen. The first branch of the nerve is the small tympanic nerve which passes through a small opening between the carotid canal and the jugular foramen orifices into the body of the temporal bone. It passes into the tympanic cavity where it forms a network of nerve fibers called the tympanic plexus. This network collects general sensory fibers from the internal surface of the tympanic membrane and surrounding tissues. The remaining fibers, that are all pre-ganglionic parasympathetic fibers, form the lesser petrosal nerve which enters the otic ganglion where they synapse. The post synaptic fibers pass into the mandibular nerve and are carried to the parotid gland on the auriculotemporal nerve.
The main trunk continues after the branch into the neck until it splits. It is formed mainly of sensory fibers that join to form the trunk. These come from general sensory fibers that come from branches in the tonsillar region, the pharynx and receptors from the carotid artery and both the somatic sensory and taste sensory fibers from the posterior one-third of the tongue. The small motor nerve to the stylopharyngeus muscle is also included in the formation of the main trunk.