THE GLOSSOPHARYNGEAL NERVE (IX)

The glossopharyngeal nerve combines fibers concerned with the innervation of structures of third pharyngeal arch origin with important visceral efferent (parasym­pathetic) and afferent components.

The glossopharyngeal nerve arises from the ventro­lateral aspect of the medulla oblongata, from the most rostral rootlets of the linear series that also gives origin to the vagus and the medullary part of the accessory nerve (Figures 8-19 and 8-21). It runs with these nerves to the jugular foramen and at about this level bears two small and rather indistinct ganglia. The first branch, the tympanic nerve, enters the tympanic cavity, where it par­ticipates with branches of the facial and internal carotid (sympathetic) nerves in forming a plexus from which a nerve leads to the otic ganglion for the supply of the parotid gland (Figure 8-70/3,14).

The main trunk cleaves for a spell to the vagus and accessory nerves and at this level detaches the carotid sinus branch, which proceeds to the carotid sinus, where it terminates in baroreceptors within the sinus wall and chemoreceptors of the carotid body. The glossopharyn­geal nerve then turns rostroventrally, parallel to the sty­lohyoid, before dividing into pharyngeal and lingual branches. The pharyngeal branches include one to the stylopharyngeus caudalis; the others become diffused within the pharyngeal plexus to which the vagus also contributes. Although most fibers are sensory to the mucosa, the possibility of a further contribution to the pharyngeal musculature seems likely.

The larger lingual branch enters the tongue parallel to the lingual artery, the lingual branch of the mandibu­lar nerve, and the hypoglossal nerve. It is sensory to the mucosa of the root of the tongue (including the taste buds in this area) and motor to the levator palatini muscle and the glands of the soft palate. Damage to the nerve, which is most common in horses as the result of inflammation of the guttural pouch, may lead to diffi­culties in swallowing. Because the vagus may also be affected, it is difficult to know the extent to which the paresis of palate and pharynx is due to glossopharyn­geal involvement. Experimental studies suggest that the role of the glossopharyngeal nerve is more important than many authors have claimed.