The Glossopharyngeal Nerve (IX)

The glossopharyngeal nerve combines fibers that innervate structures of third pharyngeal arch origin with important visceral efferent (parasympathetic) and afferent components. It is motor to part of the palatopharyngeal musculature and to certain salivary glands and sensory to mucosa of the root of the tongue, palate, and pharynx.

The glossopharyngeal nerve arises from the ventrolateral aspect of the medulla oblongata, from the most rostral rootlets of the linear series of rootlets that also give origin to the vagus and the medullary part of the accessory nerve (Figs. 8.19 and 8.20). 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 (Fig. 8.71/3 and 14).

The main trunk travels closely with 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 dispersed within the pharyngeal plexus to which the vagus also contributes. Although most fibers are sensory to the mucosa, it is likely that some fibers provide motor innervation to pharyngeal musculature.

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 glossopharyngeal 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 writers have claimed.