Nerve Supply of Mouth and Pharynx
Six of the 12 pairs of cranial nerves contribute to the nerve supply of the mouth and pharynx. The trigeminal nerve (cranial nerve V) emerges from the lateral surface of the pons as a larger sensory and a smaller motor root. A short distance from the pons the sensory root is expanded by the presence of many afferent nerve cell bodies into the trigeminal ganglion, which lies in a depression on the apex of the petrous portion of the temporal bone. From the anterior margin of this ganglion arise the ophthalmic, maxillary, and mandibular divisions of the trigeminal nerve. The motor root courses along the medial and then the inferior side of the sensory root and ganglion and joins the mandibular nerve near its beginning. The maxillary division passes through the foramen rotundum into the pterygopalatine fossa, where it gives off the following branches: (1) two or three branches to the sphenopalatine ganglion, which leave the ganglion as a pharyngeal branch passing through a bony canal to the mucous membrane of the upper part of the nasopharynx, the palatine nerves passing through the pterygopalatine canal to exit through the greater and lesser palatine foramina to supply the mucous membrane of the palate, and a sphenopalatine branch, which enters the nasal cavity and runs along the nasal septum to reach the palate through the incisive foramen; (2) the posterior superior alveolar nerves, which enter the maxilla and supply the molar teeth and related gums. The maxillary nerve continues as the infraorbital nerve, which gives off the middle and anterior superior alveolar nerves in the infraorbital canal and a branch to the upper lip after it reaches the face. The mandibular nerve reaches the infratemporal fossa through the foramen ovale and has the following branches: (1) nerves to each of the muscles of mastication (the nerve to the medial pterygoid also supplying the tensor veli palatini muscle); (2) the inferior alveolar, which, before entering the mandibular foramen, gives off the mylohyoid branch to that muscle and the anterior belly of the digastric, courses through the alveolar canal supplying the mandibular teeth, and ends as the mental nerve, which exits through the mental foramen to give sensory supply to the chin and part of the lower lip; (3) the buccal nerve giving sensory supply to the cheek; and (4) the lingual nerve, which, after receiving the chorda tympani from the facial nerve, courses inferiorly and then anteriorly on the lateral surface of the hyoglossus muscle to reach the undersurface of the tongue. The trigeminal fibers in the lingual nerve carry general sensation from the anterior two thirds of the tongue.
The facial nerve (cranial nerve VII) emerges from the lateral side of the pontomedullary junction as a larger motor and a smaller sensory root (nervus intermedius), containing general visceral efferent fibers of VII as well as afferent fibers. It leaves the cranial cavity by way of the internal acoustic meatus and then goes through the facial canal to exit the stylomastoid foramen, where it gives off branches to the stylohyoid muscle and posterior belly of the digastric. The facial nerve runs anteriorly through the substance of the parotid gland, crosses the external carotid artery, and divides in the substance of the gland into branches that leave the anterior border of the parotid gland to innervate the muscles of facial expression, of which the ones surrounding the oral orifice, including the buccinator, are of interest in this discussion. The geniculate ganglion is present within the facial canal at a sharp bend in the nerve. Proximal to the ganglion, the nerve gives off the greater petrosal nerve, which eventually reaches the pterygopalatine ganglion. Distal to the ganglion it gives off the chorda tympani, which eventually joins the lingual nerve, carrying special visceral afferent fibers, which carry taste sensation from the anterior two thirds of the tongue, as well as preganglionic parasym athetic axons, which go to the submandibular ganglion.
The glossopharyngeal nerve (cranial nerve IX) emerges by a series of rootlets from the cranial part of the groove between the inferior cerebellar peduncle and the inferior olive of the medulla. It leaves the cranial cavity through the jugular foramen near which it exhibits two ganglionic swellings, courses downward along the posterior border of the stylopharyngeus muscle, and disappears deep to the hyoglossus muscle to ramify into its terminal branches to the tongue. The tympanic branch of the glossopharyngeal nerve follows a bony canal from the margin of the jugular foramen to the tympanic cavity, where it helps to form the tympanic plexus and then continues as the lesser petrosal nerve, which eventually brings preganglionic parasympathetic axons to the otic ganglion. The pharyngeal branches of the glossopharyngeal nerve mostly join with the pharyngeal branches of the vagus and branches of the superior cervical ganglion to form the pharyngeal plexus, which supplies the muscles of the pharynx (except the stylopharyngeus, entirely glossopharyngeal) and the muscles of the soft palate (except the tensor veli palatini, mandibular branch of trigeminal). The pharyngeal plexus consists of motor axons from the vagus and sensory axons to the mucous membrane via the glossopharyngeal, except for a muscular branch of the glossopharyngeal that innervates the stylopharyngeus muscle. The tonsillar branches arise near the base of the tongue and supply also the soft palate and the fauces. The lingual and terminal branches of IX take care of both the general sense and the sense of taste of the posterior one third of the tongue and the pharyngoepiglottic folds.
The vagus nerve (cranial nerve X) emerges just inferior to the glossopharyngeal nerve. It also leaves the cranial cavity by way of the jugular foramen and has two ganglionic swellings in this region, one at the foramen and one below the foramen. Entering the carotid sheath, the vagus courses inferiorly in the neck between the internal jugular vein and the internal or common carotid artery. Some branches of the vagus contribute to the supply of the mouth and pharynx. The pharyngeal branches (variable in number) supply motor axons to the pharyngeal plexus. The superior laryngeal nerve divides into external and internal branches. The external branch runs inferiorly and anteriorly on the external surface of the inferior pharyngeal constrictor to innervate the cricothyroid and cricopharyngeus muscles and a variable amount of the inferior pharyngeal constrictor. The internal branch pierces the thyrohyoid membrane and divides into an ascending and a descending branch, the former going to the mucous membrane covering the epiglottis and a small adjacent part of the tongue and the latter supplying the mucous membrane on the pharyngeal surface of the larynx in addition to its laryngeal distribution. The recurrent laryngeal branch of the vagus gives some supply to the inferior pharyngeal constrictor muscle as the branch passes under the inferior border of this muscle in entering the larynx.
The hypoglossal nerve (cranial XII) emerges by a series of rootlets from the sulcus between the inferior olive and the medullary pyramid. It leaves the cranial cavity by way of the hypoglossal canal and runs inferiorly and anteriorly, passing lateral to the carotid bifurcation and between the internal carotid artery and the internal jugular vein. It becomes superficial to the vessels near the angle of the mandible, where it passes across the external carotid and lingual arteries deep to the digastric muscles. From here it continues anteriorly between the mylohyoid and hyoglossus muscles. The hypoglossal nerve supplies the intrinsic muscles of the tongue, as well as the styloglossus, hyoglossus, and genioglossus muscles. Fibers from the first and second cervical nerves run with the hypoglossal to supply the geniohyoid and thyrohyoid muscles.
The areas of sensory supply of the mucous membrane of the oral cavity and pharynx, shown diagrammatically in the accompanying illustration, are only approximations because no complete agreement as to their limits exists.