The hypoglossal nerve supplies all the intrinsic and all the extrinsic muscle of the tongue except the palatoglossus (innervated by Vagus nerve). The nucleus of the hypoglossal nerve is in the medulla, undedr thr forth ventricle close to the midline. The neurons that form the nucleus of the of the hypoglossus form a column. The axons of these neurons form several fascicle that transverse the medulla to exit as multiple rootlets between the pyramid and the inferior olive. These rootlets unites in the subarachnoid space shortly after leaving the medulla and enter as a single nerve into the hypoglossal foramina (close to the foramen magnum). It leaves the skull and travels in the neck towards the tongue. The nucleus of the hypoglossal nerve is supplied primarily by the contralateral precentral cortical neurons. The most important tongue muscle is the genioglossus muscle. There are two genioglossi muscles, one on each side. The isolated function of one of these muscles leads to protrusion of the tongue with deviation of the tip towards the weak side. Hence, damage to the right hypoglossal nerve produces weakness of the right genioglossus and deviation of the tongue towards the side of the weak genioglossus, to the right (Figure 242.1 [click on it for movie clip]).

Figure 242.1.— Tongue deviation to the right due to a right hypoglossal lesion.

Weakness of the muscles innervated by the hypoglossal nerve has been reported in association to Moebius syndrome and brachial plexus injury. If such association is not present a lesion in the medulla should be suspected.

A	B	C

Figure 242.2.—[A] CT of the brain demonstrating a cyst in the posterior fossa; [B] MRI of the brain hypolasia of the left cerebellum and right brain stem; [C] MRI of the brain demostrating cyst.