The hypoglossal nerve is the 12th cranial nerve and, with its approximately 9,000 nerve fibers1, provides motor innervation to the intrinsic and extrinsic tongue muscles.
Cranial nerve nucleus
The nucleus of the hypoglossal nerve is a purely somatomotor and the sole cranial nerve nucleus of the hypoglossal nerve.
Anatomical course
The nerve fibers of the hypoglossal nerve arise between the pyramid and olive on the basal surface of the medulla oblongata. The hypoglossal nerve is the only cranial nerve that exits the medulla oblongata ventral to the olive. The fibers usually pierce the dura in two bundles and leave the skull through the hypoglossal canal in the lateral margin of the foramen magnum. In its further course, the hypoglossal nerve turns dorsally and laterally around the vagus nerve and internal carotid artery (ICA), runs downward along the ICA, and arches through the carotid triangle lateral to the branches of the external carotid artery. Cranial to the hyoid bone, it passes between the hyoglossus muscle and mylohyoid muscle to the tongue and gives off a branch to the geniohyoid muscle2.
Clinical testing
In unilateral paralysis of the hypoglossal nerve, the protruded tongue deviates toward the affected side due to the predominance of the force of the genioglossus muscle on the healthy side. After such paralysis has persisted for a longer period, atrophy of the affected half of the tongue develops.
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