Hypoglossal nerve (CN XII)

The hypoglossal nerve (twelfth cranial nerve, CN XII, Latin: nervus hypoglossus) is a motor nerve that supplies general somatic efferent fibers to all intrinsic skeletal muscles of the tongue and to three of the four extrinsic tongue muscles.

The general somatic efferent neurons of the hypoglossal nerve originate in the hypoglossal nucleus, which is a cluster of motor neurons located in the medulla oblongata, in the hypoglossal trigone (a raised area in the rhomboid fossa).

The axons of these motor neurons traverse the reticular formation, pass medial to the inferior olivary nuclei, and loop laterally. Ten to fifteen bundles of fibers exit the brainstem and collect to create the hypoglossal nerve.

The hypoglossal nerve appears on the ventral surface of the brainstem between the pyramid and the olivary eminence in the anterolateral sulcus. The hypoglossal nerve passes through the hypoglossal canal and exits the canal through the hypoglossal foramen, and descends medially to the internal jugular vein and internal carotid artery.

After exiting the hypoglossal canal, the hypoglossal nerve is joined by the superior root of the cervical plexus, which is formed by the anterior root of 1st cervical spinal nerve (C1). These fibers are distributed before the hypoglossal nerve reaches the tongue.

As the hypoglossal nerve descends it passes caudally posterior to the vagus nerve in the neurovascular bundle behind the pharynx. At the level of the angle of the mandible the nerve emerges from behind the posterior belly of the digastric muscle. Then in an arch-shaped bend of 90 degrees, the nerve turns rostrally and medially. The hypoglossal nerve runs forward at the upper edge of the carotid triangle, traverses the external carotid artery at the level of the division of the lingual artery and reaches the tongue between the hyoglossus and mylohyoid muscles.

In the tongue, the hypoglossal nerve distributes into the lingual branches, which innervate all the intrinsic tongue muscles and three of the four extrinsic tongue muscles: the genioglossus, styloglossus, and hyoglossus muscles.