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Hypoglossal nerve (CN XII)
The hypoglossal nerve (Latin: nervus hypoglossus), the last and twelfth cranial nerve (CN XII), is a motor nerve that supplies general somatic efferent fibers to all intrinsic skeletal muscles of the tongue, three of the four extrinsic tongue muscles (except for the palatoglossus), and the geniohyoid muscle, providing tongue movements, speech, and such function as swallowing.
Hypoglossal nerve course and branches
The fibers composing the hypoglossal nerve arise from the hypoglossal nucleus located on the ventral aspect of the medulla oblongata of the brainstem. The twelfth cranial nerve appears on the ventral surface of the brainstem between the pyramid and the olivary eminence (olive) in the anterolateral sulcus.
The hypoglossal nerve passes through the hypoglossal canal and exits it through the hypoglossal foramen, emerging outside of the skull base and reaching the lateral pharyngeal space. Further, it descends along the medial side of the internal jugular vein and internal carotid artery. Later, after crossing the artery, it goes on the lateral side of it.
As the hypoglossal nerve descends, it passes caudally posterior to the vagus nerve (CN X) within a 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 makes a turn forward and medially. It runs forward horizontally through the carotid triangle and then via the submandibular triangle of the neck.
At the upper edge of the carotid triangle, it traverses the external carotid artery. The hypoglossal nerve crosses the posterior belly of the digastric muscle and medial aspect of the stylohyoid muscle and reaches the tongue between the hyoglossus and mylohyoid muscles. At the anterior aspect of the hypoglossal muscle, it goes into the tongue and branches into its terminal lingual branches, which innervate the muscles of the tongue.
After exiting the hypoglossal canal, the hypoglossal nerve is joined by the motor (general somatic efferent) fibers of the anterior root of the 1st and 2nd cervical spinal nerves (C1, C2). These fibers are distributed before the hypoglossal nerve reaches the tongue. Part of the mentioned fibers form the superior root of the cervical plexus and merge with the inferior root of the plexus formed by the anterior root of the 3rd and 4th cervical spinal nerves (C3, C4).
On its course, the hypoglossal nerve also gives a meningeal branch to the dura mater of the posterior cranial fossa. This branch re-enters the cranial cavity via the hypoglossal canal. 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: genioglossus, styloglossus and hyoglossus muscles, as well as the geniohyoid muscle, which belongs to the suprahyoid muscles of the neck.
Note: The exception - the fourth extrinsic muscle of the tongue - is the palatoglossus muscle, which receives innervation from the vagus nerve (CN X).