Corpus: Hypoglossal nerve

The hypoglossal nerve is the 12th cranial nerve. It is a somatomotor nerve and innervates the muscles of the tongue and floor of the mouth.

The hypoglossal nerve emerges laterally from the medulla oblongata, appearing on the brain surface with about ten to twelve fine root fibers in the anterolateral sulcus. It then travels in up to three trunks to the hypoglossal canal of the occipital bone and exits the cranial cavity through this canal, which is very close to the foramen magnum. At the level of the hypoglossal canal, the nerve gives off a meningeal branch.

Extracranially, the nerve initially runs between the internal jugular vein, the internal carotid artery, and the external carotid artery into the carotid triangle. Fibers from the anterior branches of the upper cervical nerves (C1 and C2), forming the superior root of the deep ansa cervicalis, attach to the nerve along a short section of its course. Some fibers continue to accompany the nerve and innervate parts of the hyoid muscles.

The remaining fibers bend in the cranial part of the carotid triangle and run under the stylohyoid muscle and the posterior belly of the digastric muscle to the submandibular triangle. From there, they enter the tongue from the floor of the mouth between the mylohyoid and hyoglossal muscles.

The paired hypoglossal nerve supplies the inner and outer muscles of the tongue, except for the palatoglossus muscle, which is innervated by the pharyngeal plexus. It also innervates the lower hyoid muscles via the ansa cervicalis (fibers of the cervical plexus that run with the hypoglossal nerve).

The nucleus of the hypoglossal nerve (nucleus nervi hypoglossi) is located in the caudal part of the medulla oblongata on both sides of the paramedian line, at the base of the rhomboid fossa in the hypoglossal triangle. It is approximately at the same level as the nuclei of the 10th and 11th cranial nerves.

The hypoglossal nerve develops from the first occipital somite. It becomes visible in the fourth week of development as a collection of small nerve fibers, which form a coherent nerve by the end of the fifth week. The hypoglossal nerve nucleus is derived from the basal plate of the embryonic medulla oblongata.

Damage to the hypoglossal nerve, such as from trauma or a tumor, leads to hypoglossal nerve palsy.

Unilateral damage results in paralysis of one side of the tongue. In the mouth, the tongue deviates to the affected side. When extended, it shifts to the damaged side due to the muscle tone of the opposite side. Although this causes difficulties with eating and drinking, unilateral paralysis is usually perceived as less disabling.

Note: Lateral deviation of the tongue is not necessarily a sign of paralysis if there is no accompanying muscle atrophy. It can also occur in healthy individuals as a harmless asymmetry.

Bilateral Damage: Leads to complete paralysis of the tongue, causing atrophy of the tongue muscles over time. With bilateral paralysis, the tongue rests motionless on the floor of the mouth, resulting in severe speech disorders and impaired food intake.

Both hypoglossal nerve nuclei are primarily controlled by the contralateral hemisphere, but also receive some ipsilateral innervation. Therefore, if one hemisphere is damaged (e.g., due to a stroke), only partial dysfunction occurs.