Hypoglossal nerve

Motor function

Motor innervation of all intrinsic and extrinsic muscles of the tongue except the palatoglossus muscle (vagus).

Anatomy

1. Intra-axial segment

The hypoglossal nucleus is located in the dorsal medulla between the dorsal nucleus of vagus and the midline. Efferent fibers from the hypoglossal nucleus emerge between the olivary nucleus and the pyramid, in the preolivary sulcus.

2. cisternal segment

Multiple exiting rootlets fuse to form the hypoglossal. Some hypoglossal fibers may merge with vagal fibers.

3. Skull base segment

The hypoglossal nerve exits the skull through the hypoglossal canal in the inferior occipital bone, caudal to the jugular foramen.

4. Extracranial segment

A small branch from the C1 root joins the hypoglossal nerve after it has emerged from the hypoglossal canal.

Small meningeal branches supply some of the dura mater of the posterior cranial fossa. These are C1 fibers which travel retrogradely along the hypoglossal nerve and not part of the nerve per se.

Distal to the hypoglossal canal the nerve enters the carotid space medially. It runs in the posterior part of the carotid space close to the vagus nerve and exits the carotid space between the jugular vein and the internal carotid artery at the inferior margin of the posterior belly of digastric muscle.

Branches (also from C1 fibers) are given off to ansa cervicalis (innervates sternothyroid, sternohyoid and omohyoid muscles), thyrohyoid and geniohyoid muscles.

Distal branches innervate the extrinsic (styloglossus, hyoglossus and genioglossus) and the intrinsic muscles of the tongue.

Clinical implications

• Hypoglossal injury manifests as tongue deviation towards the side of the lesion. In chronic injury the ipsilateral tongue becomes atrophic. Infrahyoid strap muscles (sternohyoid, sternothyroid, thyrohyoid and omohyoid) also atrophy.
• Isolated hypoglossal palsy is very rare. Limited data suggest the most common cause might be infection.