Hypoglossal nerve (CN XII)

Nervus hypoglossus

Also known as: CN XII, twelfth cranial nerve

A purely motor nerve (GSE) to all the intrinsic muscles of the tongue and to all its extrinsic muscles except palatoglossus. Through it, the tongue moves for speech, chewing and swallowing.

Overview

NucleusHypoglossal nucleus, dorsal medulla

ExitHypoglossal canal (occipital bone)

ExceptionPalatoglossus is supplied by CN X

The hypoglossal nucleus is a column in the dorsal medulla, close to the midline, beneath the hypoglossal trigone in the floor of the fourth ventricle. Its axons pass ventrally and emerge as rootlets in the preolivary (anterolateral) sulcus, between the pyramid and the olive. They unite and leave the skull through the hypoglossal canal in the occipital condyle.

In the neck the nerve descends between the internal carotid artery and the internal jugular vein, deep to the posterior belly of digastric. It then curves forward around the occipital artery and crosses superficial to the internal and external carotid arteries and the loop of the lingual artery. It runs above the greater horn of the hyoid on the lateral surface of hyoglossus, deep to mylohyoid, and fans out into the tongue. Fibres from the C1 ventral ramus join it and leave again as:
• the superior root of the ansa cervicalis;
• the nerves to thyrohyoid and geniohyoid.

These are not hypoglossal fibres.

Location

Medulla (preolivary sulcus) → hypoglossal canal → between the ICA and IJV → around the occipital artery → submandibular region on hyoglossus → tongue.

Parts & subdivisions

Functional components

Motor only (GSE). The tongue muscles are derived from occipital somites, which is why the nerve is "somatic" despite the tongue's position in the pharyngeal region.

Clinical testing

Inspect the tongue lying in the floor of the mouth for wasting and fasciculations. Ask the patient to protrude the tongue and observe any deviation. Test strength by asking them to push the tongue into each cheek against your finger. Rapid side-to-side movements also test speed.

Lesion signs

In an LMN lesion (nerve or nucleus), the tongue deviates toward the side of the lesion on protrusion. This happens because the intact genioglossus on the other side pushes the tongue across. The same side shows ipsilateral atrophy and fasciculation, with dysarthria for lingual sounds. In an UMN lesion (e.g. a cortical stroke), the tongue deviates away from the side of the cerebral lesion, toward the weak face, and there is no wasting.

Anatomy

Innervation

All intrinsic tongue muscles (superior and inferior longitudinal, transverse, vertical) and the extrinsic muscles genioglossus, hyoglossus and styloglossus. Palatoglossus is supplied by CN X. It also carries C1 fibres to geniohyoid, thyrohyoid and the superior root of the ansa cervicalis.

Course

Hypoglossal nucleus → preolivary sulcus → hypoglossal canal → carotid sheath region (medial to the IJV) → loops anteriorly around the origin of the occipital artery (sternocleidomastoid branch) → crosses lateral to the ICA, ECA and lingual artery → above the hyoid on hyoglossus (below the submandibular duct and lingual nerve) → intrinsic and extrinsic tongue muscles.

Clinical significance

Tongue deviation

Mnemonic: "the tongue licks the lesion" (LMN). The commonest causes of a hypoglossal palsy are skull-base tumours, carotid artery dissection, iatrogenic injury during carotid endarterectomy or neck dissection, and motor neurone disease (bilateral wasting and fasciculation, part of bulbar palsy).

Medial medullary syndrome

Occlusion of the anterior spinal artery or vertebral artery branches damages the pyramid, the medial lemniscus and the hypoglossal fibres. This produces contralateral hemiparesis, contralateral loss of vibration and proprioception, and ipsilateral tongue deviation and wasting (Dejerine syndrome).