Hypoglossal nerve (XII)

At a glance

English

Hypoglossal nerve (XII)

Latin (TA2)

Nervus hypoglossus (XII)

System

Nerve

Region

Head

Side

Paired, left and right

About the hypoglossal nerve (xii)

Cranial nerve XII, the hypoglossal nerve, takes its name from Greek words meaning under the tongue, which describes where it runs. It is purely motor and controls every muscle inside the tongue and three of its four extrinsic muscles: genioglossus, hyoglossus and styloglossus. The fourth, palatoglossus, is supplied through the pharyngeal plexus by the vagus.

Its cell bodies form the hypoglossal nucleus, a long column near the midline of the medulla under the floor of the fourth ventricle. The fibres run forwards and leave the medulla as a row of rootlets in the anterolateral sulcus, the groove between the pyramid and the olive. They gather and leave the skull through the hypoglossal canal in the occipital bone, just above the occipital condyle. Outside the skull the nerve gives a small meningeal branch back to the posterior fossa and receives fibres from the first cervical nerve, which ride with it but leave again as the superior root of the ansa cervicalis and the nerves to thyrohyoid and geniohyoid. It spirals behind and then lateral to the vagus, descends between the internal jugular vein and internal carotid artery, and appears below the posterior belly of digastric near the angle of the jaw. There it hooks around the sternocleidomastoid branch of the occipital artery, crosses the external carotid and passes forwards on the outer surface of hyoglossus, below the lingual nerve and deep to stylohyoid and mylohyoid, before running into the substance of the tongue towards its tip.

Both motor cortices feed the nucleus, but the opposite side dominates. The muscles it supplies develop from the occipital somites, which is why the nerve takes such an indirect route to reach them.

Tongue examination tests the nerve. A lesion of the nerve or its nucleus wastes the tongue on that side, which may twitch at rest, and the protruded tongue points towards the weak side because the healthy genioglossus pushes it over. A lesion above the nucleus, such as a stroke, causes deviation away from the damaged hemisphere without wasting or twitching. Causes include tumours, neck surgery such as carotid endarterectomy, trauma, stroke of the medulla and, most often among diseases, motor neuron disease, in which progressive bulbar palsy affects speech and swallowing.

Course and supply

Arises from

The medulla's hypoglossal nucleus; rootlets emerge in the anterolateral sulcus between the pyramid and the olive and leave the skull through the hypoglossal canal.

Ends in

Ends by branching into the tongue's own muscles and those that move it from outside, as it runs forwards into the tongue towards its tip.

Supplies

Motor to all intrinsic tongue muscles and to genioglossus, hyoglossus and styloglossus; the C1 fibres it carries supply thyrohyoid and geniohyoid and form the superior root of the ansa cervicalis.