Hypoglossal Nerve (CN XII): Course, Tongue Muscles and Palsy

Nuclei and fibre types

CN XII carries one fibre type: general somatic efferent (GSE) fibres, motor to skeletal muscle. The tongue muscles develop from occipital somites, not pharyngeal arches, which is why the fibre type is somatic.

The hypoglossal nucleus is a long column close to the midline in the medulla. Its upper part lies beneath the hypoglossal trigone in the floor of the fourth ventricle. The nucleus receives corticobulbar input from both hemispheres, but the part controlling genioglossus is driven mainly by the opposite hemisphere.

Emergence and skull foramen

CN XII emerges from the medulla as a row of rootlets in the preolivary sulcus (anterolateral sulcus), the groove between the pyramid and the olive. This line is continuous with the ventral roots of the spinal cord. The rootlets join and leave the skull through the hypoglossal canal in the occipital bone, just above the occipital condyle.

Course and branches

After leaving the skull, CN XII descends in the neck and then curves forward to run into the floor of the mouth.

1. Below the skull: it lies deep, medial to CN IX, X and XI, then spirals behind the vagus to lie between the internal carotid artery and internal jugular vein.
2. Upper neck: it descends to about the level of the angle of the mandible and hooks forward around the occipital artery, usually at its sternocleidomastoid branch.
3. Carotid triangle: it crosses superficially over the internal and external carotid arteries and the loop of the lingual artery, deep to the posterior belly of digastric and stylohyoid.
4. Submandibular region: it runs forward on the lateral surface of hyoglossus, deep to mylohyoid, accompanied by small veins. On hyoglossus the lingual nerve lies above it and the submandibular duct between the two; the lingual artery runs deep to hyoglossus.
5. Tongue: it ends in branches to genioglossus and the intrinsic muscles.

Branches carrying C1 fibres

Fibres from the ventral ramus of C1 join the nerve soon after it leaves the skull and travel with it. They leave again as the meningeal branch, the superior root of the ansa cervicalis (to the infrahyoid muscles), the nerve to thyrohyoid and the nerve to geniohyoid. None of these is hypoglossal in origin.

Clinical testing

CN XII is tested by inspecting the tongue at rest and on protrusion, then testing its strength.

• At rest: look at the tongue lying in the floor of the mouth for wasting and fasciculation (fine, irregular twitching). Fasciculation is best judged with the tongue at rest, because a protruded tongue often twitches normally.
• Protrusion: ask the patient to stick the tongue straight out and look for deviation.
• Power: ask the patient to push the tongue into each cheek against the examiner's finger.
• Speech: listen for difficulty with lingual sounds such as 'la la la'.

Lesions and palsies

A lower motor neurone lesion of CN XII makes the tongue deviate toward the side of the lesion on protrusion, with wasting and fasciculation on that side. Each genioglossus pushes the tongue forward and toward the opposite side, so the healthy muscle pushes the tongue across toward the weak side.

An upper motor neurone lesion, such as a stroke, causes the tongue to deviate away from the side of the brain lesion, toward the side of the limb weakness, without wasting. Bilateral upper motor neurone lesions produce a small, stiff, slow tongue (pseudobulbar palsy), while bilateral lower motor neurone disease produces a wasted, fasciculating tongue (bulbar palsy), as in motor neurone disease.