Hypoglossal Nerve

Hypoglossal nerve is the 12th cranial nerve, and is purely motor in nature. It consist of general somatic efferent fibres.

General Somatic Efferent Fibres

• Arises from the hypoglossal nucleus.
• Supplies all the intrinsic and extrinsic muscles of the tongue that develop from occipital myotomes.

Anatomy & Pathway

Origin: From the anterolateral sulcus between the pyramid and olive on the ventral aspect of medulla.

Intracranial Course

• It consists of 10-15 rootlets that are attached in line with the rootlets of the ventral root of the 1st cervical spinal nerve.
• The rootlets run laterally and pass behind the vertebral artery to form two roots, which pierce the dura mater separately separately near the hypoglossal canal.
• In the hypoglossal canal, the two roots unite to form a single trunk and exit the cranial cavity.

Extracranial Course

• The nerve lies deep to the internal carotid artery and the glossopharyngeal and vagus nerves.
• It then reaches the interval between the internal jugular vein and the internal carotid artery, and, descends vertically in front of the vagus nerve upto the level of the angle of mandible.
• At the angle of mandible, the nerve curves forward to reach the posterior margin of the hyoglossus muscle by passing deep to the tendon of the posterior belly of digastric.
• At the anterior margin of hyoglossus it lies on the genioglossus and runs forward and upward upto the tip of tongue and ends by dividing into its terminal branches which supply the muscles of the tongue.

Branches & Distribution

Branches of hypoglossal proper

• Supplies all the muscles of tongue, except, palatoglossus (supplied by the cranial root of accessory nerve).

Branches of hypoglossal nerve containing C1 fibres

• Meningeal branch: Arises from the nerve as it comes out through the hypoglossal canal, takes a recurrent course and enters the cranial cavity through the hypoglossal canal. It supplies the dura mater of the posterior cranial fossa.
• Descendens hypoglossi or Upper root of ansa cervicalis: Arises as the nerve crosses in front of the internal carotid artery, runs downward, and, joins the inferior root of ansa cervicalis at the level of cricoid cartilage.
• Nerve to thyrohyoid: Crosses the greater cornu of the hyoid bone to reach the muscle.
• Nerve to geniohyoid: Arises from above the hyoid bone.

Clinical Significance

Supranuclear lesions

• Involves corticonuclear fibres (upper motor neuron type of paralysis).
• There will be muscle paralysis and fasciculations in tongue on the affected side.
• Mucous membrane will show wrinkling due to wasting of muscles and their fasciculations.

Lower motor neuron type paralysis

• Seen if the hypoglossal nerve is cut on one side, paralysis of tongue muscles on that side.
• When the patient is asked to protrude tongue, the tip of the tongue deviates to the paralysed side, due to the unopposed action of the muscles of healthy side.

_{Image Credit: Mukherjee SK, Gowshami CB, Salam A, Kuddus R, Farazi MA, Baksh J, https://www.ncbi.nlm.nih.gov/pubmed/?term=10.1186%2F1749-7221-7-2}

Clinical Testing

• Assess the functions of genioglossus: Ask the patient to protrude tongue.
• Normal: Protruded tongue lies in the midline.
• Unilateral damage: Tongue deviates to the side of the lesion/paralysis.
• Bilateral damage: Patient cannot protrude tongue, it lies motionless, causing difficulty in speech and swallowing.

References

• The image used in the cover photo is in public domain (Source: Wikimedia Commons).

*This article is an excerpt from the above mentioned sources and Medical Sutras does not make any ownership or affiliation claims.