Hypoglossal Nerve

Overview

Has only a somatic motor (general somatic efferent) component.

Somatic motor Innervates all the intrinsic and most of the extrinsic muscles of the tongue.

CN XII supplies three of the four extrinsic muscles of the tongue including genioglossus, styloglossus, and hyoglossus.

The palatoglossus muscle is supplied by CN X (vagus nerve).

Origin and Central Course

The fibers of the hypoglossal nerve originate from the hypoglossal nucleus located in the tegmentum of the medulla.

This long thin nucleus, which extends roughly the same length as the olive, gives rise to the hypoglossal trigone in the floor of the fourth ventricle.

Fibers leaving the hypoglossal nucleus travel ventrally just lateral to the medial lemniscus to emerge from the brainstem in the groove between the pyramid and the olive - the ventrolateral sulcus.

Intracranial Course

CN XII emerges from the brainstem as a series of rootlets which converge to form the hypoglossal nerve.

The nerve exits the cranium via the hypoglossal foramen in the posterior cranial fossa.

Extracranial Course and Final Innervation

Upon exiting the skull, the hypoglossal nerve lies medial to CNs IX, X, and XI which exited the skull via the jugular foramen.

CN XII moves laterally and downward to lie between the internal carotid artery and the internal jugular vein. All of these structures are deep to the posterior belly of the digastric muscle.

The nerve then loops anteriorly (passing lateral to the bifurcation of the common carotid artery) to run along the lateral surface of the hyoglossus muscle, deep to the Mylohyoid muscle.

The fibers of CN XII then divide to supply all the internal and most of the external muscles of the tongue (three out of four).

Voluntary Control of Muscles of the Tongue

Signals for the voluntary control of the muscles of the tongue originate in the motor cortex (in association with other cortical areas) and pass via the corticobulbar tract in the posterior limb of the internal capsule to the hypoglossal nucleus in the medulla. 

These upper motor neuron fibers innervate predominantly the contralateral hypoglossal nucleus.