Hypoglossal nucleus

The hypoglossal nuclei, exist as paired nuclei within the medulla oblongata that provide motor innervation to the intrinsic and extrinsic muscles of the tongue (excluding palatoglossus) via the hypoglossal nerve.

Gross Anatomy

The hypoglossal nuclei are located within the tegmentum of upper medulla close to the midline, lying deep to the hypoglossal trigone, or hypoglossal triangle, which is the medial eminence of the floor of the fourth ventricle, inferior to the stria medullaris .

Each nucleus is approximately 2 cm in length, extending beyond the limits of the hypoglossal trigone into the region of the closed medulla. They lie ventral and medial to the dorsal vagal nucleus. Fibers pass anteriorly between the inferior olivary nucleus and pyramid and exit the medulla as the hypoglossal nerve between the pyramid and olive.

Innervation

The nuclei contain large motor neurons and myelinated fibers of the hypoglossal nerve, innervating the intrinsic and extrinsic tongue muscles, excluding palatoglossus  which is innervated by the vagus nerve.

The hypoglossal nucleus receives corticonuclear fibers, from the precentral gyrus and adjoining areas from both the ipsilateral and contralateral cerebral hemispheres, however primarily from the latter . These synaptic connections are made either directly, or indirectly via interneurons.

Functionally, the nuclei can be divided dorsally and ventrally, and into medial and lateral components to achieve a musculotopic organization of the motor neurons within. Hence the dorsolateral group innervates the muscles responsible for tongue retraction (hyoglossus, styloglossus), and the ventromedial group represents the neurons responsible for tongue protrusion (genioglossus) . The innervation to the intrinsic tongue muscles is less clear.