Cranial Nerves (V, VII – XII)
Hypoglossal Nerve (A, B)
The twelfth cranial nerve is an exclusivelysomatomotor nerve for the tongue muscles.Its nucleus, the nucleus of the hypoglossalnerve (B1), forms a column of large multi-polar neurons in the floor of the rhomboid fossa (trigon of hypoglossal nerve). It consists of a number of cell groups, each of which in-nervates a particular muscle of the tongue. The nerve fibers emerge between pyramid and olive and form two bundles that then combine into a nerve trunk.
The nerve leaves the skull through the canalof the hypoglossal nerve (B2) and descendslaterally to the vagus nerve and the internal carotid artery. It forms a loop, the arch of thehypoglossal nerve (A3), and reaches the rootof the tongue slightly above the hyoid bone between the hypoglossal muscle and the my-lohyoid muscle, where it ramifies into termi-nal branches.
Fiber bundles of the first and second cervi-cal nerves adhere to the hypoglossal nerve. They form the deep cervical ansa (branches for the lower hyoid bone muscles) by branching off again as superior root (A4) and combining with the inferior root (A5) (sec-ond and third cervical nerve). The cervical fibers for the geniohyoid muscle (A6) and the thyrohyoid muscle (A7) continue to run inthe hypoglossal nerve. The hypoglossal nerve gives off the lingual branches to the hypoglossal muscle (A8), the genioglossal muscle (A9), the styloglossal muscle (A10),and to the intrinsic muscles of the body of the tongue (A11). Innervation of the tongue muscles is strictly ipsilateral.
Clinical Note: Injury to the hypoglossal nervecauses hemilateral shrinkage of the tongue (hemi-atrophy). When the tongue is stuck out, it turns tothe affected side because the genioglossal muscle, which moves the tongue to the front, dominates on the healthy side.