Hypoglossal Nerve (CN XII) BY : DANI MAMO

Hypoglossal Nerve The nucleus of the hypoglossal nerve is located in the lower third of the medulla, abutting the midline and just below the floor of the fourth ventricle (in the so-called hypoglossal triangle or trigone).

It consists of a number of cell groups supplying the individual muscles of the tongue. The cells are analogous to the motor anterior horn cells of the spinal cord.

Supranuclear innervation of the nucleus of the hypoglossal nerve Voluntary movements of the tongue are subserved by the corticonuclear tract, which descends through the internal capsule in association with the corticospinal tract and terminates in the nucleus of the hypoglossal nerve.

The nucleus of the hypoglossal nerve derives its afferent input mainly from the contralateral cerebral hemisphere, though there is some ipsilateral input as well. It derives further input from the reticular formation, the nucleus of the tractus solitarius (taste), the midbrain (tectospinal tract), and the trigeminal nuclei. These connections participate in reflexes concerned with swallowing chewing, sucking, and licking.

Course and distribution of the hypoglossal nerve The hypoglossal nerve is a somatic efferent (motor) nerve. Its axons descend in the medulla and emerge from the brainstem as root fibers in the anterolateral sulcus between the inferior olive and the pyramid

The hypoglossal nerve exits the skull through the hypoglossal canal and runs in the lower cervical region between the jugular vein and carotid artery together with the fibers of the first three cervical segments. These fibers, which make no connection with the hypoglossal nerve, separate from it again a short distance later to supply the muscles of the hyoid bone, i.e., the thyrohyoid, sternohyoid, and omohyoid muscles.

The hypoglossal nerve proper innervates the muscles of the tongue 1- the styloglossus muscle 2-the hyoglossus muscle 3- the genioglossus muscle

Hypoglossal nerve palsy In unilateral hypoglossal nerve palsy, the tongue usually deviates a little toward the paretic side when it is protruded. The genioglossus muscle is responsible for protrusion. If the genioglossus muscle of one side is weak, the force of the opposite muscle prevails and pushes the tongue to the side of the lesion. In hemiplegia, the patient’s speech is dysarthric at first, but swallowing is not impaired. Bilateral supranuclear palsy produces severe dysarthria and dysphagia (pseudobulbar palsy).

Nuclear lesions affecting the hypoglossal nerve are usually manifested by bilateral flaccid paralysis of the tongue with atrophy and fasciculations, because the nuclei of the two sides lie so close to each other that they are usually affected together. In advanced cases, the tongue lies limply in the floor of the mouth and fasciculates intensely. Speech and swallowing are profoundly impaired. Causes include progressive bulbar palsy, amyotrophic lateral sclerosis, syringobulbia, poliomyelitis, and vascular processes.

Peripheral lesions of the hypoglossal nerve have the same consequences as nuclear lesions, but the paralysis is usually only unilateral. Causes include tumors, infection/inflammation, and vascular disease.