Hypoglossal nerve

Number: XII

Name: Hypoglossal

Sensory, motor, or both: Mainly motor

Origin/Target: Medulla

Nuclei: Hypoglossal nucleus

Function: Provides motor innervation to the muscles of the tongue (except for the palatoglossus, which is innervated by the vagus nerve). Important for swallowing (bolus formation) and speech articulation.

Description:

The hypoglossal nerve, also known as cranial nerve XII (CN XII), is the twelfth and final cranial nerve, dedicated entirely to motor function. It plays a crucial role in controlling the muscles of the tongue. It has no sensory or autonomic components.

Hypoglossal Nucleus:

The hypoglossal nucleus–the origin of the nerve–is found in the medulla oblongata within the brainstem. It is located near the midline beneath the floor of the fourth ventricle.

The hypoglossal nucleus receives corticonuclear fibers (upper motor neurons) from both cerebral hemispheres. However, the portion of the nucleus responsible for the genioglossus muscle (the primary muscle responsible for tongue protrusion) receives contralateral corticonuclear fibers from only one hemisphere. This has clinical significance in upper motor neuron lesions, as explained later.

Emergence from the Brainstem:

The hypoglossal nerve fibers exit the hypoglossal nucleus and traverse through the medulla oblongata. The nerve emerges from the anterior or ventral surface of the medulla in the preolivary sulcus, a groove located between the pyramid and the olive.

Course and Pathway:

• Intracranial Course: After leaving the anterior surface of the medulla, the hypoglossal nerve crosses the posterior cranial fossa and exits the skull through the hypoglossal canal, a bony opening at the base of the occipital bone.

• Extracranial Course: After exiting the hypoglossal canal, the nerve descends forward and laterally in the neck. It travels downward until it reaches the lower border of the posterior belly of the digastric muscle. At this point, it turns forward, crossing several structures: the internal carotid artery, the internal jugular vein, and the lingual artery loop. It then courses deep to the mylohyoid muscle and lies on the lateral surface of the hyoglossus muscle. Along its course, the hypoglossal nerve is joined by fibers from the C1 ventral ramus of the cervical plexus, which temporarily hitch a ride on the nerve. These fibers provide motor innervation to the geniohyoid and thyrohyoid muscles, but these muscles are not controlled by the hypoglossal nerve itself.

• Terminal (lingual) Branches: The hypoglossal nerve gives terminal lingual branches to all the intrinsic muscles of the tongue and all but one of the extrinsic muscles (detailed below).

Innervation:

Intrinsic Tongue Muscles: The hypoglossal nerve innervates all intrinsic muscles of the tongue, which include:

• Superior longitudinal muscle

• Inferior longitudinal muscle

• Transverse muscle

• Vertical muscle

These muscles control the shape and fine movements of the tongue.

Extrinsic Tongue Muscles: The hypoglossal nerve also innervates most extrinsic muscles of the tongue, which are involved in movement of the tongue:

• Genioglossus: Protrudes the tongue.

• Hyoglossus: Depresses the tongue.

• Styloglossus: Retracts and elevates the tongue.

• However, the palatoglossus muscle (also an extrinsic tongue muscle) is innervated by the vagus nerve (CN X).

Actions of the Hypoglossal Nerve:

The hypoglossal nerve allows for precise and coordinated tongue movements, which are essential for:

• Speech: Proper articulation of sounds.

• Swallowing (Deglutition): Moving food from the oral cavity to the pharynx.

• Mastication (Chewing): Positioning food effectively for chewing.

• Tongue Mobility: Enables protrusion, retraction, depression, elevation, and shaping of the tongue.

Clinical Correlations:

Dysfunction of the hypoglossal nerve can result from lesions anywhere along its pathway, caused by conditions such as tumors, trauma, demyelinating diseases (e.g., multiple sclerosis), or vascular damage (e.g., stroke).

Lesions can be classified as:

Lower Motor Neuron (LMN) Lesion:

• This results in weakness or paralysis of the tongue muscles on the ipsilateral side (same side as the lesion).

• Deviation of the tongue: On protrusion, the tongue deviates toward the side of the lesion because the intact genioglossus muscle on the opposite side overpowers the weak side.

• Atrophy of tongue muscles: The affected side of the tongue may appear shrunken or wasted.

• Fasciculations: Involuntary twitching may occur in the tongue muscles.

Upper Motor Neuron (UMN) Lesion:

• Lesions affecting the contralateral corticonuclear fibers (e.g., in a stroke) result in only the genioglossus muscle being affected.

• The tongue deviates away from the side of the lesion due to unopposed action of the ipsilateral genioglossus.

• No atrophy or fasciculations are observed, as the problem is central and not within the nerve itself.