The Anatomy of the Hypoglossal Nerve

The hypoglossal nerve supplies all of the motor function to your tongue. It's the 12th cranial nerve. Damage to this nerve can affect speech, chewing, and swallowing. The prefix hypo is of Greek origin and means "under." Glossal, also from the Greek, mean "tongue."

Anatomy

As your nerves run through your body, they split off and send branches out everywhere. These branches carry sensations (sensory information) between your brain and the rest of your body and allow for the motion (motor function) of all your moving parts.

Most of the nerves in your body originate in the spinal cord. The 12 nerves in your head are different. They're called cranial nerves and they emerge, as symmetrical pairs, from the brain itself.

Structure

Ten of your cranial nerves, including the hypoglossal nerve, emerge from the brainstem, which sits at the back of your brain and connects your brain to the spinal cord. Specifically, the hypoglossal nerve emerges from the hypoglossal nucleus, which is about at the midline of the medulla oblongata.

A branch of the cervical plexus runs inside the sheath of the hypoglossal nerve, but the fibers of these two nerves don't join together in any way.

The hypoglossal nerve doesn't branch off until it reaches the muscles of the tongue, at which time it sends off numerous small branches to innervate those muscles.

Location

Beginning as about a dozen small "rootlets" that combine to form two branches, the hypoglossal nerve passes through an area of the brain called the subarachnoid space, then runs through an opening in the occipital bone, which forms the back and base of the skull.

The two portions of the nerve then join together and leave the skull through the hypoglossal canal. That's where it meets up with the cervical plexus.

The hypoglossal nerve then runs between the carotid artery and the jugular vein, down into the neck, where it crosses the sternocleidomastoid muscle and runs along the mylohyoid muscle. It eventually comes up to the floor of the mouth and connects with the muscles of your tongue.

Anatomical Variations

In some people, the two branches of the hypoglossal nerve don't join together until after exiting the hypoglossal canal, which, in those people, includes two openings instead of one.

Function

The hypoglossal nerve is purely a motor nerve; it doesn't send any sensory information to and from the brain.

This nerve and the muscles it serves are at least responsible for several important functions, including:

• Talking and singing
• Chewing
• Swallowing

In addition, the hypoglossal nerve supplies movements that help you clear your mouth of saliva, aid unconscious movements involved in speech, and are involved in several automatic and reflexive motions.

The hypoglossal nerve controls two sets of muscles. One set is extrinsic (on the exterior of the tongue) while the other set is intrinsic (fully contained within the tongue).

The extrinsic muscles include:

• Genioglossus: Makes up the bulk of the tongue and allows you to stick your tongue out and move it side to side
• Hyoglossus: Comes up from the neck, depresses and retracts the tongue, and is important for singing
• Styloglossus: Above and on both sides of the tongue, allows you to retract and lift your tongue

The intrinsic muscles include:

• Superior longitudinal: A thin muscle right underneath the mucous membranes in the back of the tongue; works with the inferior longitudinal to retract the tongue and make it short and thick
• Inferior longitudinal: A narrow band under the surface of the tongue between the genioglossus and the hyoglossus muscles; along with the superior longitudinal, allows the tongue to be retracted
• Transverse: Along the sides; allows you to narrow and elongate your tongue
• Vertical: At the borders of the forepart of the tongue; allows you to flatten and broaden your tongue

The palatoglossus, which raises the back part of your tongue, is the only muscle of the tongue not innervated by the hypoglossal nerve. Instead, it's controlled by the Pharyngeal plexus of the vagus nerve, which is the 10th cranial nerve.

The hypoglossal nerve communicates with several other nerves as well, including:

• Vagus nerve
• Sympathetic trunk
• Cervical plexus
• Lingual branch of the trigeminal nerve

Associated Conditions

The hypoglossal nerve and its associated functions can be compromised by disease or injury. Medical conditions that can impair hypoglossal nerve function include:

• Amyotrophic lateral sclerosis (ALS)
• Multiple sclerosis
• Guillain-Barre syndrome
• Sarcoidosis
• Progressive bulbar palsy
• Stroke
• Tumor at the base of the skull
• Bone abnormalities at the base of the skull
• Aneurysm (bulge) at the base of the brain
• Infection of the medulla oblongata
• Supranuclear lesions
• Basilar meningitis
• Unilateral 12th nerve palsy
• Motor neuron disease
• Kennedy's disease
• Complications of surgery to remove a blockage from an artery in the neck

Injuries that may result in damage to the hypoglossal nerve are rare. Nerve damage can result from injury to the back of the head or neck. Because of its proximity to other cranial nerves, damage to the hypoglossal nerve alone is rare.

Depending on where the injury is, it may affect one or both sides of the nerve pair.

Symptoms of impaired function of the hypoglossal nerve include:

• Weakness of the tongue on the affected side
• Atrophy of the tongue on the affected side
• Impaired speech
• Difficulty chewing and swallowing

Rehabilitation

The treatment of hypoglossal nerve dysfunction depends on the cause. To make a diagnosis, a doctor will consider all of your symptoms and perform a physical exam.

Tests may include magnetic resonance imaging (MRI) to check for evidence of a stroke or tumor(s), a spinal tap (lumbar puncture) to look for cancer or infection, or specific tests for diseases the symptoms may indicate. Once a diagnosis is made, treatment can begin.

A lot of research is focusing on direct nerve repair as well as nerve transfers to correct problems such as facial paralysis, and surgical techniques are improving regarding the hypoglossal nerves as well as several other nerves of the face.

Other Clinical Significance

Hypoglossal nerve stimulation has been found to be a safe and effective treatment for obstructive sleep apnea in cases where people haven't been successful with other treatments. It's also being researched for children who have both Down syndrome and obstructive sleep apnea.