The hypoglossal nerve is one of the 12 cranial nerves arising from the brain. It's a paired nerve, and you have two — left and right. Together, the two hypoglossal nerves control the muscles of the tongue. Your tongue moves food around in your mouth and helps in chewing it. It also enables you to speak and swallow. Disorders of the hypoglossal nerve make it difficult to speak, swallow, and chew food.
What Is the Hypoglossal Nerve?
The hypoglossal nerve is also known as the 12th cranial nerve. It is chiefly a motor nerve and carries motor fibers that control muscles. It does have some sympathetic fibers that supply the blood vessels of the tongue and some glands in the mouth.
The hypoglossal nerve originates in the hypoglossal nucleus in the lower part of the medulla. It has four branches:
- Meningeal branch
- Descending branch
- Thyrothyroid branch
- Muscular/lingual branch
The hypoglossal nerve controls most tongue actions necessary for eating and speaking.
What Does the Hypoglossal Nerve Do?
The hypoglossal nerve function is to control the muscles of the tongue. The tongue has four extrinsic muscles that arise outside the tongue and are attached to it. The hypoglossal nerve controls three of these. The tongue also has intrinsic muscles:
Genioglossus. This muscle arises from the lower jaw (mandible) and attaches to the entire length of the tongue. It pushes your tongue forward and lowers its central part.
Hyoglossus. It arises from the hyoid bone and attaches to the side of your tongue. It's the muscle you use for pulling your tongue back and down. It also flattens your tongue.
Styloglossus. This muscle arises from the temporal bone and attaches to the side of your tongue. This muscle moves the tongue up and down and pulls it back into the mouth.
Intrinsic muscles. These muscles are entirely inside your tongue. They perform actions such as shortening the tongue, curling its apex, turning it downward, narrowing and elongating it, and flattening and widening it.
The only tongue muscle not innervated by the hypoglossal nerve is the palatoglossus. This muscle is controlled by the vagus nerve (cranial nerve 10).
The hypoglossal nerve also innervates the thyrohyoid, omohyoid, sternohyoid, and sternothyroid muscles, which control the mandible (lower jaw) and the hyoid bones while speaking and swallowing. But this is not truly a function of the hypoglossal nerve — the nerve fibers come from the first, second, and third cervical spinal nerves and travel with the hypoglossal nerve.
Where Is the Hypoglossal Nerve Located?
Hypoglossal nerve anatomy begins at the hypoglossal nucleus in the lowest part of the brain, the medulla. It emerges as several small rootlets that join to form the hypoglossal nerve. The nerve exits the skull through an opening called the hypoglossal canal.
Once outside the skull, the hypoglossal nerve travels downward between the internal carotid artery and the internal jugular vein. It passes around the hyoid bone and runs along the hyoglossus muscle in the floor of the mouth. Reaching the tongue, it divides into branches that supply the intrinsic muscles and three extrinsic tongue muscles.
Signs Something Could Be Wrong With Your Hypoglossal Nerve
If one of your hypoglossal nerves isn't working well, you may notice that your tongue deviates toward one side when pushed out. Your speech may be disturbed, as you need precise tongue movements to form syllables. Long-term nerve dysfunction can lead to disuse atrophy of muscles on one side of the tongue — you'll notice one side of your tongue is wasted and smaller than the other.
Damage to one hypoglossal nerve won't usually cause you serious problems. The other hypoglossal nerve can usually compensate. But if both the nerves aren't working, you will have difficulties with speaking and swallowing.
Fasciculations are flickering movements on the surface of the tongue that are seen in some disorders of the hypoglossal nerve. Difficulty speaking or swallowing, weakness of the tongue, and fasciculations could be signs of a serious disorder like nervous system disease or cancer. You should consult your doctor if you notice any of these symptoms.
Conditions Affecting the Hypoglossal Nerve
The hypoglossal nerve can be damaged by a penetrating injury to the neck below the skull. It is also at risk of injury during surgical procedures like carotid endarterectomy. Such injuries usually affect the nerve on one side. Both hypoglossal nerves are sometimes injured by radiotherapy.
Diseases that affect this nerve include:
Amyotrophic lateral sclerosis (ALS). This is usually seen in people over 60 but can happen to younger people. The motor neuron cells in the brain stop working and cause weakness that worsens with time. The motor neurons of the hypoglossal nerve may be involved in this disease process. You'll notice your speech is slurred, and you have difficulty swallowing.
Progressive bulbar palsy. This condition affects the motor neurons in the brain and leads to weakness of the muscles of the head and neck. This condition involves the nucleus of the hypoglossal nerve too. Progressive bulbar palsy manifests as wasting of the tongue muscles, weakness of throat muscles and difficulty swallowing, inability to speak, and weak jaw and face muscles. This condition sometimes progresses to amyotrophic lateral sclerosis.
Encephalitis. This is an inflammation of the brain. The swelling can create pressure on the hypoglossal nerve, causing dysfunction.
Head and neck cancer. Both the tumors and their treatment can harm tissues around them. The hypoglossal nerve can be damaged. The hypoglossal nerve itself can also have tumors like schwannoma. Magnetic resonance imaging (MRI) scans are needed to diagnose such conditions.
Sleep apnea. This is a condition in which the muscles at the back of your throat lose tone while you sleep. The tongue falls back, partially or completely blocking your airway. Sleep apnea is associated with hypertension, heart disease, stroke, and a higher risk of death. Hypoglossal nerve stimulation therapy stimulates the nerve in step with breathing to stiffen the tongue, opening the airway.
Stroke. Parts of the brain are damaged, and the hypoglossal nerve may be involved.
Keeping Your Hypoglossal Nerve Healthy
There aren't any specific measures for hypoglossal nerve health, but you can keep your nervous system healthy by:
- Having an active lifestyle and a healthy diet
- Maintaining a healthy weight, which protects you against sleep apnea
- Taking a regular treatment if you have high blood pressure (hypertension), diabetes, or a chronic condition like ALS