Hypoglossal Nerve

The nervous system is an interconnected network for carrying various signals to different parts of our body. The cranial nerves and spinal nerves are two set of nerves that communicate the signals. Spinal nerves are nerves emerging from the spinal cord, and there are 31 pairs of them, while the cranial nerves emerge from the brain and connect various parts of the body, like the eyes, ears, throat, head, neck and trunk. There are 12 pairs of cranial nerves, and the hypoglossal nerve is the twelfth nerve that connects from the brain to below the tongue.

The axons of the nerve cells form root like structures which emerge from its nucleus located in the medulla oblongata. The nerve passes through the internal carotid artery (major arteries of the head that supply blood to the brain), hyoid bone (a floating U-shaped bone in the throat which is situated just under the chin). This is precisely how the hypoglossal nerve got its name. 'Hypo'- Greek word meaning under, and 'Glossal'- from a 'glossa' meaning tongue. The nerve gets further streamed in four branches, which spread around the tongue to innervate the complete tongue area. This nerve forms an important part in the vital functions of the tongue.

Functions

It is also known as the motor nerve. This nerve carries motor fibers to three of the four vital muscles of the tongue. It is responsible for everyday activities like eating, speech and swallowing. The muscles of this nerve are the origin of proprioceptors (muscles that define the movement of the body and position in space), which commence the sensory fibers, they wrap along these fibers to reach the medulla oblongata, the impulses of these muscles carry signals regarding the use and movement of the tongue. Activities like swallowing saliva to clear the mouth occur without any thought or conscious command. However, other activities have to be taught to, and learned by this nerve to perform. Speech requires the maximum movement of the tongue in a specific manner, languages also demand unusual movements of the tongue for creating certain sounds. Learning to move the tongue to create these sounds is necessary for children to be able to talk in that language. Our tongue is used to moving specifically while speaking in a certain language. It is therefore difficult for adults to adjust to the sounds of another language. That is the reason we usually take some time to pronounce clear sounds in another language.

Hypoglossal Nerve Damage

Let us look at the reasons that can cause damage to this nerve.

• Since this nerve originates in the medulla oblongata in the brain, a stroke or any infection in the brain stem can cause severe damage.
• An injury to the cranial nerves caused by surgery around the neck or the head, removing a blockage from the artery located in the neck, can result in severe damage.
• Any bone abnormality or tumor at the base of the skull.
• Diseases of the lower brain stem like Ischemia and motor neuron disease may affect the hypoglossal nerve.
• Other diseases like Amyotrophic lateral sclerosis (Lou Gehrig's disease) can also damage this nerve.

Severe damage to the core of the nerve or the trunk may cause paralysis. It affects the tongue, and as a result, the tongue becomes weak on the paralyzed side. The injury may lead to speech difficulties like unclear speech or stumbling while talking (dysarthria). One may have a problem with chewing and even swallowing food. In case of injury due to Amyotrophic lateral sclerosis (Lou Gehrig's disease), slight tiny twitching movements, also known as fasciculations, may occur on the tongue surface. The paralysis of the nerve results in a tongue that deviates to the paralyzed side, and with time the tongue atrophies (diminishes in size).

Testing Damaged Hypoglossal Nerve

We are aware of the complications due to any injury to this nerve. Let us look at how you can test this nerve at home.

• You can simply check if the tongue deviates more towards either side when poked out.
• You can test the strength of the tongue, poke the tongue on the inside of the cheek and push a finger on the outer side of the cheek against it. Feel how strong you can push the finger on the cheek. This is a more decent way, than touching the tongue directly to check it.
• In case of partial paralysis of the nerve, the tongue will move towards the impaired side.

When checking disorders, usually an MRI (magnetic resonance imaging ) is done, to check if there is tumor or evidence of a stroke. A lumbar puncture or a spinal tap (procedures that help examine the fluid surrounding the spine and the brain) is also used to diagnose any infections, cancer, etc. After proper medical examination, the treatment is prescribed. It varies depending on the cause of injury or damage.

Treatment

• Surgery can be prescribed if there are penetrating wounds to the nerve.
• Exercises for the palate and the pharynx, if there is difficulty in speaking (dysarthria ), for any injury caused by blunt trauma.
• A treatment to improve communication skills and behavioral impairments like dysphagia (disorder for difficulty in swallowing food) through various specialist teams is also advised.
• Practice of various conducive strategies for implementing safe and apt intake of food.

We now know that the hypoglossal nerve is responsible for the many and most important functions of our tongue. We can't even dream of living without being able to eat or talk. Amazing that the gift of words and sounds is safely wrapped by one of these nervous networks!