The hypoglossal nerve, also known as cranial nerve XII, is one of the twelve paired nerves emerging directly from the brain. Its name originates from Greek words, where “hypo” means under and “glossal” refers to the tongue, aptly describing its position and primary function. This nerve originates from the medulla oblongata, a part of the brainstem, and extends to the muscles of the tongue.
Controlling Tongue Movement
The hypoglossal nerve is solely responsible for controlling the movements of the tongue. It innervates the tongue’s intrinsic and extrinsic muscles. The intrinsic muscles, located entirely within the tongue, allow it to change shape, such as shortening, narrowing, or curling.
The extrinsic muscles, which originate outside the tongue and attach to it, are responsible for moving the tongue within the mouth. These include the genioglossus, which pushes the tongue forward; the hyoglossus, which pulls it back and flattens it; and the styloglossus, which moves it up and back. The only tongue muscle not controlled by the hypoglossal nerve is the palatoglossus, which is innervated by the vagus nerve. This coordinated control enables a wide array of movements, fundamental for various oral functions.
Its Role in Speech and Swallowing
The precise movements facilitated by the hypoglossal nerve are fundamental for speech articulation. The tongue’s ability to change shape and position within the mouth, interacting with the teeth, palate, and lips, is essential for forming different speech sounds. Without this fine motor control, producing clear and understandable words would be challenging.
Beyond speech, the hypoglossal nerve also plays a significant role in swallowing, a complex process known as deglutition. The tongue, guided by this nerve, manipulates food within the mouth, forming it into a compact mass called a bolus. The nerve then orchestrates the tongue’s movements to propel this bolus efficiently toward the back of the throat, initiating the swallowing reflex. The hypoglossal nerve’s function is directly linked to both communication and nutrition.
When It Doesn’t Work Properly
Damage or impairment to the hypoglossal nerve can lead to noticeable signs and symptoms affecting tongue function. Weakness or paralysis of the tongue muscles often occurs on the side corresponding to the damaged nerve. Over time, this can result in atrophy, where the affected side of the tongue appears smaller or wasted.
One common manifestation of hypoglossal nerve dysfunction is a visible deviation of the tongue. When an individual attempts to protrude their tongue, it will typically point toward the side of the nerve damage. This occurs because the healthy genioglossus muscle on the unaffected side pushes the tongue forward without opposition from the weakened muscle on the compromised side.
Impairment of the hypoglossal nerve also impacts daily activities. Difficulty with speech, known as dysarthria, may arise, particularly affecting sounds that require precise tongue movements. Swallowing difficulties, or dysphagia, can also occur, making it challenging to manipulate food or clear the throat effectively. Small, involuntary muscle twitches on the tongue’s surface, called fasciculations, can also be present.