The hypoglossal nerve, known as the twelfth cranial nerve (CN XII), is classified as a purely motor nerve. Its primary function is to transmit signals from the brain to muscles, controlling their action. This nerve is singularly responsible for governing the complex movements of the tongue, a function that is indispensable for a variety of daily activities. Without its coordinated signaling, fundamental human actions like speaking and swallowing would be nearly impossible. The nerve’s name itself, derived from Greek, means “under the tongue,” accurately describing its location and purpose.
Where the Hypoglossal Nerve Travels
The hypoglossal nerve begins within the brainstem, originating from the hypoglossal nucleus in the medulla oblongata. From this nucleus, the nerve fibers emerge as rootlets from the anterior surface of the medulla. It then travels laterally across the posterior cranial fossa before reaching its exit point.
The nerve leaves the skull by passing through a bony opening called the hypoglossal canal, which is located in the occipital bone. Once outside the skull, the nerve enters the upper part of the neck. It descends briefly, passing close to major blood vessels of the neck, such as the internal carotid artery and internal jugular vein.
The hypoglossal nerve then loops forward, moving toward the front of the neck and the floor of the mouth. It travels deep to the muscles under the chin, ultimately reaching the base and core of the tongue, where its fibers branch out to innervate the target muscles. This extended pathway makes the nerve susceptible to injury from various sources outside the brain.
Controlling the Muscles of the Tongue
The primary role of the hypoglossal nerve is to provide motor control to nearly all the muscles that make up the tongue. These muscles are divided into two functional groups: the extrinsic muscles, responsible for gross movements, and the intrinsic muscles, which manage the tongue’s shape.
The extrinsic muscles—the genioglossus, hyoglossus, and styloglossus—allow the tongue to be protruded, retracted, and moved side-to-side. For example, the genioglossus is the primary muscle responsible for pushing the tongue forward, while the hyoglossus retracts and flattens it.
The intrinsic muscles include the superior and inferior longitudinal, vertical, and transverse muscles, which are contained within the tongue structure. Activation of these muscles allows for fine motor control, such as curling the tongue tip, making it narrow, or flattening its surface. This precise shaping facilitates the complex movements necessary for daily functions.
The coordinated action of these two muscle groups is fundamental for proper speech articulation, particularly for forming consonants. The rapid and subtle adjustments in tongue shape and position are what distinguish sounds like “t,” “d,” and “l”. Without the hypoglossal nerve’s innervation, the ability to rapidly change the vocal tract shape would be lost, leading to slurred speech.
Beyond speech, the nerve’s control is important for the initial stage of swallowing, known as the oral phase. The tongue muscles manipulate food within the mouth, mixing it with saliva and compressing it against the hard palate to form a cohesive ball called a bolus. The nerve then directs the tongue to push this bolus backward into the pharynx, initiating the involuntary stages of the swallow reflex.
When the Nerve is Injured
Damage to the hypoglossal nerve results in a distinct set of symptoms that directly impair tongue movement. Because the nerve controls the muscles on its own side, a unilateral injury causes weakness and eventual wasting, or atrophy, of the tongue on the affected side. As the muscle loses bulk, involuntary, fine twitching movements called fasciculations may become visible.
When a patient is asked to stick their tongue out, the tongue deviates toward the side of the injury. This happens because the genioglossus muscle on the healthy side is unopposed and pushes the tongue forward, while the weakened muscle on the injured side cannot push back. A physician often tests for this deviation to assess the nerve’s integrity.
The functional consequences of nerve damage primarily manifest as difficulty speaking and swallowing. Impaired tongue movement leads to dysarthria, or slurred speech, because the patient cannot properly articulate consonants that require precise tongue placement. Swallowing difficulty, or dysphagia, occurs because the weak tongue struggles to form a cohesive food bolus and propel it backward, which can increase the risk of choking or aspiration.
Causes of hypoglossal nerve injury are varied and can occur anywhere along its path. Sources of damage include strokes that affect the brainstem, tumors or bone abnormalities at the base of the skull, and trauma to the neck. The nerve is also sometimes injured unintentionally during surgical procedures in the neck, such as a carotid endarterectomy. Furthermore, neurodegenerative conditions, like amyotrophic lateral sclerosis (ALS), can progressively damage the nerve’s nucleus in the brainstem, leading to chronic weakness and atrophy.