Areas Innervated by the Hypoglossal Nerve

Areas Innervated by the Hypoglossal Nerve
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The hypoglossal nerve is the twelfth and final cranial nerve (XII). Cranial nerves are nerves that originate from the brain and brainstem rather than from the spinal cord. In evolutionary terms, they are shared by many other vertebrates. The hypoglossal is found in mammals, birds, and reptiles, but not in amphibians or fishes.

Background

The hypoglossal nerve originates in the hypoglossal nucleus, located in the dorsal portion of the medulla oblongata in the brainstem. The axons of the nerve cells form “rootlets” that emerge from the medulla oblongata and merge to form the paired hypoglossal nerves. It passes under the hyoid bone, a “floating” U-shaped bone in the throat just under the chin, then under several of the tongue muscles. This portion of the nerve gives it its name, which means “under tongue.” Finally, it separates into branches to innervate most of the tongue.

Most of the tongue’s muscles are innervated by the hypoglossal nerve. These include both extrinsic muscles, which originate on a bone outside the tongue, and intrinsic muscles, which have both ends within the tongue.

At right: Plate from Grey’s Anatomy showing the location of the hypoglossus nerve.

Extrinsic Muscle Areas Innervated by the Hypoglossal Nerve

The extrinsic muscles innervated by the hypoglossal are the styloglossus, hyoglossus, and genioglossus muscles. (A fourth extrinsic muscle, the palatoglossus, is the only tongue muscle not innervated by the hypoglossal nerve.)

The two different motor functions of extrinsic tongue muscles, protrusion and retraction, are controlled by separate parts of the hypoglossal nerve. The medial (inner) part of the nerve originates in the ventral (bottom) part of the brainstem’s hypoglossal nucleus; it serves the muscle that protrudes the tongue, the genioglossus muscle. The lateral (outer) part of the nerve originates in the dorsal (top) part of the hypoglossal nucleus; it innervates the muscles that retract the tongue, the styloglossus and hyoglossus.

Intrinsic Muscle Areas Innervated by the Hypoglossal Nerve

All the intrinsic muscles of the tongue are innervated by the hypoglossal nerve. These muscles are the transversus, verticalis and longitudinalis linguae muscles. The intrinsic tongue muscles are used to elongate the protruding tongue and to control fine shaping movements needed for manipulating food and for speech.

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