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Anatomy of the spinal accessory (CN XI) and hypoglossal (CN XII) nerves

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The spinal accessory nerve, or the eleventh cranial nerve, and the hypoglossal nerve, or the twelfth cranial nerve, are two purely motor cranial nerves that innervate muscles in the neck, as well as the tongue.

Specifically, the spinal accessory nerve provides somatic motor innervation to the sternocleidomastoid and trapezius muscles, while the hypoglossal nerve provides somatic motor innervation to almost all the muscles of the tongue.

Let’s start with the spinal accessory nerve. The somatic motor pathway of the spinal accessory nerve starts in the cerebral cortex. Then, the axons travel to the spinal accessory nucleus, which is located in the cervical spinal cord, specifically in the posterolateral aspect of the anterior horn.

The axons travel through the rootlets from the first five or six cervical segments of the spinal cord and form the spinal accessory nerve. The spinal accessory nerve ascends through the foramen magnum, enters the posterior cranial fossa, and then exits through the jugular foramen.

It continues its journey down along the internal carotid artery and runs deep to the sternocleidomastoid muscle, which it supplies. Then, it emerges from the sternocleidomastoid muscle through its posterior border, and continues inferiorly in the posterior triangle of the neck to reach and supply the trapezius muscle through multiple branches.

The spinal accessory nerve is located superficially in the posterior triangle of the neck, and you can identify it using two landmarks: one third of the way down from the posterior border of the sternocleidomastoid muscle and one third of the way up from the anterior border of the trapezius muscle.