The 11th cranial nerve. The accessory nerve originates from neurons in the medulla and in the cervical spinal cord. It has a cranial root, which joins the vagus (10th cranial) nerve and sends motor fibers to the muscles of the larynx, and a spinal root, which sends motor fibers to the trapezius and the sternocleidomastoid muscles. Damage to the nerve produces weakness in head rotation and shoulder elevation.