The accessory nerve is the eleventh paired cranial nerve. It has a purely somatic motor function, innervating the sternocleidomastoid and trapezius muscles.This nerve supplies the sternocleidomastoid and trapezius muscles, which have the following functions: Rotation of head away from the side of the contracting sternocleidomastoid muscle. Tilting of the head toward the contracting sternocleidomastoid muscle. Flexion of the neck by both sternocleidomastoid muscles.The Spinal Accessory Nerve (SAN) or Cranial Nerve 11 is termed a cranial nerve as it was originally believed to originate in the brain. It has both a cranial and a spinal part, though debate still rages regarding if the cranial part is really a part of the SAN or part of the vagus nerve.