The eleventh cranial nerve is exclusively amotor nerve; its external branch supplies the sternocleidomastoid muscle (D12) and the trapezius muscle (D13). Its nucleus, the spinal nucleus of the accessory nerve (C14),forms a narrow cell column from C1 to C5 or C6. The large multipolar neurons lie at the lateral aspect of the anterior horn. The cells of the caudal section supply the trapezius muscle, and those of the cranial section supply the sternocleidomastoid muscle. The nerve fibers emerge from the lateral aspect of the cervical spinal cord between poste-rior root and anterior root and combine to form a bundle that enters the skull as the spinal root (C15) alongside the spinal cordthrough the foramen magnum. Fiber bundles from the caudal part of the ambigu-ous nucleus join the nerve here ascranialroots (C16). Both components pass throughthe jugular foramen (C17). Immediately after passing, the fibers change from the ambiguous nucleus as internal branches (C18) over to the vagus nerve (C19). The fibers from the cervical spinal cord form the exter-nal branch (C20), which supplies asaccessorynerve the sternocleidomastoid muscle andthe trapezius muscle. It passes through the sternocleidomastoid muscle and reaches the trapezius muscle with its terminal branches.
Clinical Note: Injury to the accessory nervecauses the head to tilt (plagiocephaly). The arm can no longer be lifted above the horizontal.