Accessory (XI) Nerve

The cranial root is the smaller of the two portions of the accessory nerve. Although it is discussed in this section, it is often considered as a part of the vagus nerve rather than the accessory nerve proper because the cranial component rapidly joins the vagus nerve and serves the same function as other vagal nerve fibers. The cranial nerve root fibers, classified as special visceral efferent, arise mainly from neurons in the caudal half of the nucleus ambiguous of the medulla, with probable minor contributions from the dorsal vagal nucleus. The fibers of the cranial root emerge as four to six rootlets from the dorsolateral sulcus, posterior to the olive, below the roots of the vagus nerve. The cranial root runs laterally to briefly join the larger spinal root before passing through the jugular foramen in the same dural and arachnoid sheath as the vagus nerve. The cranial root communicates by one or two filaments with the superior vagal ganglion; however, most of its fibers continue as the internal branch of the accessory nerve, which joins the vagus nerve at or near its inferior ganglion and provides most of the motor fibers to the pharynx and larynx. The pharyngeal branches supply the muscles of the soft palate (except the tensor veli palatini) and contribute motor fibers to the pharyngeal plexus. The fibers in the recurrent laryngeal vagal branches supply all the intrinsic laryngeal muscles except the cricothyroid.

Course of Accessory Nerve. The cranial and spinal root fibers separate distal to the jugular foramen to form the internal and external branches of the accessory nerve. The internal branch joins the vagus nerve as described above. The external accessory branch innervates the sternocleidomastoid and trapezius muscles. The external branch of the accessory nerve usually passes between the internal carotid artery and the internal jugular vein and runs obliquely downward and backward over the transverse process of the atlas and deep to the styloid process, occipital artery, and posterior belly of the digastric muscle before piercing the deep surface of the sternocleidomastoid muscle. It passes through and supplies this muscle and emerges from the midpoint of the posterior sternocleidomastoid border. The external branch then descends across the posterior cervical triangle and crosses over the levator scapulae muscle to disappear under the trapezius muscle about 2 cm above the clavicle. Along its course, the external branch receives branches from the second, third, and fourth cervical nerves.

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