The accessory nerve has a cranial root and a spinal root, both of which consist of branchial motor fibers.
Branchial motor - cranial root
|Innervates muscles of larynx and pharynx.|
|Branchial motor - spinal root
(special visceral efferent)
|Innervates the trapezius and sternocleidomastoid muscles.|
Origin and Central Course
The fibers of the cranial root originate in the caudal nucleus ambiguus and travel anteriorly and laterally to exit the medulla between the olive and inferior cerebellar peduncle (just below the exiting fibers of CN X).
Upon emerging from the medulla, the cranial root fibers momentarily join the spinal root fibers of CN XI and enter the jugular foramen. Within the foramen, the cranial root fibers split from the spinal root fibers and join the vagus nerve to exit the skull.
Extracranial Course and Final Innvervations
The cranial root of CN XI is accessory to the vagus nerve by providing part of its motor innervation of the larynx and pharynx. In fact, some texts treat the cranial root as part of CN X since it arises from the nucleus ambiguus, travels with CN X fibers (except for a few millimeters intracranially), and innervates the same structures as the vagus nerve.
CN XI cranial root fibers follow the same extracranial course as that described for the branchial motor component of the vagus nerve.
Origin and Central Course
The fibers of the spinal root originate in the lateral portion of the ventral grey matter of the upper six segments of the spinal cord (C1- C6). For reference, the accessory nucleus is approximately in line with the nucleus ambiguus of the medulla.
Note that the spinal roots of CN XI exit the cord laterally between the dorsal and ventral spinal roots.
Upon emerging from the cervical spinal cord, the spinal root fibers form a trunk that ascends through the foramen magnum to enter the posterior cranial fossa of the skull.
These fibers then momentarily join the cranial root of CN XI and loop downward to enter the jugular foramen. Within the jugular foramen, the cranial root fibers separate and join CN X.
Extracranial Course and Final Innervations
The spinal root of CN XI exits the jugular foramen medial to the styloid process and travels obliquely downward to enter the upper portion of the sternocleidomastoid muscle on its deep surface.
Some of the nerve fibers terminate in the sternocleidomastoid while others exit its posterior surface, travel through the posterior triangle of the neck (in close approximation to the superficial cervical lymph nodes), and pass deep to the anterior border of the trapezius before innervating this muscle.
Voluntary Control of the Trapezius and Sternocleidomastoid Muscles
areas remain ipsilateral as they pass through the posterior limb of the internal capsule, cerebral peduncle, basal pons, and medullary pyramids before decussating at the junction of the medulla and cervical spinal cord.
The crossed fibers make up the lateral corticospinal tract. Some of these fibers terminate in the accessory nucleus located at the level of C1- C6 to provide voluntary control of the trapezius and sternocleidomastoid muscles.
Damage to the spinal root of CN XI is a lower motor neuron lesion and results in weakness or flaccid paralysis of the sternocleidomastoid and/or trapezius muscles.
Contraction of the sternocleidomastoid muscle pulls the mastoid process toward the ipsilateral clavicle resulting in rotation of the head and an upward tilting of the chin to the opposite side. Weakness of the sternocleidomastoid will result in difficulty in turning the head opposite the side of the lesion. This will be most noticeable when attempted against resistance.
Contraction of the trapezius raises and medially rotates the scapula.Weakness of the trapezius will result in shoulder drop and downward displacement and lateral rotation of the scapula on the affected side.