Spinal Accessory Nerve

Cranial nerve XI (the accessory nerve) has traditionally been considered to comprise two parts, a cranial part that arises from neuron cell bodies in the caudal part of nucleus ambiguus (which lies in the lateral region of the medulla) and a spinal part that arises from neuron cell bodies located in the lateral part of the gray matter within upper segments of the cervical spinal cord. The laterally displaced position of the latter cell group is similar to that of the corresponding cell column in the brain stem. The so-called cranial part of XI (not included in Table II) is in fact merely the more caudal part of the vagus nerve. It supplies innervation to the intrinsic muscles of the larynx on the ipsilateral side. The spinal part of XI innervates the ipsilateral sternocleidomastoid muscle and the upper parts of the trapezius muscle. Contraction of the sternocleidomastoid muscle on one side causes the mastoid process (which lies behind the ear) to approach the sternum and medial part of the clavicle

("cleido"), thus causing the head to turn toward the contralateral side. Innervation of the spinal nucleus of XI from motor cortex appears to be predominantly ipsilateral. Damage to the spinal accessory nerve results in weakness of the ipsilateral muscles, which is revealed when trying to turn the head toward the opposite side against resistance.

The accessory nerve is sometimes unclassified, but it is a branchial motor nerve and belongs in the traditional category of SVE. The sternocleidomastoid and trapezius muscles evolved from the levator muscles of the branchial arches.

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