The spinal accessory (SA) nerve, cranial nerve XI (CN XI), is actually two nerves that run together in a common bundle for a short distance. The smaller cranial portion is a special visceral efferent accessory to the vagus. The cranial root exits through the jugular foramen separately from the spinal portion then blends with the vagus. It is distributed principally with the recurrent laryngeal nerve. The major part of CN XI is the spinal portion. The fibers of the spinal root arise from motor cells in the SA nuclei in the ventral horn from C2 to C5, or even C6. Its axons emerge as a series of rootlets laterally between the anterior and posterior roots. These unite into a single trunk which ascends between the denticulate ligaments and the posterior roots. The nerve enters the skull through the foramen magnum, ascends the clivus for a short distance, then curves laterally. The spinal root joins the cranial root for a short distance, probably receiving one or two filaments from it. It exits through the jugular foramen in company with CNs IX and X.

The supranuclear innervation of CN XI arises from the lower portion of the precentral gyrus. Fibers from the lateral corticospinal tract in the cervical spinal cord communicate with the SA nucleus. There is some controversy, but the bulk of current evidence indicates that both the sternocleidomastoid (SCM) and trapezius receive bilateral supranuclear innervation, but the input to the SCM motor neuron pool is predominantly ipsilateral and that to the trapezius motor neuron pool is predominantly contralateral. The SCM turns the head to the opposite side, and its supranuclear innervation is ipsilateral; therefore the right cerebral hemisphere turns the head to the left.