Lab 10 - Cranial Nerve Nuclei and Brain Stem Circulation

Cranial Nerve XI - Spinal Accessory Nerve

The spinal accessory nucleus extends from the caudal medulla to about C5. Axons from the spinal accessory nucleus travel in the spinal accessory nerve to end in part of the trapezius and sternocleidomastoid muscles. Cranial Nerve XI-Spinal Accessory NerveThe movements these muscles generate are head rotation, flexion and shoulder shrug.

Destruction of the spinal accessory nerve produces weakness in rotating the head against resistance toward the side opposite the lesion and atrophy of the sternocleidomastoid. The patient cannot shrug the affected shoulder and exhibits a mild drooping of the affected shoulder with the scapula displaced downward.

Fibers from the nucleus ambiguus travel in the spinal accessory nerve within the skull, join the vagus nerve outside the skull, and end in the intrinsic muscles of the larynx.

The cortical input to the spinal accessory nucleus is bilateral and direct for volitional control of head rotation. Interruptions of the corticobulbar fibers (upper motor neuron lesions) produce very mild symptoms that are difficult to recognize unless the fibers are injured bilaterally. Diseases of the corpus striatum may result in torticollis, a repeated involuntary movement of the head toward the normal side.

Clinical Testing of Cranial Nerve XI: