Diseases of the eleventh cranial (spinal accessory) nerve. This nerve originates from motor neurons in the lower medulla (accessory portion of nerve) and upper spinal cord (spinal portion of nerve). The two components of the nerve join and exit the skull via the jugular foramen, innervating the sternocleidomastoid and trapezius muscles, which become weak or paralyzed if the nerve is injured. The nerve is commonly involved in motor neuron disease, and may be injured by trauma to the posterior triangle of the neck.