Cranial Nerve XI
Accessory Nerve
BACKGROUND
The spinal accessory nerve arises from the medulla and has contributions from the spinal route rising from C2 to C4. It is purely motor and innervates the sternocleidomastoid and the trapezius.
The ipsilateral cerebral hemisphere supplies the contralateral trapezius and the ipsilateral sternocleidomastoid. Thus, a single upper motor lesion can give rise to signs on both sides.
