The accessory nerve, also known as cranial nerve XI, is a motor nerve that primarily controls the muscles involved in head movement and shoulder elevation. It has two parts: the cranial part, which innervates some muscles of the larynx, and the spinal part, which innervates the sternocleidomastoid and trapezius muscles. This nerve originates from the brainstem and the upper spinal cord, traveling through the neck to reach its target muscles. Damage to the accessory nerve can lead to weakness in shoulder elevation and difficulty turning the head, affecting overall neck and shoulder function.