The spinal accessory nerve supplies motor innervation to the sternocleidomastoid and trapezius muscles.
Traditionally, the spinal accessory nerve is divided into the spinal and cranial parts. Although the cranial part unites with CN XI briefly, most authors agree that it actually belongs to the vagus nerve (CN X), as it unites with it later. Therefore, we refer to the spinal part as the spinal accessory nerve proper.