Accessory Nerve

In anatomy, the accessory nerve is a nerve that controls specific muscles of the shoulder and neck. As part of it was formerly believed to originate in the brain, it is considered a cranial nerve. Based on its location relative to other such nerves, it is designated the eleventh of twelve cranial nerves, and is thus abbreviated CN XI.

Traditional descriptions of the accessory nerve divide it into two parts: a spinal part and a cranial part. But because the cranial component rapidly joins the vagus nerve and serves the same function as other vagal nerve fibers, modern descriptions often consider the cranial component part of the vagus nerve and not part of the accessory nerve proper. Thus in contemporary discussions of the accessory nerve, the common practice is to dismiss the cranial part altogether, referring to the accessory nerve specifically as the spinal accessory nerve.

The spinal accessory nerve provides motor innervation from the central nervous system to two muscles of the neck: the sternocleidomastoid muscle and the trapezius muscle. The sternocleidomastoid muscle tilts and rotates the head, while the trapezius muscle has several actions on the scapula, including shoulder elevation and adduction of the scapula.

Range of motion and strength testing of the neck and shoulders can be measured during a neurological examination to assess function of the spinal accessory nerve. Limited range of motion or poor muscle strength are suggestive of damage to the spinal accessory nerve, which can result from a variety of causes. Injury to the spinal accessory nerve is most commonly caused by medical procedures that involve the head and neck.

The accessory nerve is derived from the basal plate of the embryonic spinal segments C1–C6.