The accessory nerve (CN XI) is a motor (efferent) nerve supplying the sternocleidomastoid muscle and trapezius muscle.

Traditionally, the accessory nerve is described as arising from both - the medulla oblongata and spinal cord, consisting of a cranial root and a spinal root. However, the existence of a cranial contribution to the accessory nerve is controversial, and many neuroanatomists no longer consider this nerve to be a true cranial nerve, rather as a unique spinal nerve, and the previously thought cranial root to be part of the vagus nerve (CN X).

The accessory nerve, also referred to as the spinal accessory nerve, originates from the spinal accessory nucleus located in the posterolateral aspect of the anterior horn of the spinal cord at the levels C1 - C5 (or C6, C7).

The fibers coming from the nuclei form delicate spinal rootlets emerging from the lateral surface of the spinal cord, and all together, they collect to form the accessory nerve.

The nerve ascends parallel to the spinal cord, enters the cranial cavity through the foramen magnum, and continues on the lateral aspect of the medulla oblongata. The nerve then joins with the aberrant fibers of the vagus nerve as they emerge from the medulla oblongata on its posterolateral sulcus as a distinct rootlet.

These two groups of nerve fibers travel alongside for a short distance through the jugular foramen and then separate to go in different directions. Those separate divisions are known as the internal branch of the accessory nerve and external branch of the accessory nerve.

The aberrant vagal fibers or the internal branches (previously called the "cranial root" of the accessory nerve) join the vagus nerve on the border between its superior ganglion and inferior ganglion to ultimately reach the pharyngeal muscles (except the stylopharyngeus muscle), muscles of the soft palate (except the tensor veli palatini muscle) and intrinsic laryngeal muscles.

The external branch or the accessory nerve exits the skull via the jugular foramen and enters the anterior triangle of the neck. It then travels downward, going anterolateral to the internal jugular vein. The occipital artery crosses the nerve as it goes further, and the nerve goes medially behind the digastric muscle and stylohyoid muscle to reach the upper part of the sternocleidomastoid muscle.

It enters the deep surface of the muscle and provides motor branches to innervate it, also forming anastomosis with fibers from spinal nerves C2 and/or C3 in the posterior triangle of the neck, and, below the trapezius muscle, it anastomoses with the fibers from the spinal nerves C3 and/or C4.

After piercing the sternocleidomastoid, the accessory nerve descends further, exiting from the posterior border of the muscle. The nerve crosses over the posterior triangle of the neck, lying deep to the cervical plexus. It goes between the layers of the deep cervical fascia and reaches the deep surface of the upper part of the trapezius muscle, and supplies it with motor innervation.

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Accessory nerve (CN XI)