CN XI: Accessory nerve

In this note, we are going to discuss the anatomy and function of the accessory nerve (n. accessorius), or the 11th cranial nerve (CN XI).

It innervates the sternocleidomastoid and trapezius muscles.

Sternocleidomastoid muscle (m. sternocleidomastoideus)

According to the generally accepted classification of functional components, it contains general somatic efferent fibers (GSE) and special visceral efferent fibers (SVE). That means that it is a motor nerve.

Some authors recognize only one of the listed types of fibers in the accessory nerve (either GSE or SVE). This is due to the fact that the sternocleidomastoid and trapezius muscles are not directly derived from the pharyngeal arches. In classical anatomy, the nerve was described only as a general (or somatic) sensory efferent nerve (GSE). However, according to modern data, certain parts of these muscles are still derived from the pharyngeal arches. In this regard, we cannot exclude the presence of SVE fibers.

The fibers of the accessory nerve arise from the two nuclei and form two divisions.

The cranial division arises from the nucleus ambiguus (nucleus ambiguus), which is located in the dorsal part of the medulla oblongata.

The axons of these neurons exit the brain along the posterior border of the olive and form the cranial root of the accessory nerve (radix cranialis).

Сranial root of the accessory nerve (radix cranialis)

The spinal division arises from the spinal accessory nucleus (nucleus spinalis nervi accessorii), which is located in the cervical spinal cord, specifically in the posterolateral aspect of the anterior horn.

The axons of these neurons form the spinal root of the accessory nerve (radix spinalis).

It ascends inside the vertebral canal (canalis vertebralis) as the spinal accessory nerve,

enters the cranial cavity through the foramen magnum (foramen magnum) and merges with the cranial root to form the common trunk of the accessory nerve.

The accessory nerve exits the cranial cavity through the jugular foramen (foramen jugulare).

Location of the spinal root in the posterior cranial fossa

Then it splits into two branches: the internal branch (r. internus), and the external branch (r. externus).

The internal branch is formed by the fibers of the cranial root and continues its course along the pharyngeal and laryngeal branches of the vagus nerve.