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.
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).
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).
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.
Some authors still consider only its spinal division to be the accessory nerve and attribute the cranial division directly to the vagus nerve.
The external branch consists of the fibers originating from the spinal root.
It travels inferiorly, along the internal carotid artery and runs deep to the sternocleidomastoid muscle, providing motor innervation to it.
Reaching the anterior border of the trapezius muscle, the nerve divides into its terminal branches.
CN XI: Accessory nerve
- Accessory nerve
- n. accessorius
- Nucleus ambiguus
- nucleus ambiguus
- Cranial root
- radix cranialis
- Spinal nucleus
- nucleus spinalis
- Spinal root
- radix spinalis
- Vertebral canal
- canalis vertebralis
- Foramen magnum
- foramen magnum
- Jugular foramen
- foramen jugulare
- Internal branch
- ramus internus
- External branch
- ramus externus
- Vagus nerve
- nervus vagus
- Sternocleidomastoideus muscle
- m. sternocleidomastoideus
- Trapezius muscle
- m. trapezius