In anatomy, the accessory nerve is a nerve that controls specific muscles of the neck. As a 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. Although anatomists typically refer to the accessory nerve in singular, there are in reality two accessory nerves, one on each side of the body.
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 upper part of the trapezius muscle. The sternocleidomastoid muscle tilts and rotates the head, while the trapezius muscle has several actions on the scapula, including shoulder elevation.
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.
Like other cranial nerves, the spinal accessory nerve begins in the central nervous system and exits the cranium through a specialized hole (or foramen). However, unlike all other cranial nerves, the spinal accessory nerve begins outside the skull rather than inside. In particular, in the majority of individuals, the fibers of the spinal accessory nerve originate solely in neurons situated in the upper spinal cord. These fibers coalesce to form spinal rootlets, roots, and finally the spinal accessory nerve itself, which enters the skull through the foramen magnum, the large opening at the base of the skull. The nerve courses along the inner wall of the skull towards the jugular foramen, through which it exits the skull with the glossopharyngeal (CN IX) and vagus nerves (CN X). Owing to its peculiar course, the spinal accessory nerve is notable for being the only cranial nerve to both enter and exit the skull.
Traditionally, the accessory nerve is described as having a small cranial component that descends from the medulla oblongata and briefly connects with the spinal accessory component before branching off of the nerve to join the vagus nerve. A recent study of twelve subjects suggests that in the majority of individuals, this cranial component does not make any distinct connection to the spinal component; the roots of these distinct components were separated by a fibrous sheath in all but one subject.
Once the cranial component has detached from the spinal component, the spinal accessory nerve continues alone and heads posteriorly (backwards) and inferiorly (downwards) upon exiting the skull. It pierces the sternocleidomastoid muscle while sending it motor branches, then continues inferiorly until it reaches the trapezius muscle to provide motor innervation to its upper portion.
The fibers that form the spinal accessory nerve are formed by lower motor neurons located in the upper segments of the spinal cord. This cluster of neurons, called the spinal accessory nucleus, is located in the lateral horn of the spinal cord. This is in contrast to most other motor neurons, whose cell bodies are found in the spinal cord's anterior horn. The lateral horn of high cervical segments appears to be continuous with the nucleus ambiguus of the medulla oblongata, from which the cranial component of the accessory nerve is derived.
Injury to the spinal accessory nerve can cause an accessory nerve disorder or spinal accessory nerve palsy, which results in diminished or absent function of the sternocleidomastoid muscle and upper portion of the trapezius muscle. Patients with spinal accessory nerve palsy often exhibit signs of lower motor neuron disease such as diminished muscle mass, fasciculations, and partial paralysis of the sternocleidomastoid and trapezius muscles.
In Neuroanatomy and the Neurologic Exam, Terence R. Anthoney provides a historical account of the usage of the terms accessory nerve, spinal accessory nerve, and cranial accessory nerve, summarized in the following excerpt: