The accessory nerve controls specific muscles of the neck. It begins in the central nervous system and exits the cranium through a foramen. Unlike other cranial nerves, it begins outside the skull rather than inside, originating in the upper spinal cord. It innervates the sternocleidomastoid and trapezius muscles. Injury can cause diminished function of these muscles.

1. thethe accessory nerveaccessory nerve is ais a nervenerve that controlsthat controls
specificspecific musclesmuscles of the neck.of the neck. ..

2. AnatomyAnatomy
 Like otherLike other cranial nervescranial nerves, the spinal accessory nerve begins in the, the spinal accessory nerve begins in the central nervous systemcentral nervous system andand
exits theexits the craniumcranium through a specialized hole (orthrough a specialized hole (or foramenforamen). However, unlike all other cranial). However, unlike all other cranial
nerves, the spinal accessory nerve begins outside the skull rather than inside. In particular, in thenerves, 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 inmajority of individuals, the fibers of the spinal accessory nerve originate solely in neuronsneurons
situated in the uppersituated in the upper spinal cordspinal cord..These fibers coalesce to form spinal rootlets, roots, and finallyThese fibers coalesce to form spinal rootlets, roots, and finally
the spinal accessory nerve itself, which enters the skull through thethe spinal accessory nerve itself, which enters the skull through the foramen magnumforamen magnum, the large, the large
opening at the base of the skull. The nerve courses along the inner wall of the skull towards theopening at the base of the skull. The nerve courses along the inner wall of the skull towards the
jugular foramenjugular foramen, through, through which it exits the skull with thewhich it exits the skull with the glossopharyngealglossopharyngeal (CN IX) and(CN IX) and vagusvagus
nervesnerves (CN X). Owing to its peculiar course, the spinal accessory nerve is notable for being the(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.only cranial nerve to both enter and exit the skull.
 Traditionally, the accessory nerve is described as having a small cranial component thatTraditionally, the accessory nerve is described as having a small cranial component that
descends from thedescends from the medulla oblongatamedulla oblongata and briefly connects with the spinal accessory componentand briefly connects with the spinal accessory component
before branching off of the nerve to join thebefore branching off of the nerve to join the vagusvagus nervenerve. A recent study of twelve subjects. A recent study of twelve subjects
suggests that in the majority of individuals, this cranial component does not make any distinctsuggests 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 aconnection to the spinal component; the roots of these distinct components were separated by a
fibrous sheath in all but one subject.fibrous sheath in all but one subject.[4][4]
 Once the cranial component has detached from the spinal component, the spinal accessoryOnce the cranial component has detached from the spinal component, the spinal accessory
nerve continues alone and heads posteriorly (backwards) and inferiorly (downwards) upon exitingnerve continues alone and heads posteriorly (backwards) and inferiorly (downwards) upon exiting
the skull. It pierces thethe skull. It pierces the sternocleidomastoidsternocleidomastoid musclemuscle while sending it motor branches, thenwhile sending it motor branches, then
continues inferiorly until it reaches thecontinues inferiorly until it reaches the trapeziustrapezius musclemuscle to provide motor innervation to its upperto provide motor innervation to its upper
portion.portion.

3. nucleusnucleus
The fibers that form the spinal accessory nerve are formedThe fibers that form the spinal accessory nerve are formed
byby lower motor neuronslower motor neurons located in the upper segments oflocated in the upper segments of
thethe spinal cordspinal cord. This cluster of neurons, called the. This cluster of neurons, called the
spinal accessory nucleusspinal accessory nucleus, is located in the, is located in the lateral hornlateral horn ofof
the spinal cord. This is in contrast to most other motorthe spinal cord. This is in contrast to most other motor
neurons, whose cell bodies are found in the spinal cord'sneurons, whose cell bodies are found in the spinal cord's
anterior hornanterior horn. The lateral horn of high cervical segments. The lateral horn of high cervical segments
appears to be continuous with theappears to be continuous with the nucleusnucleus ambiguusambiguus ofof
thethe medulla oblongatamedulla oblongata, from which the cranial, from which the cranial
component of the accessory nerve is derived.component of the accessory nerve is derived.

4. InjuryInjury
Injury to the spinal accessory nerve can cause anInjury to the spinal accessory nerve can cause an
accessory nerve disorder or spinal accessoryaccessory nerve disorder or spinal accessory
nerve palsy, which results in diminished ornerve palsy, which results in diminished or
absent function of the sternocleidomastoidabsent function of the sternocleidomastoid
muscle and upper portion of the trapeziusmuscle and upper portion of the trapezius
muscle. Patients with spinal accessory nervemuscle. Patients with spinal accessory nerve
palsy often exhibit signs of lower motor neuronpalsy often exhibit signs of lower motor neuron
disease such as diminished muscle mass,disease such as diminished muscle mass,
fasciculations, and partial paralysis of thefasciculations, and partial paralysis of the
sternocleidomastoid and trapezius muscles.sternocleidomastoid and trapezius muscles.