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The accessory nerve is primarily considered a motor nerve that supplies two muscles: the
sternomastoid muscle and the trapezius muscle.
The sternomastoid helps to turn your head to the side, as well as bend your neck down
or to the side.
The trapezius is involved with the movement of the neck and scapula, or shoulder blade.
The trapezius is important to a variety of shoulder and arm movements, such as shrugging
your shoulders or raising your arms above your head.
Traditionally, the accessory nerve has been divided into a spinal component, which originates
in the accessory nucleus in the spinal cord, and a cranial component, which originates
in the nucleus ambiguus in the medulla.
The fibers that make up the spinal portion of the nerve leave the spinal cord as a series
of rootlets, which then come together to form what is known as the spinal root.
The spinal root ascends to join the cranial root that extends from the medulla.
The cranial and spinal roots travel together briefly as they exit the skull, but then the
cranial portion travels with the vagus nerve while the spinal portion extends to the sternomastoid
and trapezius muscles.
Because the cranial portion can be functionally considered part of the vagus nerve, often
the spinal portion is considered the true accessory nerve.
Damage to the accessory nerve can cause weakness or paralysis of the sternomastoid and/or trapezius
muscles, but trapezius dysfunction is typically the more incapacitating problem.
Trapezius dysfunction may impair arm and shoulder movement, cause the shoulder to droop, and
cause the shoulder blade to abnormally protrude from the back--a condition known as a “winged
scapula.”
The patient may also experience shoulder and neck pain, along with muscle fatigue because
other muscles in the shoulder and arm that are unaccustomed to supporting the shoulder
must be utilized.