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.

5 thoughts on “2-Minute Neuroscience: Accessory Nerve (Cranial Nerve XI)”

Leave a Reply

Your email address will not be published. Required fields are marked *