The accessory nerve is the eleventh of twelve cranial nerves. It leaves the cranium through the foramen magnum, and innervates the sternocleidomastoid muscle (sternomastoid) and trapezius muscle on the ipsilateral side.
There are two parts to the accessory nerve, a spinal part (innervates the muscles around neck), and a cranial part, that splits off, and quickly combines with the vagus nerve. The cranial part of nerve XI can be thought of doing the exact same things as the vagus.
Nuclei of the spinal accessory nerve
The spinal accessory nucleus lies within the cervical spinal cord (C1-C5) in the ventral horn.
Testing the accessory nerve
Getting a person to shrug their shoulders while you push down tests trapezius. When a person turns their head, especially against force, sternocleidomastoid should be prominent.
