THE ACCESSORY NERVE:
Netter Plate Number: 121

The spinal accessory nerve is derived from motor neurons found in the spinal nucleus, extending down to C5.

This nerve enters the skull through the foramen magnum, then exiting the skull via the jugular foramen.
It usually emerges posterior and lateral to the internal jugular vein, however 30% of the time it exits medial to the IJ, whereas 3-5% of the time it actually splits and travels through the vessel.
It then pierces the deep portion of the sternocleidomastoid muscle to exit the posterior portion of the sternocleidomastoid, traversing the posterior triangle where it then disappears under the anterior surface of the trapezius to innervate this muscle.

This nerve innervates the sternocleidomastoid and trapezius muscles.

A special anatomical relationship called "Erb's point" is noted where the greater auricular, transverse cervical and lesser occipital nerves exit the SCM posteriorly at about the midpoint of this muscle. The accessory nerve can be identified approximately 1 cm superior to this point.

This accessory nerve can also be found entering the anterior portion of the trapezius muscle approximately 5 cm above the clavicle.