Accessory nerve (CN XI)

Nervus accessorius

NervousNervesWhole bodyLeft and right

View left in 3D View right in 3D

What it is
Formed mainly by a spinal root that rises from the upper five cervical cord segments, enters the skull through the foramen magnum and leaves again through the jugular foramen. It then runs down and back across the side of the neck, piercing the sternocleidomastoid and crossing the posterior triangle to the trapezius.
Function
Motor to the sternocleidomastoid (turns the head) and trapezius (shrugs the shoulder and rotates the shoulder blade).
Size
About 1.5 mm thick; ~12 cm from foramen magnum to trapezius
Clinical note
Its superficial course through the posterior triangle makes it the nerve most often injured in lymph-node biopsies of the neck, leaving a dropped shoulder and difficulty lifting the arm.

Nearby nervous structures Brachial plexus, Cardiac plexus, Cervical plexus, Left phrenic nerve, Left recurrent laryngeal nerve, Left vagus nerve (CN X), Right phrenic nerve, Right recurrent laryngeal nerve, Right vagus nerve (CN X), Spinal cord