Accessory nerve (XI)

At a glance

English

Accessory nerve (XI)

Latin (TA2)

Nervus accessorius (XI)

System

Nerve

Region

Head

Side

Paired, left and right

About the accessory nerve (xi)

The eleventh cranial nerve, called accessory, is a motor nerve for two large muscles of the neck and shoulder: sternocleidomastoid, which turns and tilts the head, and the upper part of trapezius, which lifts and rotates the shoulder blade. Older books describe a cranial root joining a spinal root; today the cranial fibres, which come from the nucleus ambiguus and go to the palate, pharynx and larynx, are usually treated as part of the vagus, and the name accessory nerve usually means the spinal part alone.

Its cell bodies form the spinal accessory nucleus in the outer part of the upper cervical cord's anterior horn, from the junction with the medulla down to about the sixth cervical segment, and develop from the basal plate of those segments. The axons leave the side of the cord as rootlets between the anterior and posterior roots and join into a trunk that ascends behind the denticulate ligament. It passes up into the cranium by way of the foramen magnum, runs along the inside of the occipital bone to the jugular foramen and leaves again alongside nerves IX and X.

In the neck the nerve runs backwards and downwards. Near the posterior belly of digastric it crosses the internal jugular vein, usually in front of it but in a minority behind. It then pierces and supplies sternocleidomastoid and emerges from its back edge into the posterior triangle, where it lies just under the skin and fascia, receiving sensory fibres from C3 and C4, before passing under the front edge of trapezius. The trapezius is controlled mainly by the opposite cerebral hemisphere, while sternocleidomastoid is thought to be driven mainly by the same side.

The nerve is tested by shrugging the shoulders and turning the head against resistance; weak turning to one side points to the sternocleidomastoid of the other side. Because it is so superficial in the posterior triangle, it is the nerve most often damaged in lymph node biopsy and neck dissection. The result is a dropped, aching shoulder, wasting of trapezius, mild winging of the scapula and difficulty raising the arm above the head. Nerve repair or muscle transfer may help. Weakness of both muscles on both sides suggests a general disorder such as motor neuron disease or Guillain-Barre syndrome.

Course and supply

Arises from

The spinal accessory nucleus in the anterior horn of the cervical cord from C1 to about C6; rootlets leave the lateral cord, unite and ascend through the foramen magnum.

Ends in

Leaves the skull through the jugular foramen, supplies sternocleidomastoid, crosses the posterior triangle and ends in the deep surface of trapezius.

Supplies

Motor to sternocleidomastoid (tilting the head and turning the face to the other side) and to trapezius (elevating, retracting and rotating the scapula).