Accessory Nerve (CN XI): Course, Posterior Triangle and Palsy

Nuclei and fibre types

CN XI is a motor nerve. It is classically described as having two roots, though most modern texts treat only the spinal root as the true accessory nerve.

The fibre type of the spinal root is debated. Many texts call it special visceral efferent (SVE), grouping it with branchial motor nerves; others call it general somatic efferent (GSE), because sternocleidomastoid and trapezius develop at least partly from somites. Because the cranial root is functionally part of the vagus, the nerve described below is the spinal part.

Emergence and skull foramen

The spinal root has an unusual route: it enters the skull before leaving it. Its rootlets emerge from the lateral surface of the upper cervical cord, between the dorsal and ventral roots and behind the denticulate ligament. They join into a trunk that ascends alongside the cord and passes up through the foramen magnum, behind the vertebral artery.

Inside the posterior cranial fossa it turns laterally and leaves through the jugular foramen, sharing a dural sheath with the vagus. The cranial rootlets, where described, emerge from the postolivary sulcus of the medulla below the vagal rootlets and join it here.

Course and branches

Below the skull, CN XI runs backward and downward through sternocleidomastoid and then across the posterior triangle to trapezius.

1. Upper neck: it passes backward, usually superficial to the internal jugular vein (sometimes deep to it), deep to the styloid process and the posterior belly of digastric, and is crossed by the occipital artery.
2. Sternocleidomastoid: it enters the deep surface of the upper part of the muscle, supplies it, and emerges from its posterior border roughly a third of the way down, a little above the point where the great auricular nerve winds round the muscle.
3. Posterior triangle: it crosses the triangle obliquely downward and backward, lying on levator scapulae within or just under the investing layer of deep cervical fascia, with only skin and fascia above it.
4. Trapezius: it passes under the anterior border of trapezius a few centimetres above the clavicle and runs on its deep surface.

In the posterior triangle it receives branches from the cervical plexus (C2 to C4). These carry proprioception and may add some motor supply to trapezius, which partly explains the variable weakness after injury. A surface marking runs from the junction of the upper and middle thirds of the posterior border of sternocleidomastoid to the junction of the middle and lower thirds of the anterior border of trapezius.

Clinical testing

CN XI is tested by asking the patient to shrug the shoulders and turn the head against resistance.

• Trapezius: inspect for wasting and a drooping shoulder, then ask the patient to shrug against downward pressure. Ask them to abduct the arm above 90 degrees; trapezius rotates the scapula upward to allow this.
• Sternocleidomastoid: ask the patient to turn the head to one side against the examiner's hand on the chin. The contracting muscle is on the opposite side, so turning to the left tests the right sternocleidomastoid. Pushing the head forward against resistance tests both muscles.
• Scapula: look from behind for winging.

Lesions and palsies

Injury to CN XI in the posterior triangle weakens trapezius but spares sternocleidomastoid, because the nerve supplies sternocleidomastoid before it enters the triangle. The patient has a drooping shoulder, weak shrug, difficulty lifting the arm above shoulder height and a dull ache around the shoulder.

The scapula sits lower and further from the midline, with lateral winging that is most obvious on abduction. This differs from serratus anterior palsy (long thoracic nerve), which causes medial winging, most obvious when pushing against a wall.