Accessory nerve (CN XI)

Nervus accessorius

Also known as: CN XI, eleventh cranial nerve, spinal accessory nerve

A motor nerve to sternocleidomastoid and trapezius. It arises from the upper cervical spinal cord, ascends into the skull through the foramen magnum and leaves again through the jugular foramen. Its superficial course across the posterior triangle of the neck makes it vulnerable during surgery.

Overview

OriginSpinal accessory nucleus, C1–C5(6)

Skull passageIn via foramen magnum, out via jugular foramen

Muscles suppliedSternocleidomastoid, trapezius

The functional accessory nerve is the spinal accessory nerve. Its cell bodies form the spinal accessory nucleus in the lateral anterior horn of segments C1–C5 (C6). Rootlets leave the lateral surface of the cord between the dorsal and ventral roots and unite into a trunk. The trunk ascends through the foramen magnum behind the vertebral artery and then exits through the jugular foramen with IX and X. A small "cranial root" from the nucleus ambiguus joins the vagus almost immediately and is functionally part of CN X; many modern texts regard CN XI as purely spinal.

Below the skull the nerve passes deep to the posterior belly of digastric and supplies and usually pierces sternocleidomastoid. It then crosses the posterior triangle of the neck obliquely in the investing fascia, superficial to levator scapulae, where it is covered only by skin and fascia. It enters the deep surface of trapezius about 5 cm above the clavicle. Cervical plexus fibres (C2–C4) join it and are mainly proprioceptive.

Location

Upper cervical cord (C1–C5) → foramen magnum → jugular foramen → sternocleidomastoid → posterior triangle → trapezius.

Parts & subdivisions

Functional components

Motor only. It is usually classified as branchial motor (SVE), because SCM and trapezius are thought to come partly from occipital somites and pharyngeal-arch mesoderm. Some texts classify it as GSE.

Clinical testing

Shrug the shoulders against resistance (trapezius). Turn the head against resistance while palpating the SCM. The right SCM turns the face to the left, so a right XI lesion weakens turning to the left.

Lesion signs

Ipsilateral shoulder droop, weak shoulder shrug, difficulty abducting the arm above 90° (loss of scapular upward rotation), and mild lateral scapular winging. If the lesion is proximal, there is also weakness of head turning to the opposite side and visible wasting of SCM.

Anatomy

Innervation

Sternocleidomastoid and trapezius (motor). Proprioception is carried with C2–C4 contributions.

Course

Spinal accessory nucleus (C1–C5/6) → rootlets ascend in the subarachnoid space → foramen magnum → posterior cranial fossa → jugular foramen → descends posterolateral to the IJV → deep to and through sternocleidomastoid → emerges near the junction of the upper and middle thirds of the posterior border of SCM (above Erb's point) → crosses the posterior triangle on levator scapulae → deep surface of trapezius.

Clinical significance

Iatrogenic injury in the posterior triangle

The nerve lies superficially in the posterior triangle and is one of the nerves most often injured iatrogenically. Injury occurs during cervical lymph node biopsy and neck dissection. Typical symptoms are a dragging shoulder ache, droop and weakness of shoulder abduction. Trapezius is affected but SCM is spared, because its branch arises higher.

Torticollis

Cervical dystonia (spasmodic torticollis) involves involuntary contraction of SCM and trapezius. It is treated with botulinum toxin injections into these muscles, and occasionally with selective denervation of the accessory nerve.