Accessory Nerve (CN XI)

The accessory nerve is the eleventh cranial nerve, unique in having both cranial and spinal roots. The spinal accessory nerve (from cervical spinal cord) is clinically significant, innervating the sternocleidomastoid and trapezius muscles for head and shoulder movement.

Origin

Spinal root: C1-C5 spinal cord (spinal accessory nucleus); Cranial root: nucleus ambiguus (joins vagus)

Foramina

Jugular foramen

Course

Spinal root ascends through foramen magnum → joins cranial root → exits via jugular foramen → descends through posterior triangle of neck

Functions

•Head rotation (sternocleidomastoid)
•Shoulder elevation and scapular rotation (trapezius)
•Cranial portion contributes to vagal functions

Branches

Cranial root (joins vagus for pharyngeal/laryngeal function)

Spinal root to SCM and trapezius

Structures Innervated

→Sternocleidomastoid muscle
→Trapezius muscle

Clinical Testing

Test shoulder shrug against resistance (trapezius). Test head turn against resistance (SCM turns head to OPPOSITE side). Look for shoulder droop and scapular winging.

Clinical Relevance

CN XI is superficial in the posterior triangle, vulnerable to surgery (lymph node biopsy) and trauma. Injury causes shoulder droop, scapular winging, weakness turning head to opposite side, and difficulty shrugging.

Study Tips

✓SCM turns head to OPPOSITE side (left SCM turns head right)
✓CN XI = SCM and Trapezius (think "11" looks like two vertical muscles)
✓Vulnerable in posterior triangle - warn before lymph node biopsy
✓Trapezius weakness causes shoulder droop and scapular winging

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Accessory Nerve (CN XI) FAQs

Common questions about this cranial nerve

The sternocleidomastoid attaches from the sternum and clavicle to the mastoid process. When one SCM contracts, it pulls the mastoid (behind the ear) forward, rotating the face to the opposite side. Bilateral contraction flexes the neck.

After exiting the jugular foramen, CN XI runs superficially through the posterior triangle of the neck before innervating the trapezius. Surgical procedures like lymph node biopsies or trauma in this region can damage the nerve.

Trapezius weakness from CN XI damage causes the scapula to protrude ("wing") when pushing against a wall. The upper trapezius normally holds the scapula against the thorax. Note: serratus anterior weakness (long thoracic nerve) also causes winging with different mechanism.