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Accessory Nerve (CN XI) — Full

nervus accessorius (cranialis et spinalis)

The accessory nerve has two functional components: the cranial part joins the vagus for laryngeal innervation, and the spinal part supplies the SCM and trapezius. The spinal accessory nerve crosses the posterior triangle superficially between these two muscles — the most vulnerable major nerve to surgical injury in the neck. Its injury produces trapezius palsy with shoulder drop and winged scapula from trapezius weakness.

Region: Head & Skull

Anatomical Data

Origin, Insertion & Supply

Origin	Cranial part: nucleus ambiguus in the medulla. Spinal part: C1-C5 anterior horn (ascends through foramen magnum to join the cranial root briefly)

Clinical Relevance

Clinical Notes

The spinal accessory nerve is at risk during posterior cervical lymph node biopsy, neck dissection, and internal jugular vein cannulation. Trapezius palsy produces: shoulder drop, loss of shoulder shrug, difficulty abducting the arm above 90 degrees, and the characteristic lateral scapular winging of trapezius weakness (distinct from the medial winging of serratus anterior palsy). Physiotherapy and Eden-Lange transfer for permanent palsy.

Pathology

Common Injuries & Conditions

SAN Palsy from Neck Dissection

Spinal accessory nerve injury producing trapezius paralysis and shoulder morbidity managed with nerve repair or Eden-Lange transfer for permanent cases.