MedCourseFinder

Cranial nerves · Motor

XI. Accessory nerve

Not medically reviewed · Content last updated · 2 min read ·

This is the date the draft was written, not a medical review date.

The accessory nerve (CN XI) supplies the sternocleidomastoid and trapezius muscles. Its spinal root arises from the upper cervical cord, ascends through the foramen magnum and then leaves the skull with the vagus and glossopharyngeal nerves.

Terminologia Anatomica (Latin): Nervus accessorius

Function

Special visceral efferent (classically described) to sternocleidomastoid and trapezius. The cranial root, which joins the vagus nerve to supply laryngeal muscles, is now generally regarded as part of the vagus. Proprioceptive fibres arrive from the cervical plexus.

Nuclei

Spinal accessory nucleus in the lateral part of the anterior horn of the upper cervical cord (roughly C1 to C5 or C6). The cranial fibres originate from the nucleus ambiguus. Supranuclear control for the sternocleidomastoid is ipsilateral in effect (turns the head to the opposite side), and for the trapezius is mainly contralateral.

Exit from the skull

The spinal roots ascend alongside the cord, enter the skull through the foramen magnum, and leave through the jugular foramen. The nerve then passes to the sternocleidomastoid and crosses the posterior triangle of the neck superficially to reach the trapezius.

Testing

Ask the patient to shrug the shoulders against resistance (trapezius) and to turn the head against the examiner's hand (sternocleidomastoid; the muscle on the side opposite to the direction of turning is tested). Inspect for wasting, shoulder drooping and winging.

Signs of a lesion

Ipsilateral weakness of the trapezius (drooping shoulder, weak shrug, difficulty abducting the arm above horizontal) and of the sternocleidomastoid (weak head rotation toward the opposite side). A jugular foramen lesion usually also involves CN IX and X.

Clinical correlations

The spinal accessory nerve is vulnerable in the posterior triangle of the neck, where it lies superficially, during lymph node biopsy, neck dissection or penetrating trauma, producing shoulder drop and winging. Stroke affecting the supranuclear pathway produces different patterns of weakness.

Common exam points

  • Supplies sternocleidomastoid and trapezius.
  • Spinal root ascends through the foramen magnum and leaves via the jugular foramen.
  • Superficial in the posterior triangle: at risk in lymph node biopsy and neck surgery.
  • Lesion: drooping shoulder and weak shrug; weak head turn to the opposite side.

References

  1. Drake RL, Vogl AW, Mitchell AWM. Gray's Anatomy for Students. Elsevier — Tier 3 (textbook)
  2. Moore KL, Dalley AF, Agur AMR. Clinically Oriented Anatomy. Wolters Kluwer, 8th edition (2018) — Tier 3 (textbook)
  3. Haines DE, Mihailoff GA. Fundamental Neuroscience for Basic and Clinical Applications. Elsevier, 5th edition (2018) — Tier 3 (textbook)
  4. Bickley LS. Bates' Guide to Physical Examination and History Taking (Bickley). Wolters Kluwer, 13th edition (2020) — Tier 3 (textbook)

References are listed only where the article cites them. See the sources page for the full policy. How sources are chosen and credited

Back to the index