Medicine & USMLE

Accessory Nerve (CN XI)

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Cranial Nerves
  1. Olfactory Nerve (CN I)
  2. Optic Nerve (CN II)
  3. Oculomotor Nerve (CN III)
  4. Trochlear Nerve (CN IV)
  5. Trigeminal Nerve (CN V)
  6. Abducens Nerve (CN VI)
  7. Facial Nerve (CN VII)
  8. Vestibulocochlear Nerve (CN VIII)
  9. Glossopharyngeal Nerve (CN IX)
  10. Vagus Nerve (CN X)
  11. Accessory Nerve (CN XI)
  12. Hypoglossal Nerve (CN XII)

Summary

The accessory nerve, also known as the spinal accessory nerve, is the 11th cranial nerve or cranial nerve number eleven (CN XI). The accessory nerve is a motor nerve that sends nerve signals to the sternocleidomastoid and trapezius muscles to produce the movements of head turning and shoulder shrugging. Despite its historical categorization as a cranial nerve, this nerve actually originates from the upper spinal cord, just below the medulla, enters the skull via the foramen magnum, and then exits the skull again at the jugular foramen.

Key Points

  • (Spinal) Accessory Nerve (Cranial Nerve XI)
    • Nerve type
      • Motor
    • Function
      • Head turning and shoulder shrugging, abduction of arm above horizontal
        • Innervates sternocleidomastoid (SCM)
        • Innervates trapezius muscle
        • Test by assessing strength of head turn and shoulder shrug against resistance
    • Nerve Entry/Exit
      • Exits from upper spinal cord
        • The spinal accessory nucleus is located in anterior horn of spinal cord from medulla-spinal cord junction to C6
    • Cranial foramen
      • Jugular foramen
    • Reflexes
      • None
    • Other information
      • May be affected by injury to posterior triangle of neck
        • E.g. penetrating injury (knife) or surgery (e.g. cervical lymph node dissection)
        • Damage also results in winging of the scapula via loss of innervation to trapezius