Accessory nerve (CN XI)
Nerve · CN XI Motor
The spinal root arises from anterior horn cells of C1–C5 (some texts to C6), ascends through the foramen magnum, then leaves through the jugular foramen and crosses the posterior triangle of the neck just under the skin and platysma. The cranial root, from the nucleus ambiguus, simply joins the vagus. Purely motor to sternocleidomastoid and trapezius.
Traced from the start
- Medulla oblongata
- Accessory nerve (CN XI)
Detail
- Motor
- Sternocleidomastoid (turns the head to the opposite side and tilts it to the same side; C1–C2 fibres) and trapezius (elevates and rotates the scapula; shrug and abduction above 90 degrees; C3–C4 fibres)
- Course
- Enters the deep surface of sternocleidomastoid, emerges from its posterior border at the junction of upper and middle thirds, crosses the posterior triangle superficially on levator scapulae and enters trapezius two fingerbreadths above the clavicle
- Test
- Shoulder shrug against resistance (trapezius); turn the head against the examiner's hand and palpate the opposite sternocleidomastoid
When it goes wrong
Iatrogenic injury during lymph node biopsy or neck dissection in the posterior triangle, where the nerve lies superficially
Shoulder droop, difficulty shrugging and abducting the arm above the shoulder, mild scapular winging and chronic shoulder pain; sternocleidomastoid is spared because its branch has already left
Jugular foramen lesion (with IX and X signs)
Weakness turning the head to the opposite side and of shoulder elevation on the side of the lesion, with wasting of sternocleidomastoid and trapezius
Practise this structure
4 questions in the bank tagged Cranial Nerves.
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A 44-year-old man has had 18 months of worsening hearing on the left with constant ringing in that ear. He still follows conversation in a quiet room but cannot…
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A 58-year-old man has sudden vertigo and a hoarse voice, and chokes when he drinks. The palate elevates to the right and there is no gag response when the left…
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A 27-year-old man was struck on the head at football a week ago. He now sees two images, one above the other, worst when reading or going down stairs. The verti…
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