Nerve
Accessory nerve (CN XI)
Trapezius and sternocleidomastoid
High-yield
Purely motor to SCM and trapezius; vulnerable in the posterior triangle of the neck → shoulder droop and weak head turning.
Overview
The eleventh cranial nerve, supplying the sternocleidomastoid and trapezius via its spinal root that ascends into the skull and re-exits.
Reference facts
- Roots
Motor (spinal root C1–C5/C6)
- Course
The spinal root ascends through the foramen magnum, joins the cranial root briefly, exits the jugular foramen, and crosses the posterior triangle of the neck
- Motor
Sternocleidomastoid and trapezius
- Sensory
None
Clinical correlation
Iatrogenic injury during lymph node biopsy in the posterior triangle causes trapezius weakness and shoulder droop.