Nerve · Neck

Accessory nerve

Nervus accessorius

The accessory nerve is unusual: its motor fibres arise not from the brain but from the upper cervical spinal cord, climb up through the foramen magnum into the skull, and then leave it again to supply just two muscles — sternocleidomastoid and trapezius. Between those two muscles it crosses the posterior triangle of the neck lying in superficial fascia, covered by little more than skin. That exposed stretch matters clinically: the nerve is easily injured there, and the result is a drooping shoulder, difficulty lifting the arm overhead, and a deep ache through the shoulder girdle.