The Spinal Accessory Nerve (Cranial Nerve XI)

The spinal accessory nerve (cranial nerve XI) provides motor innervation of the sternocleidomastoid and trapezius muscles with essentially no sensory component. The nerve emerges under the sternocleidomastoid muscle to lie on the levator scapulae and middle scalene muscles ventral to the anterior border of the trapezius muscle. Variations in the course and surface landmarks of the nerve have led investigators to study its sonographic anatomy. Because of its superficial location near lymph nodes, the spinal accessory nerve can be injured during cervical lymph node biopsy.

The spinal accessory nerve is among the smallest nerves imaged with ultrasound, with a transverse diameter of about one mm and monofascicular in sonographic appearance (Video 768 KB).