Which cranial nerve innervates the sternocleidomastoid and trapezius muscles?

• A. Accessory nerve
• B. Facial nerve
• C. Trochlear nerve
• D. Glossopharyngeal nerve

Answer: (A)

The accessory nerve (cranial nerve XI) supplies the sternocleidomastoid and trapezius muscles. These two muscles are responsible for turning the head and shrugging the shoulders, and they receive motor input from CN XI, which is primarily motor and uniquely involved with these neck muscles. Clinically, testing this nerve involves having the patient shrug the shoulders against resistance and turn the head against resistance to assess strength of the trapezius and sternocleidomastoid, respectively.

The other nerves listed do not innervate these muscles: the facial nerve controls facial expression muscles; the trochlear nerve innervates the superior oblique muscle of the eye; and the glossopharyngeal nerve supplies stylopharyngeus (and several sensory functions), not the SCM or trapezius.