Which muscles are innervated by the accessory nerve (CN XI)?

• A. Deltoid and pectoralis major
• B. Trapezius and sternocleidomastoid
• C. Sternohyoid and omohyoid
• D. Masseter and temporalis

Answer: (B)

The accessory nerve provides motor innervation to two neck muscles that control head and shoulder movements: the sternocleidomastoid and the trapezius. This explains why these two muscles are the ones affected when CN XI is involved. The sternocleidomastoid enables you to rotate the head and flex the neck, while the trapezius helps elevate the shoulders and rotate the scapula, contributing to shoulder shrug. The other muscles listed are innervated by different nerves: deltoid and pectoralis major by the axillary and pectoral nerves, respectively; sternohyoid and omohyoid by the ansa cervicalis; and masseter and temporalis by the trigeminal nerve (mandibular division).