Which two muscles are innervated by CN XI?

Which two muscles are innervated by CN XI?

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

Answer: (C)

The main idea here is understanding which muscles receive motor input from the spinal accessory nerve (CN XI). This nerve innervates two key muscles of the neck and shoulder girdle: the sternocleidomastoid and the trapezius. The sternocleidomastoid helps you turn your head to the opposite side and flex the neck, while the trapezius elevates and retracts the scapula, aiding shoulder shrug and shoulder girdle stabilization. The other muscles mentioned are innervated by different nerves: the masseter and temporalis by the trigeminal nerve (mandibular division), the deltoid by the axillary nerve and the pectoralis major by the pectoral nerves from the brachial plexus, and the sternohyoid and omohyoid by branches of the cervical plexus (ansa cervicalis). Therefore, the muscles innervated by CN XI are the trapezius and sternocleidomastoid.

The main idea here is understanding which muscles receive motor input from the spinal accessory nerve (CN XI). This nerve innervates two key muscles of the neck and shoulder girdle: the sternocleidomastoid and the trapezius. The sternocleidomastoid helps you turn your head to the opposite side and flex the neck, while the trapezius elevates and retracts the scapula, aiding shoulder shrug and shoulder girdle stabilization. The other muscles mentioned are innervated by different nerves: the masseter and temporalis by the trigeminal nerve (mandibular division), the deltoid by the axillary nerve and the pectoralis major by the pectoral nerves from the brachial plexus, and the sternohyoid and omohyoid by branches of the cervical plexus (ansa cervicalis). Therefore, the muscles innervated by CN XI are the trapezius and sternocleidomastoid.