The deltoid muscle, a primary shoulder abductor, is innervated by which nerve?

• A. Musculocutaneous nerve
• B. Radial nerve
• C. Axillary nerve
• D. Median nerve

Answer: (C)

The deltoid muscle is innervated by the axillary nerve. This nerve comes from the posterior cord of the brachial plexus (C5–C6) and travels through the quadrangular space with the posterior circumflex humeral artery to reach the deltoid. The deltoid is the main mover for shoulder abduction after the first 15 degrees, with the initial 0–15 degrees of abduction primarily performed by the supraspinatus. Therefore, when the axillary nerve is intact, the deltoid can abduct the arm from about 15 to 90 degrees and beyond with trapezius and serratus anterior assistance. If the axillary nerve is compromised, abduction is weak beyond the initial portion, reflecting its role. The other nerves supply different muscle groups: musculocutaneous to the anterior arm, radial to posterior arm and forearm extensors, and median to many forearm flexors and hand muscles.