Which nerve innervates the deltoid?

• A. Medial pectoral nerve
• B. Suprascapular nerve
• C. Dorsal scapular nerve
• D. Axillary nerve

Answer: (D)

Deltoid motor function comes from the axillary nerve. This nerve, arising from the posterior cord of the brachial plexus (C5–C6), travels through the quadrangular space with the posterior circumflex humeral vessels to reach the deltoid and provide its motor innervation (as well as sensory supply to the skin over the shoulder via the superior lateral cutaneous nerve of the arm). Because the axillary nerve supplies the deltoid, damage to it impairs the ability to abduct the arm beyond the initial 15 degrees that is started by the supraspinatus, making deltoid the primary mover for later abduction. The other nerves listed—medial pectoral (to pectoralis major/minor), suprascapular (to supraspinatus and infraspinatus), and dorsal scapular (to rhomboids and levator scapulae)—do not innervate the deltoid.