Muscles of the thumb

The most superficial muscle in the thenar group is the abductor pollicis brevis. The abductor brings the thumb away from the other four fingers. The flexor pollicis brevis, which lies next to the abductor, will flex the thumb, curling it up in the palm. The opponens pollicis lies deep to abductor pollicis brevis. As its name suggests, it opposes the thumb, bringing it against the fingers. This is a very important movement, as most of human hand dexterity including grip comes from this action.

The abductor pollicis brevis originates on the scaphoid tubercle and the flexor retinaculum. It inserts to the radial sesamoid bone and the proximal phalanx of the thumb. It is innervated by the median nerve (C8 and T1).[6]

The flexor pollicis brevis is a two-headed muscle. The superficial head arises on the flexor retinaculum, while the deep head originates on three carpal bones: the trapezium, trapezoid, and capitate. The muscle is inserted onto the radial sesamoid bone of the metacarpophalangeal joint. It acts to flex, adduct, and abduct the thumb, and is therefore also able to oppose the thumb. The superficial head is innervated by the median nerve, while the deep head is innervated by the ulnar nerve (C8-T1).[6] Due to a common interconnection between the median and ulnar nerves in the hand (Riche-Cannieu interconnection), the median nerve may innervate the flexor pollicis brevis in 35% of people. It is innervated by the ulnar nerve in 50% of people and by both the median and ulnar nerves in 15%.

The opponens pollicis originates on the tubercle of the trapezium and the flexor retinaculum. It is inserted onto the radial side of the first metacarpal. It opposes the thumb and assists in adduction. It is innervated by the median nerve.[6] There are normal variations in the muscles nerve innervation. In a Cannieu-Riche anastomosis, fibers from the deep palmar branch of the ulnar nerve innervate the opponens pollicis and/or abductor pollicis brevis. Regardless of their final innervation, the nerves that reach the thenar muscles arise from the C8 and T1 roots, pass through the lower trunk of the plexus, and then through the medial cord of the plexus.

The adductor pollicis also has two heads. It lies deeper and more distal to flexor pollicis brevis. Despite its name, its main action is mainly rotation and opposition. The transversal head originates along the entire third metacarpal bone, while the oblique head originates on the carpal bones proximal to the third metacarpal. The muscle is inserted onto the ulnar sesamoid bone of the metacarpophalangeal joint. It adducts the thumb, and assists in opposition and flexion. It is innervated by the deep branch of the ulnar nerve (C8-T1).[6]

The first dorsal interosseous, one of the central muscles of the hand, extends from the base of the thumb metacarpal to the radial side of the proximal phalanx of the index finger.[7]