Muscles flexing the thumb

Flexor pollicis longus

Flexor pollicis brevis

Flexor pollicis brevis

Flexor pollicis brevis is the most medial of the three thenar muscles, usually being partly fused with opponens pollicis. It arises from the distal border of the flexor retinaculum and tubercle of the trapezium(the superficial part), with a deeper attachment to the capitate and trapezoid bones. Both parts form a single tendon containing a sesamoid bone which inserts into the radial side of the base of the proximal phalanx of the thumb.

Nerve supply

Flexor pollicis brevis is usually supplied by the median nerve, root value T1, but frequently the deep part is supplied by the ulnar nerve, root value T1. The skin over the muscle is supplied by root C6. 

Action

Flexor pollicis brevis produces flexion of the thumb at the metacarpophalangeal and carpometacarpal joints. Its continued action tends to produce medial rotation of the thumb, since flexion of the carpometacarpal joint automatically involves this movement.

Palpation

Flexor pollicis brevis can be palpated in the medial part of the thenar eminence if flexion of the thumb is resisted.