Origin: Trapezoid, flexor retinaculum
Insertion: Thumb, proximal phalanx
Nerve: Median nerve, deep branch of ulnar nerve(medial head)
Action: Flexion of thumb
Antagonist: Extensor pollicis longus muscle, Extensor pollicis brevis muscle
The Flexor pollicis brevis consists of two portions, superficial and and deep head:
- The superfical head (lateral and more superficial portion) arises from the lower border of the transverse carpal ligament and the lower part of the ridge on the greater multangular bone; it passes along the radial side of the tendon of the Flexor pollicis longus, and, becoming tendinous, is inserted into the radial side of the base of the first phalanx of the thumb; in its tendon of insertion there is a sesamoid bone.
- The deep head (medial and deeper portion) of the muscle is very small, and arisesfrom the ulnar side of the first metacarpal bone between the Adductor pollicis (obliquus) and the lateral head of the first Interosseous dorsalis, and is inserted into the ulnar side of the base of the first phalanx with the Adductor pollicis (obliquus). The medial part of the Flexor brevis pollicis is sometimes described as the first Interosseous volaris.