The muscles of thumb of the human hand are eight in number. There are three flexors, one adductor, two abductors, and two extensors. They are voluntarily controlled with precision by a large area in the precentral central gyrus (motor strip) of the brain frontal lobe. The great precision and strength of these muscles are the result of the human brain cortex evolution. Thus, the muscles that move the thumb are:
1) the Flexor pollicis longus.- It flexes the thumb when you close it tightly, drawing it towards the palm of hand. It is supplied by a branch of the median nerve;
2) the flexor pollicis brevis.- It also flexes the thumb to close it, pulling it inwardly towards the center of hand; it usually works together with the one mentioned above and the flexors of the four fingers when you grasp something. The first one is innervated by the anterior interosseous nerve, which is a branch of the median nerve, while the flexor pollicis brevis is supplied by a branch of the ulnar nerve;
3) oppenens pollicis muscle.- It arises from the crest of the trapezium bone and the flexor retinaculum, and it is inserted into the first metacarpal bone. It pulls the ball of thumb towards the center of palm.
4) adductor pollicis.- It is a short broad muscle that adducts the thumb, drawing it sideways and inwardly towards the hand and base of the forefinger. It originates from by two heads from the capitate bone of wrist (the largest carpal bone) and the base of second metacarpal bone (of forefinger) and it is attached to the medial side of proximal phalanx of thumb. It is supplied by a deep branch of the ulnar nerve;
5) abductor pollicis brevis.- It opens up the thumb, pulling it away from the forefinger. It is innervated by a recurrent branch of the median nerve.
6) the abductor pollicis longus.- This muscle works together with the brevis to open up the thumb, but it is innervated by a branch of the radial nerve.
7) the extensor pollicis brevis.- It extends and abducts the thumb, pulling it away and backwards. It arises from dorsal aspect of distal radius and is inserted in the posterior side of base of thumb proximal phalanx. It is innervated by a branch of the posterior interosseous nerve.
8) the extensor pollicis longus.- It extends the distal phalanx of thumb and is supplied also by a branch of the posterior interosseous nerve, which arises from the radial.
Below, view of right hand showing five muscles of thumbs.
