The thenar eminence is made up of the three short thumb muscles whose origin is essentially from the flexor retinaculum. The most radial of these is abductor pollicis brevis. It arises from the flexor retinaculum and the tubercle of the scaphoid and is inserted into the radial side of the base of the proximal phalanx and the tendon of extensor pollicis longus.
Flexor pollicis brevis lies to the ulnar side of the abductor. It arises from the flexor retinaculum and adjacent trapezium and is inserted into the radial sesamoid of the thumb and so to the radial border of the proximal phalanx. Irregularities of origin are fairly common and have been given many names. The least confusing descriptive nomenclature is to call any fibers that are inserted into the radial sesamoid flexor pollicis brevis, no matter what their origin; fibers inserted into the ulnar sesamoid should not be named flexor pollicis brevis, but considered part of either adductor pollicis or the first palmar interosseous.
Opponens pollicis lies deep to the former two muscles. It arises from the flexor retinaculum and the adjacent trapezium and is inserted into the whole of the radial border of the metacarpal bone of the thumb.
Nerve supplies of thenar muscles:
All three muscles are supplied by the muscular (recurrent) branch of the median nerve (mainly T1 but probably with some component from C8), but it is necessary to qualify this simple statement. The nerve supply of flexor pollicis brevis is subject to more variation than that of any other muscle in the body. It may be from the muscular branch of the median nerve or the deep branch of the ulnar nerve, or it may have a double supply from both nerves. The opponens usually has a double supply, and there may be other anomalies including the supply of all three by the ulnar nerve alone. All these possibilities must be kept in mind when testing for nerve injuries.
Actions of thenar muscles:
The actions of the three muscles are indicated by their names. The abductor abducts the thumb (moving it in a plane at right angles to the palm), and it also abducts the proximal phalanx at the metacarpophalangeal joint and slightly rotates the phalanx. It is thus an indispensible aid in opposition of the thumb. The flexor flexes the proximal phalanx and draws the thumb across the palm, and the opponens opposes the metacarpal of the thumb.