Flexor pollicis longus

The flexor pollicis longus muscle is a deep muscle of the anterior compartment of the forearm, exclusively responsible for flexing the thumb.

Origin

Its primary origin is from the anterior surface of the radius and the adjacent interosseous membrane. The accessory heads might arise from the medial epicondyle of the humerus or the coronoid process of the ulna.

Course and tendon

The flexor pollicis longus muscle lies laterally to the flexor digitorum profundus. Its tendon passes through the carpal tunnel, specifically lateral to the tendons of the flexor digitorum profundus, before reaching the thumb.

Insertion

The tendon of the flexor pollicis longus inserts onto the base of the distal phalanx of the thumb.

Innervation

The flexor pollicis longus is innervated by the anterior interosseous nerve, which is a branch of the median nerve. The anterior interosseous nerve also supplies the lateral part of the flexor digitorum profundus and the pronator quadratus, interlinking these three muscles functionally and anatomically.

Actions

Given its insertion onto the distal phalanx of the thumb, the flexor pollicis longus's primary action is to flex the interphalangeal joint of the thumb. It also assists in flexing the metacarpophalangeal and carpometacarpal joints of the thumb, as well as the wrist.