The extensor pollicis longus muscle begins at the ulna and the interosseous membrane, a tough fibrous tissue that connects the ulna and the radius in the lower arm. It is a skeletal muscle and is controlled by the motor nervous system. Its fibers are striated, which means they are arranged in parallel. The extensor pollicis longus muscle ends at the distal phalanx (the tip) of the thumb. The muscle serves to extend the thumb. When moving the thumb, the muscle uses the radial tubercle as a pulley. It crosses the tendons of the extensores pollicis brevis, one of the five main muscles involved with movement of the wrist. It also passes over the abductor pollicis longus muscle, one of the muscles of the hand. Motor nerve functions are supplied by the posterior interosseous nerve, which is a branch of the radial nerve. The extensor pollicis longus muscle can spontaneously rupture (especially with suddenly applied force). It is also subject to other trauma injuries of the hand, wrist, and digits.