Significance of the innervation pattern of the human abductor pollicis longus muscle.

The abductor pollicis longus muscle (APL) can be separated into 2 divisions, deep and superficial. The deep division has more bellies, each of which has its own site of insertion. The superficial division is situated on top of the deep tendon. Each division is innervated by a separate branch of the radial nerve. This pattern of innervation may indicate independent actions by individual muscle parts. The deep division, inserting around the carpometacarpal joint of the thumb, has a nerve supply that reaches each separate muscle belly. This division should have a stabilising action on the joint. The superficial division, together with the extensor pollicis brevis, is innervated from the ulnar side by a terminal branch of the radial nerve. This division should, in cooperation with the extensor pollicis brevis and longus muscles and the thenar muscles, act to move the thumb in various directions.