Abductor pollicis longus

The abductor pollicis longus (APL) is a muscle found in the deep layer of the posterior compartment of the forearm. As it descends, it becomes superficial and passes under the extensor retinaculum and through the 1st extensor compartment of the wrist before attaching distally. It is one of the extrinsic muscles of the hand.

Summary
origin: posterior surfaces of ulna, radius and interosseous membrane
insertion: via tendon, usually dividing into two slips, one attaching to the base of 1st metacarpal, the other attaching to the trapezium
arterial supply: proximally by the posterior interosseous artery and distally by a branch from the anterior interosseous artery
innervation: posterior interosseous nerve (C7 and C8), a continuation of the deep branch of the radial nerve (C7, C8)
action: abducts and extends the thumb at the carpometacarpal joint
Variant anatomy
presence of multiple slips instead of a single APL tendon 4
 a vertical septum spliting the 1st extensor compartment into two distinct subcompartments (i.e. APL lies in its own tunnel) 4.
References
Related Radiopaedia articles
Anatomy: Upper limb
skeleton of the upper limb[+]
joints of the upper limb[+]
spaces of the upper limb[+]
muscles of the upper limb
shoulder girdle[+]
anterior compartment of the arm[+]
posterior compartment of the arm[+]
anterior compartment of the forearm[+]
posterior compartment of the forearm (extensors)
superficial[+]
deep
supinator[+]
abductor pollicis longus
extensor pollicis brevis
extensor pollicis longus
extensor indicis
hand (mnemonic)[+]
accessory muscles[+]
blood supply to the upper limb[+]
innervation of the upper limb[+]
lymphatic drainage of the upper limb[+]