What is the anatomy and location of the Extensor pollicis longus muscle in the human body?

The Extensor pollicis longus (EPL) is a narrow, elongated muscle located in the deep posterior compartment of the forearm. It originates from the middle third of the posterior surface of the ulna and the adjacent interosseous membrane. The muscle belly runs distally and converts into a tendon that passes through a separate compartment (the third dorsal compartment) of the extensor retinaculum at the wrist. This tendon then crosses obliquely over the tendons of the extensor carpi radialis longus and brevis muscles, creating a visible anatomical landmark known as the "anatomical snuffbox" on the lateral aspect of the wrist. The EPL tendon ultimately inserts onto the base of the distal phalanx of the thumb. Functionally, this muscle extends the thumb at both the interphalangeal and metacarpophalangeal joints, and also assists in abduction and extension of the wrist. The EPL is innervated by the posterior interosseous nerve, a branch of the radial nerve (C7-C8), and receives its blood supply from branches of the posterior interosseous artery. Understanding the anatomy of this muscle is important when evaluating thumb extension weakness, tendon ruptures (which can occur after distal radius fractures), or when identifying the anatomical snuffbox during physical examination.