Posterior interosseous nerve

After crossing the supinator muscle, the deep branch of the radial nerve, considerably diminished in size, descends as the posterior interosseous nerve (or dorsal interosseous nerve), on the interosseous membrane, in front of the Extensor pollicis longus, to the back of the carpus, where it presents a gangliform enlargement from which filaments are distributed to the ligaments and articulations of the carpus.

It supplies all the muscles on the radial side and dorsal surface of the forearm, excepting the Anconæus, Brachioradialis, and Extensor carpi radialis longus.