Nerve · Forearm

Posterior interosseous nerve

Nervus interosseus antebrachii posterior

The motor nerve of the back of the forearm. It is the deep branch of the radial nerve renamed at the moment it emerges from the far side of supinator, having spiralled round the neck of the radius inside that muscle. It then runs down on the interosseous membrane, deep to the superficial extensors, supplying the muscles that straighten the fingers and thumb and pull the wrist backwards, and ends in a small swelling on the back of the wrist that supplies the carpal joints. Like its counterpart on the front of the forearm it reaches no skin, so weakness of finger extension without any numbness points to this nerve rather than to the radial nerve as a whole.