Deep Radial Nerve

The Deep Radial nerve penetrates the supinator muscle, and wraps around the lateral aspect of the neck of the radius. It is entirely muscular in its distribution. The posterior interosseus is the continuation of the deep radial nerve at the interval between the abductor pollicis longus and extensor indicis.

When the radial nerve is injured in the radial groove, the triceps are weakened because of loss of the medial head, but the posterior forearm muscles are paralyzed, resulting in wrist drop. Direct severance of the deep branch of the radial nerve results in an inability to extend the thumb and the metacarpophalangeal joints.