The radial nerve derives its innervation from the C5-T1 nerve roots and all the three trunks of the brachial plexus. The posterior divisions from all the three trunks unite to form the posterior cord. This posterior cord gives off the axillary, thoracodorsal, and subscapular nerves before it forms the radial nerve.

In the proximal upper arm, the radial nerve gives off the posterior cutaneous nerve of the arm, the lower lateral cutaneous nerve of the arm, and the posterior cutaneous nerve of the forearm. The muscular branches are then given off to the three heads of the triceps and the anconeus muscle at the elbow, then the radial nerve wraps around the posterior humerus in the spiral groove and descends into the region of the elbow. Muscular branches are given off to the brachioradialis and the extensor carpi radialis (longus and brevis). Approximately 3–4 cm distal to the lateral epicondyle the radial nerve divides into a superficial and a deep branch. The superficial branch is the superficial radial sensory nerve, which travels distally into the forearm over the radial bone and supplies sensation to the lateral dorsum of the hand, as well as part of the thumb and the dorsal proximal phalanges of the index, middle, and ring fingers. This superficial radial sensory nerve terminates over the thumb see Fig. 15.1.

The deep radial motor branch supplies the extensor carpi radialis brevis and the supinator muscles before it passes under the arcade of Frohse, which can be a site of entrapment. This arcade is formed by the proximal border of the supinator muscle. As the nerve enters the supinator, it is called the posterior interosseous nerve (PIN). This nerve then supplies all the extensors of the wrist and the digits, such as the extensor digitorum, extensor carpi ulnaris, abductor pollicis longus, extensor indicis proprius, extensor pollicis longus, and extensor pollicis brevis. While there are no cutaneous sensory branches in the PIN, it does supply sensory fibers to the interosseous membrane and the radioulnar joints.

When the radial nerve is injured at the spiral groove a wrist drop results. This is a frequent presentation when a person wraps their arm over a chair or a bench during deep sleep or when in a state of intoxication (Saturday night palsy). A wrist drop can also result from strenuous muscular effort, fracture of the humerus, injury to the posterior cord, C7 radiculopathy, or from a vasculitic lesion. A wrist drop can also occur with the isolated involvement of the PIN that involves only the motor fibers and spares the superficial radial sensory nerve.