Nerve · Upper arm

Radial nerve

Nervus radialis

The radial nerve is the great extensor nerve of the arm: it straightens the elbow, the wrist and the fingers, and it supplies the skin of the back of the arm, forearm and the back of the thumb side of the hand. It spirals around the back of the humerus in a groove in the bone, and that groove is its most famous vulnerability — sustained pressure there (an arm draped over a chair back through a heavy sleep, or a mid-shaft humeral fracture) produces wrist drop, where the hand hangs and the fingers cannot be straightened, with numbness over the back of the hand. Lower down it splits: the posterior interosseous branch dives through the supinator under a fibrous arch and is a pure motor nerve, so compression there causes finger-drop without numbness and, because one wrist extensor is supplied above the split, the wrist still extends but deviates to the thumb side. A third pattern, radial tunnel syndrome, produces aching in the outer forearm just below the elbow that mimics tennis elbow but is tender a little further down.