Nerve · Upper arm

Ulnar nerve

Nervus ulnaris

The ulnar nerve is the nerve of the hand's fine machinery: it supplies almost all the small muscles between the bones of the palm — the ones that spread and close the fingers and give grip its precision — plus the little-finger side of the hand's skin. Everyone has met it, because it runs in a shallow groove behind the bony bump on the inside of the elbow with nothing but skin over it: knocking the "funny bone" is a direct hit on the nerve. That same groove, the cubital tunnel, is the second most common entrapment site in the body: the tunnel narrows as the elbow bends, so symptoms come on with a bent elbow held for a long time — phoning, or sleeping curled up — and give tingling in the ring and little fingers, a weakening pinch, and eventually visible hollowing between the thumb and index finger on the back of the hand. It can also be squeezed at the wrist in Guyon's canal, classically by handlebar pressure in cyclists; there the back of the hand keeps its feeling, because the branch supplying that skin leaves before the canal.