which nerve extends down the thumb side of the arm into the back of the hand
The superficial branch of the radial nerve is responsible for delivering sensation from the back of the hand, thumb, and first two fingers to the brain. If the radial nerve is interrupted, numbness of the back of the hand can result. Damage to the radial nerve is less common than the medial nerve. The trauma is also more obvious, as well. Rather than a subtle swelling pinching the nerve, the cause may be a bone fracture in the hand, for example.