The radial nerves are each one of the primary nerves in the arm. They branch off from a larger bundle of nerves, the brachial plexus, and run down the back of the arm, terminating in the nerves that reach smallest finger of each hand. There are several diseases that affect this nerve which all have similar symptoms, such as difficulty in straightening out the arm, hand and finders, and lack of sensation on the back of the arm and hand.