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The sensory nerves of the thumb emerge from the brachial plexuses. These networks of nerve fibers are located deep within the shoulders between the neck and the axillae (armpits). The major branches emerging from the brachial plexuses include musculocutaneous nerves, which supply the arms on the front side and skin of the forearms; ulnar nerves, which supply the forearms and hands and the skin of the hands; medial nerves, which supply the forearms and muscles and skin of the hands; radial nerves, which supply the back of the arms and the skin of the forearms and hands; and axillary nerves, which supply the upper, lateral, and posterior regions of the arms.