The brain sends messages with the nerves telling the muscles of your forearm and hand how and when to maneuver. Moreover, the deep head from the flexor pollicis brevis is innervated because of the deep department of the ulnar nerve. Since the abductor pollicis longus intact (posterior interosseous nerve from https://www.theorthopaedicandpainpractice.com/what-are-the-different-bones-found-in-my-hand/