[Surgeons] cut the nerves to two chest muscles, the pectoralis and serratus, at a point where those nerves have branched to go to different parts of the muscles, but far "upstream" from the point where the nerves divide into tiny fibers that attach to individual bundles of muscle fiber. . . . They then sew the stumps of the large nerves that once went to the arm and hand to the cut ends of the chest-muscle nerves. In the same operation, the nerves carrying sensation from the skin over the pectoral muscle are also sewn into the arm nerves.Thanks to electrodes positioned over the area and wired to the prosthesis, when Mitchell attempts to move her missing arm, the prosthetic arm moves in its place (a video of the arm in action can be seen here.) Future iterations of the arm will have more motors in addition to electrodes in the hand that send signals to the user's chest skin, emulating sensory input from the missing hand.
Over several months, the arm nerves grow down the sheaths of the motor fibers and attach to the muscles. . . . Simultaneously, the sensory nerves grow down the sensory sheaths and into the skin. . . . If all goes well, a person is left with chest muscles that twitch in different places in response to such thoughts as "bend the wrist back," "move the thumb" and "clench the fingers." The person also ends up with a patch of skin about the width of a baseball that, when stroked, warmed or pricked, feels like a hand rather than part of the chest.