Image: National Academy of Sciences

30 November 2007—Over the past two years, neuroscientists have found a way to redirect the arm nerves of amputees to their chest muscles, allowing them to use the chest to intuitively control a prosthetic arm and even to feel some pressure applied to the limb. More recently, they found that the patients could actually feel touch on the skin of the chest as if it were on the skin of the missing hand. Now doctors at the Rehabilitation Institute of Chicago have mapped the sensitive spots on the chest to specific parts of the missing fingers and hands, and have even found that the missing fingers can “feel” heat, cold, and pain. With such a map at their disposal, researchers are a big step closer to making artificial limbs that patients can not only control but feel as well. They reported their results in this week’s Proceedings of the National Academies of Science.


“What we’re doing is creating a portal to the nervous system,” says Todd Kuiken, director of the Neural Engineering Center for Bionic Medicine at the RIC, one of several organizations funded by a large U.S. Defense Department project called Revolutionizing Prosthetics. Kuiken pioneered the surgical procedure that led to the breakthrough. Called targeted muscle reinnervation (TMR), it involves surgically rewiring the entire shoulder by redirecting what are called residual nerves. These are bundles of nerves that connect the upper spinal cord to the 70 000 nerve fibers in the arm. In their normal layout, these travel from the upper spinal cord, across the shoulder, down into the armpit, and into the arm.


Even after the arm is severed, these nerve bundles are still functional. They just don’t have any sensory input. The pectoral muscles, too, are still intact, but they no longer have an arm to move. Kuiken rewired the residual nerves by pulling them away from the armpit and threading them under the clavicle to connect with the pectoral muscles.


The goal of the initial research was to make using an artificial arm an intuitive process. It worked better than Kuiken could have expected. The patients are able to use the arm just by thinking about it, thus sending signals down the nerves formerly connected to the arm but that are now connected to the chest. The chest muscles contract in response to the nerve signals, the contractions are sensed by electrodes on the chest, and the electrodes send signals to the motors of the prosthetic arm. 


But Kuiken was surprised to find that the nerves that had been redirected to the muscle had also meanwhile started to reinnervate a saucer-sized patch of skin on the chest. Because of the complex rewiring scheme, touching that skin feels to the patient as if he or she is being touched on both the chest and the missing limb.


So Kuiken and his group began the difficult task of mapping exactly how this worked. They determined which areas of the chest skin corresponded to feelings of touch on the missing limb and mapped the thresholds at which uncomfortable heat, warmth, cold, and painful electric shocks translated into these sensations in the missing fingers and hands. 


It turned out that those thresholds were not too different from those on other areas of the patients’ skin, says Kuiken. “Our patients can feel touches as slight as a tap on the hand,” he says.


Kuiken thinks that these unexpected results are due to the brain’s ability to adapt to a changed body. So far, the sensory stimulation has been performed only on the chest and not on a prosthetic arm electronically linked to the chest. Engineers at Johns Hopkins University, in Baltimore, have been working on a robotic arm that senses pressure and, in response, affects a device that pushes on the chest and gives the wearer the sensation of pressure on the fingers. “Action, stimulus, reaction, stimulus,” says Hopkins program manager Stuart Harshbarger, whose team is also funded by the Defense Department. “It’s a self-contained closed-loop system.” The Hopkins device could eventually translate the temperature and tactile senses from the prosthetic limb back to the nerves in the chest.


Kuiken expects that with time, the brain will learn to associate areas of the artificial limb with its sense of the limb that was lost. Strong evidence suggests that the brain can “remodel” itself, he says, “so that those spots start to feel more and more like natural fingers.”