Rewiring Nerves: How to Rewire the Nervous System
It is known as "phantom limb syndrome" or "phantom pain". But this strange phenomenon feels all too real to the people it affects, and can be agonisingly painful. Amputees and people who have become paralysed may still "feel" a missing limb or a part of their body, even though it is no longer connected to their nervous system.
Yet such sensations offer confirmation that even when a limb has been severed or cut off from the nervous system, the nerves that once serviced it remain alive and well. Doctors are now finding ways to put these nerves to good use, by rewiring them to control prosthetic limbs or reanimate paralysed limbs.
Moreover, rewiring the nervous system should allow amputees to gain a sense of "embodiment" of a prosthetic. That is, by controlling and sensing the prosthetic using the same neural pathways and parts of the brain that once governed the real limb, the prosthetic can be made to feel and act like a genuine extension of the user's body. And by stimulating the nerves in the legs or arms of paralysed patients—nerves that have been cut off from the central nervous system—it is possible to create co-ordinated movement of great subtlety. For example, the hands of paralysed patients have been stimulated to enable them to grasp and turn door knobs. And with careful control and co-ordination of the muscle groups in their legs, patients can even rise from their wheelchairs and take steps.
Prosthetic limbs are becoming increasingly sophisticated, but it can be very difficult to control them in a natural way, says Paul Marasco, a biomedical engineer at the Louis Stokes Cleveland Department of Veteran Affairs Medical Center, in Ohio. For example, patients control some motorised devices by flexing muscles in their remaining stump, shoulder or chest. These muscle movements are detected by electromyography (EMG) sensors on the skin, and the signals are translated into movements by the prosthetic.
But external stimulation is less than ideal, says Dustin Tyler, a biomedical engineer at Case Western Reserve University in Cleveland, Ohio. The muscles in the legs are so large that the whole muscle does not contract, he says. So he and his colleagues have been looking at ways to activate these muscles by tapping into the femoral nerve, in the groin. "By moving back to the nerve you get the whole muscle," he says. The femoral nerve is divided into several dozen separate bundles of nerves, called fascicles, each of which contains hundreds if not thousands of individual nerves. Different fascicles lead to different muscles, so stimulating groups of fascicles at different times and by different degrees should enable co-ordinated leg movements.