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 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 coordinated 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 sensors on the skin, and the signals are translated into movements by the prosthetic.

This approach can provide incredible control over an artificial arm, but patients often prefer to use simpler, mechanical prosthetics. For one thing, such devices allow the patient to sense the movement of the arm through a system of cables which are used to control the device, usually by attaching them to the opposite shoulder. So even when their eyes are closed they can get a sense of whether an artificial arm is extended, or if there is resistance to a grasping motion, making the limb feel less detached and unnatural than an EMG device. Another problem with EMG prosthetics is that patients literally have to retrain their brains to make new associations between muscle movements and their outcomes— a shoulder flex could become a grasp motion, for example, while a twitch of pectoral muscle in the chest may extend the artificial arm. But there is a way to overcome both these difficulties with motorised limbs, using a technique called “targeted reinnervation”.

Pioneered by Todd Kuiken, director of the Neural Engineering Centre for Artificial Limbs at the Rehabilitation Institute of Chicago, along with his colleagues Aimee Schultz, Blair Lock and Marasco (who was formerly at NECAL), this involves rerouting the nerves that would have originally controlled and sensed the missing limb and connecting them instead to other parts of the body. By rewiring a missing arm’s motor nerves to muscles in the remaining stump, shoulder or chest, for example, and rewiring the arm’s sensory nerves to the skin in these regions, a channel is opened to the part of the brain that once controlled the missing limb.

It is a strange and slightly ghoulish idea, because it means that if a patient tries to flex his missing finger, for example, a muscle in another part of his body (which is now connected to the nerves that used to control the finger) contracts instead. “When the amputee wants to open or close their hand, these muscles twitch,” says Marasco. EMG sensors detect these signals and translate them into control signals that cause the mechanical hand to open and close. The patient can then open and close his prosthetic hand simply by trying to move the fingers that are no longer there.

The sensory side of things works in a similar fashion, but instead of reconnecting nerves to different muscles, they are instead rewired to the skin’s underlying sensory systems. “They hook themselves up to the receptors in the skin,” says Marasco. So when the reinnervated skin (on the chest or shoulder, perhaps) is touched, it registers to the patient as a sensation in the missing limb. “They have very distinct sensations that they can feel: vibrations, temperature and pressure,” he says. This means sensors in a prosthetic limb could, in theory, stimulate the reinnervated skin to cause realistic sensations.

Targeted reinnervation is now available as a treatment. More than 40 people around the world have undergone the procedure so far. Even though patients can currently use it only to control their artificial limbs, sensory feedback is coming. And these are merely the first examples of what can be done by rewiring the nervous system, and linking nerves to electronic and robotic devices.