Although modern prosthetic devices are more lifelike and easier for amputees to control than ever before, they still lack a sense of touch. Patients depend on visual feedback to operate their prostheses – they know that they’ve touched an object when they see their prosthetic hand hitting it. Without sensation, patients cannot accurately judge the force of their grip or perceive temperature and texture.
Todd Kuiken, a professor at Northwestern University and director of the Neural Engineering Center for Artificial Limbs at the Rehabilitation Institute of Chicago, has led the development of a new technique known as targeted reinnervation, which can help amputees control motorized prosthetic arms. He and his team now hope to extend the applications of targeted reinnervation to help patients regain sensory capabilities.
In targeted reinnervation, the motor nerves of a nearby target muscle (usually the chest) are deactivated. Then the residual motor nerves at the end of an amputated arm are transplanted from the stump to the chest. The nerves rewire themselves and grow into the chest muscle. Since amputation of a limb does not prevent the nerves left in the residual limb from signaling, the reinnervation procedure simply gives the signals a new destination.
After the procedure, when a person thinks about moving a muscle in the missing arm or hand, the chest muscle twitches. Electrodes pick up these signals and pass them on to a motorized prosthetic arm, allowing patients to control multiple motor functions like the simultaneous movement of both the elbow and hand to throw a ball.
The regrowth of sensory nerves after this procedure was discovered by accident. The first patient to undergo targeted reinnervation told Kuiken and his other doctors about an interesting sensation he experienced: when someone touched the area of his chest where his nerves had regrown, he felt as if someone was touching his missing hand. The sensory nerves from his arm stump had reinnervated the skin above his chest muscle. He was experiencing touch to the reinnervated skin as being applied to his missing limb. It turned out that sensory reinnervation such as this was common following the procedure.
Kuiken and his colleagues are currently exploring how to take advantage of sensory reinnervation to build prosthetic arms with sensors on the fingers that can transfer touch information from the prosthetic to the chest, allowing patients to “feel” what they are touching with their prostheses.
The next step is to figure out the mechanisms that guide reinnervation, with the hope of someday being able to direct the regrowth of nerves for more refined results. To better understand how sensory reinnervation affects brain reorganization, Kuiken and his colleague Paul Marasco examined the brains of rats after amputation and targeted reinnervation. In this experiment, published in The Journal of Neuroscience, Marasco and Kuiken looked at how the somatosensory cortex, the brain area that receives and processes input from sensory organs, changed in rats following forelimb amputation with and without the targeted reinnervation procedure.
One group of rats underwent forelimb amputation and then targeted reinnervation, while another group of rats underwent only the amputation. The rats that did not undergo targeted reinnervation effectively had the input between the cortex and the forepaw silenced. After thirteen weeks of recovery, the experimenters recorded brain activity in the primary somatosensory cortex of all the animals. Marasco and Kuiken were especially interested in the region known as the forelimb barrel subfield, which would normally process touch input from the amputated forepaw.
As expected, the rats that underwent amputation without targeted reinnervation showed an almost complete silencing of brain activity in the forelimb barrel subfield. The receptive fields for the few active areas in this region were located on the residual shoulder.
In contrast, the rats that underwent targeted reinnervation showed extensive activity in the forelimb barrel subfield. The receptive fields for the active sites in these rats were small and densely clustered on the far end of the stump, and differed in proportion from the large and diffuse receptive fields observed on the residual limb of the amputation-only rats. It appeared that the sensory input from the reinnervated skin was processed within the cortical representation of the missing forepaw.
This helps explain why Kuiken’s earlier human patient reported feeling a touch on his chest as occurring on his missing hand. His somatosensory cortex, in particular the area devoted to the missing limb, had reorganized to accommodate the new sensory input. Sensations from the skin on his chest were being processed within the hand representation area of his somatosensory cortex.
Further somatosensory reorganization was evident in the rats. In most of the animals that underwent targeted reinnervation following amputation, there were regions of the forelimb barrel subfield (called dual receptive fields) that were responsive to both the stump and other regions of the body (the whiskers, lower lip, and hindlimb). The presence of dual receptive fields in these rats, but not in the amputation-only rats, suggests that the adjacent brain areas expanded into the denervated regions following the amputation. The sharing of space allowed those sensory nerves to keep transmitting signals, even after amputation.
Marasco and Kuiken’s results provide important insights into the sensory phenomena observed in human targeted reinnervation patients. The reorganization of somatosensory cortex in rats following the procedure supports the hypothesis that the reinnervated skin is able to act as a direct line of communication from a prosthetic device to the regions of the brain that process hand and limb sensations. This is likely the mechanism by which targeted reinnervation provides sensation that is perceived as coming from an amputated limb.
Ultimately, Marasco and Kuiken hope that this experiment will contribute to the building of better prosthetic limbs. Motorized prostheses that also provide sensory feedback have the potential to be more effective, capable of more functions, and easier to manipulate. Most importantly, they would not only function like a real human arm but also feel like one, allowing the prosthetic to be integrated more naturally into the patient’s self image.
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