New Control System Allows Greater Control Over Prosthetic Limbs
November 15, 2007 11:09 AM
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Research promises a more capable prosthetic limb
There may be new hope for patients suffering from limb loss. A new control system pioneered by Todd A. Kuiken, M.D., Ph.D., a professor at
and physiatrist at the Rehabilitation Institute of Chicago, allows fine manipulation of prosthetic arms and suggests that even more control is possible.
Targeted muscle reinnervation (TMR) allows prosthesis to respond directly to the brain's signaling, rather than relying on muscle movements like in typical prosthetics. Kuiken came up with the idea for TMR as a graduate student in the 1980s. Rather than learning to control muscle groups in the chest, patients who have undergone TMR simply think of opening their hand, and the hand on the prosthetic opens.
TMR works by rerouting the remaining nerves from the amputated limb to muscles in the chest. Instead of controlling the chest muscles, when a patient wants to grasp something with his hand, the signals from the rerouted nerves cause the muscles in the chest to flex. An electromyogram picks up the electrical signals from the muscle's contraction, and sends it to a microprocessor in the prosthesis. The processor decodes the signal and moves the limb in the intended manner.
So far the system is capable of opening and closing a hand, and bending and straightening an elbow. The microprocessor can be programmed for more signals and further research being done on 16 different movements in the arm and hand promises more range of movement than just the elbow and hand. Fine hand movements such as grasping a pen or tool are not out of the question.
Kuiken and colleagues have also begun work with the military to help soldiers have lost limbs. Brooke Army Medical Center and the Walter Reed Army Hospital are actively involved in the project.
"We're excited to move forward in doing this surgery with our soldiers some day. We've been able to demonstrate remarkable control of artificial limbs and it's an exciting neural machine interface that provides a lot of hope," Kuiken said of the project.
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RE: why couldn't they just read the signals?
11/15/2007 3:46:45 PM
From all I understand, nerve signals aren't simply electronic. There is an action potential, but the real signal is sent by the type of neural transmitters a particular nerve is set up to release at a particular time (which is why drugs affect nerves--a drug isn't electrical, but it can mimic a neural transmitter and screwup all the signals in the brain). The action potential will be identical in either case that you want to relax or contract a muscle, it's just different nerves will be used, or the same nerve got a different nerve's signal and switched which neural transmitter it was shipping to the synapse. You can't just hook up an electrode to a nerve and have it make sense. This isn't even to mention the ratio of transmitters released, and the overall concentrations all cause slightly different outputs. Neural signaling is incredibly complex and more like analogue instead of digital; the action potential is only a "release what's been prepared" signal, not the signal itself.
So then, by simply watching the twitch initiated by the proper signals in the chest muscles, the processor can fine tune the specific movement,
of movement, and so forth. There's a lot of translation that still has to go on, which is why it's a breakthrough in its own right. None the less, there's no way yet to make
sense of a nerve impulse by itself as it can mean so many different things (some nerves are much easier to translate than others just by the degree and frequency of their action potentials; such as optical nerves which are committed to one signal so you don't have to worry so much about which neural transmitter is being use, but in general this is not the case); not till there are systems to detect neural transmitters and translate that correctly.
RE: why couldn't they just read the signals?
11/15/2007 8:26:30 PM
Measuring the twitch of the muscle is used because measuring the contraction is far easier than decoding the nerves signal itself. Also measuring these is easy using a couple of leads.
The muscle acts as a sort of potentiometer. The electrical signal from the muscle can be measured using electromyography. They can measure Voltage and Frequency. Measuring these they can mimic to a degree how fast(frequency) how much force(voltage) etc.
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