How do prosthetic limbs work?

We posed this question to our brain panel.

Mike - There are prosthetic limbs now that do respond to thought for example and how that works is pretty complicated. I've just been looking it up myself and as far I understand when you amputate the limb, say you amputate the lower leg, what you do is preserve the nerves that are supplying the bit that you chopped off by attaching them to muscles and parts of their remains. And then the limb is then basically responding to changes in the muscle that's being re-innovated. Their limb is being controlled by a computer and they're sort of learning that when a particular part of a muscle mass twitches, that's a signal to make a particular movement. So you can actually have a prosthetic arm with a hand which will allow you to pick up objects by replicating the movement that would've been there if the arm and the hand have been there.

Katie - My understanding is that it's sort of prosthetic limbs where they can receive some of the signals the brain is that it acts like a pattern analyser effectively. So, their nerves are reconnected to a muscle higher up and then as you think about doing something like moving your arm, opening your hand, closing your hand, those sorts of things, the computer learns. It has sensors in the prosthetic arm and it learns what those pattern of the muscle contractions in the higher up muscle, what that means in terms of what movement you're thinking of. As the sensors pick these up, the computer looks at the pattern of muscle contractions and reads that off as which movement is intended and then that creates in the prosthetic limb.

Bill - There's also been some progress in a kind of EEG control of prosthetic devices. For example, a patient in a wheelchair could control the direction of the wheelchair by thinking about it. Want to go right and the wheelchair turns to the right. It depends on the sophisticated ability to analyse electric patterns in EEG type recordings from kind of a bicycle helmet connected to the skull. Well, just a helmet you could wear that would pick up your brain waves and interpret them and then you learn to think left in a way that the wheelchair understands and it will go left.

Katie - The sort of very, very early research being done into actually using the electric signal of muscles contracting and sending signals back so you get a very rudimentary sensation or stimulation that could be interpreted as touch on the brain. So, using a prosthetic limb to actually get some sort of sensory feedback as well.