Can we create artificial nerve signals?

17 January 2010

Question

Can we create artificial nerve signals?

Answer

Chris - Yes we can. We actually know quite a lot about how nerve information travels. If you can imagine a nerve cell as a bit like a long straw, with sides and a space in the middle - what nerves do is to move positively charged ions, in this case sodium ions, from the inside of the nerve, to the outside world. So the inside of the nerve is a bit negative compared to the surroundings. When a nerve impulse travels along a nerve, what happens is that some "positive" (some sodium) goes back inside the nerve through tiny pores, which are on the surface of the nerve. This is called de-polarising the nerve, and it makes that section of the nerve become, transiently, a bit positive.

Now this does two things; it starts an electrical signal rather like a Newton's Cradle running inside the nerve, but it also activates other little channels and pores on the surface of the nerve a bit further downstream. They open and let in some more "plus" to sustain and maintain the propagation of the signal. In a big nerve, this signal is actually travelling along at something like 100m/s. The impulse (or action potential) only goes in one direction though because, in the opposite direction - where it's just come from, the nerve pumps the "plus" back out again, so it goes back to being net minus and the nerve resets itself.

This process happens in milliseconds, so you can literally conduct hundreds if not thousands of these impulses down a nerve in less than a second. The information can travel very, very fast.

You can also make this happen by artificially stimulating the nerve. If you apply a little bit of electricity to the wall of the nerve fibre itself you can actually make that process trigger off, and then it self-sustains. The signal will propagate along the nerve to wherever it goes - in both directions.

Scientists use this for a number of reasons; one is that you can artificially activate muscles that way - so if you've been paralysed, for example, you can use techniques like this to restore movement to certain muscle groups by electrically stimulating certain nerves that supply those muscles. Another reason is to use brain stimulation - this has been done quite effectively in Parkinson's disease. Scientists implant a little electrode in a part of the brain which makes movements and is involved in the same circuit as is affected by Parkinson's disease. If you stimulate those bits of the brain electrically, you can trigger off impulses in the right way and the right rate to help people who have Parkinson's symptoms to overcome their symptoms and move a bit more easily.

So, it is possible to re-create nerve signals. At the moment it's fairly low resolution: you're not stimulating individual nerve cells, you're stimulating clusters of nerve cells. But at the same time, you can do that.

Also, if you listened to last week's programme, you'll know that we talked about cochlear implants, which are things that basically stimulate the nerve that conveys hearing information into the brain stem. They're doing effectively the same thing - stimulating nerve cells directly to send sound information into the brain.

Diana - I remember about a year ago we had a story on Breaking Science with the researcher Andrew Schwartz who monitored the signals being sent from Chimp's brains, and then translated them using a computer into muscle movements.

Chris - I guess that's going the other way - rather than putting input into a cluster of nerve cells, he was recording the activity, understanding how nerves are encoding information and using that (once it's been decoded by a computer) to then drive muscle movements in an arm.

I think they were making the Chimpanzees reach out and grab things just by the power of thought.

Diana - Yeah, that's right.

Comments

Is there a citation/paper for the study where they induce an action potential with an electrical impulse?

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