US scientists are reporting that one tiny brain cell is all it takes to restore voluntary movement of paralysed muscles.
In experiments pointing to new treatments for paralysis caused by spinal cord injury or stroke, monkeys learned within minutes to harness the power of a single neuron to activate muscles immobilised by drugs.
There are some 100 billion neurons in the human brain, and the study suggests an unsuspected degree of flexibility in the kinds of tasks they can perform.
"Nearly every neuron we tested could be used to control this type of stimulation," Chet Moritz, lead author and a researcher at the University of Washington, told journalists in a conference call.
If a monkey can do it, a human should be able to do it even better, he said.
Clinical trials, however, are at least several years away, perhaps longer, Moritz added.
Spinal cord injuries cripple hundreds of thousands of people worldwide every year, rendering the simplest of actions - opening a door, scratching an itch, drinking a glass of water - frustratingly difficult, or simply impossible.
Those afflicted with the most severe form of paralysis, known as lock-in syndrome, are fully conscious prisoners inside a body that no longer responds to commands.
While the brain activity that would normally result in a voluntary movement is still present, the instructions simply don't reach the muscles.
Moritz and two colleagues at the University of Washington found a way to bypass the kind of nerve damage that can result in such paralysis.
They first connected electrodes to individual neurons inside the motor cortex of monkey's brain and recorded the electrical activity.
These signals were then routed in real-time to a computer, and from there through a stimulator to another set of electrodes attached directly to wrist muscles that had been artificially blocked further up the arm along the normal neural pathway.
Because little processing power is needed, the computer is the size of a cell phone, and can be attached to the animal's body.
Future versions will be wireless and small enough to implant directly in the body, the researchers said.
The monkey had already mastered a simple video game, grasping targets shown on a video screen with a control device manipulated by a single hand.
"But once he was paralysed, the only way to move his wrist was to change the activity of individual neurons in his brain," Moritz explained.
On average it took about 10 minutes for the monkeys to "train" the neuron well enough to play the video game again.
"The brain can very rapidly learn to control new cells and use them to generate movements," said co-author Eberhard Fetz.
Earlier experiments enabling monkeys to manipulate prosthetic devices or computer cursors using only electrical impulses coming from the brain were based on a fundamentally different premise, according to the new study.
"They tried to read the mind of the money and figure out what he was planning to do," a technique that required massive computing power, said Moritz.
"Our approach is to recreate the raw connectivity between single neurons in the brain and muscles, and let the monkey's nervous system learn how to use that connectivity."
This is also the first study to show that a one neuron can control a muscle - and possibly a whole group of muscles.
Electrodes connected to a single location in the spinal cord below an injury may be able to activate 10 or 15 muscles that are already precisely balanced for, say, grasping a coffee mug or walking, the researchers said.
And if a stroke has damaged the motor cortex, patients might be able to commandeer other brain cells that do not usually play a role in controlling muscles.
Several obstacles remain, however, before this new technique can be tested in humans, he said.
To avoid infections, the system would have to become fully implantable so that no wires passed through the skin. And electrodes would need to be made more stable so that they could record the activity of neurons over a period of years, rather than weeks.