By Michelle Roberts
Health reporter, BBC News
Scientists have shown it is possible to harness brain signals and redirect them to make paralysed limbs move.
The technology bypasses injuries that stop nerve signals travelling from the brain to the muscles, offering hope for people with spinal damage.
So far the US team from the University of Washington have only tested their "brain-machine interfaces" in monkeys.
The hope is to develop implantable circuits for humans without the need for robotic limbs, Nature reports.
Spinal cord injuries impair the nerve pathways between the brain and the limbs but spare both the limb muscles and the part of the brain that controls movement - the motor cortex.
Recent studies have shown that quadriplegic patients - people who have paralysis in all four limbs - can consciously control the activity of nerve cells or neurons in the motor cortex that command hand movements, even after several years of paralysis.
Using a gadget called a brain-machine interface, Dr Chet Moritz and colleagues re-routed motor cortex control signals from the brains of temporarily paralysed monkeys directly to their arm muscles.
The gadget, which is the size of a mobile phone, interprets the brain signals and converts them into electrical impulses that can then stimulate muscle to contract.
By wiring up artificial pathways for the signals to pass down, muscles that lacked natural stimulation after paralysis with a local anaesthetic regained a flow of electrical signals from the brain.
The monkeys were then able to tense the muscles in the paralysed arm, a first step towards producing more complicated goal-directed movements, such as grasping a cup or pushing buttons, say the researchers.
Lead researcher Dr Chet Moritz said: "This could be scaled to include more muscles or stimulate sites in the spinal cord that could activate muscles in a coordinated action.
"Similar techniques could be applied to stimulate the lower limb muscles during walking."
The scientists found the monkeys could learn to use virtually any motor cortex nerve cell to control muscle stimulation - it did not have to be one that would normally controlled arm movement. And their control over the muscles improved with practice.
The researchers say they need to do trials in humans, meaning a treatment could be decades away.
Dr Mark Bacon, head of research at the UK charity Spinal Research, said: "This is clearly a step in the right direction and proves the principle that artificially transducing the will to move generated in the brain with relevant motor activity can be achieved.
"However, these results have been produced in experimental models where there is no injury per se."
He said injury-induced changes to the nerve circuits might hinder the technology's application in real life.
Also, brain-machine interfaces communicate in only one direction - in this case from the brain to the muscle.
"Sensory feedback, so important for fine control of movements and dexterity, is still some way away," he said.