THE past few months have been a busy time for research into ways to help people paralysed by spinal injury use signals from their brains to control mechanical limbs. If that could be done routinely, it would be an enormous boon to the disabled. But not, perhaps, as enormous as what is being proposed by Grégoire Courtine, of the Federal Institute of Technology in Lausanne, Switzerland. For he and his colleagues have just published a paper in Science in which they explain how they coaxed the paralysed to walk again. And not just to walk, but to run, avoid obstacles and even sprint up a staircase. Unfortunately for those crippled by injury, the paralysed creatures in question were rats. But the results are so extraordinary that they should give hope to human sufferers. Dr Courtine paralysed his rats by cutting their spinal cords in two places, so that the animals could no longer move their hind legs. The cuts he made did not completely sever the cord, so some nerve fibres still ran from the rats" brains to their lower bodies, but the signals transmitted by these fibres were not powerful enough to allow a rat to use its hind legs. These injuries are similar to those of many people who have suffered spinal damage. Having disabled his rats, Dr Courtine then attached to each animal's spine a small device designed to stimulate its nerves. The stimulation came in two forms: pulses of electricity and pulses of chemicals that mimic the action of serotonin and dopamine, two molecules that transmit messages between nerve cells. The question was, would this stimulation be enough to get the rats walking again? To find out, he used a special harness which held the rats upright over a treadmill, with their hind feet touching the mill. He then switched on the treadmill while the stimulator was running, to try to encourage the rats to walk using their back legs alone. And he found that, after seven days or so, they could. But only when the treadmill was running and the stimulator working. The experiment, in other words, had shown that paralysis can be reversed, but not in a way that is particularly useful. This first experiment, however, had engaged only the animals" spinal cords. To push things further, Dr Courtine and his team now tried engaging their brains as well - with astonishing results. Once again, the rats were placed on the treadmill, in the harness, and their spinal cords were tickled by the stimulator. This time, though, a piece of food was dangled in front of each rat, to give it a motive to move. And move it did. After an average of six weeks of training, food-motivated rats were able to walk on their hind legs, even with the treadmill switched off. Two or three weeks later they could not only walk but could also dodge round obstructions and run upstairs. On level ground, they were able to travel at a speed of seven metres a minute. Nor was this a fluke. Dr Courtine tried it with 100 different rats and got the same result every time. The reason, he found when he dissected the animals, was that the density of nerve fibres in their spines had increased almost fourfold in response to the combination of electricity, stimulating chemicals and greed. Each rat had, in effect, grown a new nervous system which ran from the motor cortexes of its brain, down its spinal column, past the breaks in its spine and into its back legs. The next step, Dr Courtine hopes, will be to put the implant directly under the rat's control, so that it is switched on automatically when the animal thinks about moving. That should not be too hard. The motor cortex is one of the better-understood parts of the brain. When its owner wants to walk, it sends out what is called a tonic signal ordering forward movement and the spine does the rest. Tapping into that tonic signal with an external electrode is simple. The more difficult trick will be replicating the experiment in people, but Dr Courtine hopes this will be possible within a year or two. His team is already collaborating with researchers at University College, London, and Finetech Medical, a British firm, to design a human version of the stimulator. If they can pull it off, then many lives will be improved enormously.