Paralysed rats 'learn to walk'
Paralysed rats have been able to walk again after their spinal cords were bathed in chemicals and zapped with electricity, scientists have shown.
An injury to the spinal cord stops the brain controlling the body.
The study, in the journal Science , showed injured rats could even learn to sprint with spinal stimulation.
Experts said it was an "exceptional study" and that restoring function after paralysis "can no longer be dismissed as a pipedream".
In 2011, a man from Oregon in the US was able to stand up again while his spinal cord was stimulated with electricity. Rob Summers had been paralysed from the chest down after being hit by a car.
Now researchers at the Swiss Federal Institute of Technology (EPFL) say they have restored far more movement in rats which became able to run and climb stairs.
The spinal cord of the rats was cut in two places. It meant messages could not travel from the brain to the legs, but the spinal cord was still in one piece.
The researchers then tried to repair the damage. In an advance from the Rob Summers study, the spinal cord was first injected with chemicals to stimulate the nerves in the spine. The base of the spinal cord was then electrically stimulated as well. The scientists say they were reawakening the "spinal brain".
However, this was not sufficient to restore movement. The rats were supported in a robotic harness and were shown a treat which they needed to "learn" to walk towards.
The lead researcher, Prof Gregoire Courtine, said: "Over time the animal regains the capacity to perform one, two steps, then a long run and eventually we gain the capacity to sprint over ground, climb stairs and even pass obstacles."
He told the BBC: "It is completely unexpected to see this recovery, they walk and climb stairs voluntarily."
The scientists showed that new nerves were forming across the injury and there were also changes in the brain.
This is not, however, a cure for spinal cord injuries in people.
Prof Reggie Edgerton, from the University of California Los Angeles, was part of the team which helped Rob Summers stand again.
He told the BBC the study was "important" and that it was becoming clear that engaging the brain was the key. "You've got to make the rat want to step, it demonstrates the importance of training and rehabilitation," he said.
Both procedures only work while the spinal cord is still being stimulated, even though new nerves have bridged the injury.
Why this is the case is still unknown. Prof Edgerton speculated that "we are activating the spinal cord to a critical level" close to the level which would trigger movement, and a small signal from "the brain pushes it over" leading to movement.
Dr Bryce Vissel, from the Garvan Institute of Medical Research in Sydney, said he was "excited" by the research.
He said: "The major advance of this new study is to show that it is possible to stimulate almost complete functional recovery in rats with profound injury, using a combination of therapeutic drugs injected into the spinal cord, electrical stimulation of the spinal cord and initial assistance to walk.
"We are on the edge of a truly profound advance in modern medicine: the prospect of repairing the spinal cord after injury."
Dr Mark Bacon, the director of research at the charity Spinal Research, said: "This is a robust demonstration that medical research is moving in the right direction and restoring function after paralysis can no longer be dismissed as a pipedream.
"For all its complexity, the important message here may be that our standard approach to rehab may not be making the most of the potential to restore function if we don't provide appropriate 'rewarding' feedback to every part of the nervous system, including the brain."
However he warned that "real world" injuries might be more complicated, with less tissue for new nerves to grow through.