The combination of electrochemical stimulation and active training, which included walking up stairs and around obstacles, resulted in new neuronal connections that bypassed the site of injury. “We promoted extensive remodeling of the neuronal connections not only at the site of the injury but throughout the central nervous system, including in the brain,” says Courtine. What was most surprising, he says, was the fourfold increase in neuronal projections sent to the brain stem from the motor cortex, which provides conscious control of movements. “The motor cortex becomes the maestro of the reorganization process.
The conscious intent of the rats was necessary for the remodeling as well. The nervous systems of rats that received the electrochemical stimulation but trained only on treadmills did not demonstrate the anatomical changes. “You need to incorporate an input from the brain,” says Heutschi. “It doesn’t work if the rat is on a treadmill; you have to force them to use the brain to control their hind limbs.”
The clinical significance of the findings is unclear, according to Rutgers University neuroscientist Wise Young, because of the unusual surgical injury to the experimental rats (two cuts on each side of the cord at different heights). A more relevant injury would have been a spinal-cord contusion or bruise, he says.
However, other experts think the results are promising for those spinal-cord-injured patients who do not have a complete cut through the cord. Even though all connections between the brain and the lower spinal cord were disrupted in the experimental rats, “there are some remaining fibers, so the beauty of their technology is using the robotic training system to activate those remaining connections that can allow the cortex to control the limbs and to regain voluntary movement,” says Zhigang He, a neuroscientist at Harvard Medical School. “This robotic training system makes that happen,” he says.
Plans are under way to develop a human-sized version of the training system and to test its effects in clinical trials in Europe. Researchers at the Swiss Federal Institute of Technology and other European institutions are also working on an improved, implantable version of the electrical spinal stimulation system that may find its way into humans next year.