People with a paralyzing spinal cord injury can walk again with the help of medical devices that put electricity on their nerves. But the designers of these new implants weren’t entirely sure how they restored motor function over time — now a new study provides clues.
The new study of humans and laboratory mice, published Nov. 9 in the journal Nature (opens in new tab), points to a specific population of nerve cells that appears to be key to restoring the ability to walk after a paralyzing spinal cord injury. With an electric shock, an implant can switch on these neurons, triggering a cascade of events in which the very architecture of the brain is disrupted nervous system changes. This cellular remodeling restores the lost lines of communication between the brain and the muscles needed to walk, allowing once paralyzed people to walk again, the researchers concluded.
Understanding how the nerve zapping system, called epidural electrical stimulation (EES), “reshape spinal circuits may help researchers develop targeted techniques to restore walking and potentially enable recovery of more complex movements,” Eiman Azim (opens in new tab)a principal investigator at the Salk Institute for Biological Studies in La Jolla, California, and Kee Wu Huang (opens in new tab)a postdoctoral researcher in Azim’s lab, wrote in a commentary (opens in new tab).
Nine people with paralyzing spinal cord injuries participated in the new study. Six were largely or completely unable to move their legs, but retained some sensation in the limbs; the other three participants had no motor control or sensation from the waist down.
Related: A woman fainted when she tried to get up. New implant lets her walk.
The nine participants underwent surgery in which electrodes were implanted on top of their lower spinal cord, below the muscles and bones but outside the membrane that encloses the nervous system. Each participant then trained with their implant for five months. They began practicing standing, walking, and performing various exercises indoors in a weight-bearing harness, eventually graduating to exercise outdoors using a walker for stability.
These exercises were completed with the EES implant turned on, but over time, four of the nine participants were able to bear weight and walk with the device turned off, the researchers wrote in their report.
The team also found that, as each participant was able to walk again, their overall spinal cord activity decreased in response to the EES – what initially seemed like a roaring fire of nerve cell activation dwindled to a smolder. This indicated that the combination of rehabilitation and electrical stimulation reorganized the nervous system so that fewer and fewer cells were needed to perform the same action.
“If you think about it, it shouldn’t come as a surprise, because in the brain, when you learn a task, that’s exactly what you see — fewer and fewer neurons are activated” as you improve, co-senior author Gregoire Courtine (opens in new tab)a neuroscientist and professor at the Swiss Federal Institute of Technology, Lausanne (EPFL), nature told (opens in new tab).
The team used rodent-sized EES implants to study how this reorganization unfolds mice with paralyzing spinal cord injuries. The mice completed a rehabilitation course similar to the human participants, and throughout the time, the researchers tracked which of their nerve cells responded to the treatment by changing which genes they turned on.
This analysis revealed a range of neurons in the lumbar spinal cord that consistently responded to therapy, even as other neurons became less active. Blocking the activity of these neurons in uninjured mice did not affect their ability to walk, but in injured mice with paralysis, silencing the cells prevented them from walking again. This suggests that while other nerve cells may play their own roles in recovery, this particular group is especially important, Courtine said Science (opens in new tab).
“The findings are consistent with the idea that certain types of spinal neurons[s] those who have lost their input from the brain after an injury can be ‘reawakened’ or used again to restore movement if given the right combination of stimulation and rehabilitation,” Azim and Huang wrote. Assuming the findings from the mouse studies transfer to humans, the experiments could lay the groundwork for new and improved devices aimed at restoring the spinal cord after injury, they said.