Electric zaps can ‘wake up’ lost neural connections, helping paralyzed people walk again

People with crippling spinal injuries can walk again using medical devices that zap their nerves with electricity. But the designers of these new implants weren’t completely sure how they restored motor function over time – now a new study is providing clues.

The new study in humans and lab mice, published Nov. 9 in the journal Nature (opens in a new tab), identifies a specific population of nerve cells that appears essential for regaining the ability to walk after a crippling spinal cord injury. With an electrical jolt, an implant can fire these neurons and thus trigger a cascade of events in which the very architecture of the the nervous system changes. This cellular remodeling restores the lost lines of communication between the brain and the muscles necessary for walking, allowing people who were once paralyzed to walk again, the researchers concluded.

Understanding how the nerve-zapping system, called epidural electrical stimulation (EES), “remodels spinal circuitry could help researchers develop targeted techniques to restore gait and potentially allow recovery of more complex movements.” Eiman Azim (opens in a new tab)senior researcher at the Salk Institute for Biological Studies in La Jolla, California, and Kee Wu Huang (opens in a new tab)a postdoctoral fellow in Azim’s lab, wrote in a comment (opens in a new tab).

Nine people with crippling spinal injuries took part in the new study. Six were mostly or completely unable to move their legs but retained some sensation in their limbs; the other three participants had no motor control or waist-down sensation.

Related: A woman passed out every time she tried to stand. The new implant allows him to walk.

diagram shows a person in a weight harness alongside an illustration of their nervous system, with a charged electrode device implanted in the lower spinal cord

The implant delivers electrical stimulation to nerves in the lower spinal cord. (Image credit: NEURORESTORE/JIMMY RAVIER)

All nine participants underwent surgery to have electrodes implanted at the top of their lower spinal cord, below the muscles and bones, but outside the membrane that envelops the nervous system. Each participant then trained with their implant for five months. They started by practicing standing, walking and performing various exercises indoors in a weight-bearing harness, and eventually managed to practice outdoors with a walker for more stability.

These exercises were performed with the EES implant turned on, but over time, four of the nine participants were able to bear their weight and walk with the device turned off, the researchers wrote in their report.

The team also found that as each participant regained their ability to walk, their overall spinal cord activity decreased in response to the SEA – what initially sounded like a roaring fire of nerve cell activation s is reduced to a smoldering fire. This suggested 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.

“When you think about it, it shouldn’t be a surprise because in the brain, when you learn a task, that’s exactly what you see — there are fewer and fewer neurons activated” as you go. as one improves, senior co-author Gregoire Courtine (opens in a new tab)neuroscientist and professor at the Federal Polytechnic School of Lausanne (EPFL), says Nature (opens in a new tab).

The team used rodent-sized EES implants to study how this reorganization takes place in mouse with paralyzing spinal cord injuries. The mice went through a rehabilitation course, similar to the human participants, and throughout, the researchers tracked which of their nerve cells responded to the treatment by altering the genes they had turned on.

This analysis revealed a set 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 paralyzed injured mice, cell silence prevented them from walking again. This suggests that while other nerve cells may play their own role in recovery, this particular group is particularly important, Courtine said. Science (opens in a new tab).

“The results are consistent with the idea that certain types of spinal neurons[s] who have lost their brain inputs after injury can be ‘awakened’ or reused to restore movement if they receive the appropriate combination of stimulation and rehabilitation,” Azim and Huang wrote. Assuming the results of the mouse studies carry over to humans, the experiments could lay the groundwork for new and improved devices aimed at repairing the spinal cord after injury, they said.