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What’s in a stimulator
In most instances, including for six patients in the more recent Nature study, the implanted electrode arrays were repurposed from machines originally designed to treat neuropathic pain. But three people in this report had an experimental device, under commercial development by Onward Medical and based on research by the Swiss company NeuroRestore, of which Squair is a part.

Because this device was tailored to help revive function in patients with paralysis, Squair says the stimulator can restore sensation more quickly than older implants. “To activate the muscles that you want to activate, you need to target the right spots on the spinal cord,” he says. “The leads that have been originally designed for pain don’t provide the ability to do this.” The stimulators can also be programmed to deliver signals timed to the pattern of locomotion.

Hofstöetter agrees that, compared with versions meant for pain relief, this is a “more sophisticated, high-end technological” implant. But, she notes, no study “has specifically investigated whether such level of technological complexity would be” better than conventional devices.

No matter the device—there are about a half-dozen companies working on these kinds of spinal stimulators, Oh points out—the effect is not like turning on a switch. As the Nature report describes, for the patients with chronic paralysis to walk, it required five months of physical therapy, with one- to three-hour sessions up to five times a week. Restoration of function may also mean training for specific motions.

“Getting people out of wheelchairs and walking independently is a miracle,” Oh says. In the current study, all nine participants could walk, some independently and others with assistance. But this stimulation is not a panacea for paralysis, he notes: It cannot repair the ability to run, dance, or kick.

An unusual pattern occurs in the spinal nerve systems of patients who receive this stimulation: Their movement may improve even when the stimulators are turned off. What’s more, the overall level of cell activity in the spinal networks appears to decrease, says study author Jordan Squair, a postdoctoral scientist at the Swiss Federal Institute of Technology. Essentially, he says, the spines are becoming “more efficient”—unlike at the start of rehabilitation, they no longer need to activate a whole bunch of cells to create movement.

That led Squair and his colleagues to investigate which neurons wind up taking the lead. After a spinal cord injury, healthy circuits become disrupted, says Michael Oh, a neurosurgeon at the University of California, Irvine. The activated cells identified in this study are probably intermediaries between the neurons responsible for sensation and those that govern motion, smoothing out that disorganization and streamlining firing circuits.