Scientists from the University of Montreal designed a neuroprosthetic to stimulate the brain’s cortex and to speed recovery of patients with spinal cord injuries. The device may improve how we treat spinal cord injuries in the future — as long as the effects seen in rats appear in humans as well.

Marina Martinez, associate professor in the department of neuroscience at the University of Montreal, and her team described the neuroprosthetic in the journal Science Translational Medicine in March. The authors revealed that when they implanted the device in rats with spinal cord injuries, the rats could move their leg again as soon as the device was "turned on." 

Previously developed neuroprosthetics constantly stimulated the brain, even when the rat was immobile. This device detected muscle movements that occurred when a rat tried to move its leg and responded by stimulating neurons in the brain's cortex.

“Overall, I’m pretty impressed by this study,'' said Christian Ethier, an assistant professor in the department of neuroscience at Laval University, who was not involved in this work. “The neuroprosthesis field sometimes just focuses on either restoring movement, like immediate function, or they focus on the decoding side and predicting behavior. This one does both.”

Neuroprosthetics aren’t new. Cochlear implants have treated profound hearing loss since the 1950s. Like cochlear implants, most neuroprosthetics function by stimulating nerves and muscles. Devices for spinal injury patients often directly stimulate the spinal cord. Instead, this newly developed prosthetic stimulates the brain’s motor control center, the cortex.

“There are a lot of people asking, ‘what is the difference between your work and [previous] work in…prosthetic control of locomotion?’” Martinez said. “The main difference is that we don’t tap into the same structure.” 

After the researchers implanted the neuroprosthetic in injured rats, they “turned it on” and tested how well it worked by putting the rats through rehabilitation. They trained the rats to run on treadmills for thirty minutes every day for months. Not only did the rats learn to run, but their fine motor skills improved. They could climb ladders, a behavior that they were never taught.

Most surprisingly, the rats continued to improve after the neuroprosthetic was “turned off.” This was the most important finding, according to Martinez. Either agreed: “They turn the stimulation off, and a few weeks later, the rats are better… This aspect of the therapeutic application is the most interesting.”

This finding indicates that the device may do more than teach a paraplegic rat to walk; it may help its brain talk to the spinal cord again. Some connections between the brain and the spinal cord still exist after an incomplete spinal injury, and re-establishing communication between the two could significantly improve patient recovery.

“I think the paper offers extremely interesting insight into the fact that stimulating the cortex promotes some level of circuitry reorganization at the spinal cord, not only inside the brain,” said Marco Capogrosso, assistant professor of neurological surgery and director of the spinal cord stimulation laboratory at the University of Pittsburgh, who was not involved in this study. “Of course, there is a limitation. This reorganization is not perfect. At the end of the treatment, the rats are not cured. But it’s absolutely interesting.”

Martinez recognizes this limitation. She emphasized that this treatment will not work for patients with complete spinal cord injuries. The spinal cord and brain don’t communicate anymore in these patients.

It will take some time and more research before physicians can implant these neuroprosthetics in human brains, but Martinez is eager to move the device towards the clinic. She is currently developing a non-invasive version of the neuroprosthetic, and believes that she can start a clinical trial in a couple of years.

“Making rats walk again is not my goal. My goal is really to have this strategy be in the clinic at some point because we want to promote recovery of locomotion for people who are impacted by [spinal cord injury]. It's a lot of people,” said Martinez.

Reference

1. Bonizzato, M. and Martinez, M. An intracortical neuroprosthesis immediately alleviates walking deficits and improves recovery of leg control after spinal cord injury. Sci. Trans. Med  13 (586): eabb4422. (2021).