A group of Swiss researchers is showing that even chronic spine injuries can be treated to restore a patient’s walking ability using a mix of electrical stimulation and intense physical therapy.

The findings are based on the successful treatment of nine people with chronic spinal injuries. They had previously lost their ability to walk due to nerve damage sustained as part of these injuries, suffering either severe or complete paralysis. All patients saw improvements immediately after starting the treatment, and continued to see progressive improvement by the end of the trial.

Although previous research has shown that electrical stimulation of the spinal cord could reverse injury-related paralysis, it wasn’t yet clear why this happened. The current study identified the exact nerve groups that are stimulated by such treatment, filling in an important gap in our scientific understanding of spinal function.

Powering it back up

The nerve bundles that handle walking are concentrated in a section of the spinal cord that runs through our lower back. That’s why injuries to the spine in this area often lead to some level of impairment in walking ability. Damage here can cut the link between our brain and the muscles in our legs, which can prevent individuals from walking even if their muscles and motor neurons are healthy and whole. Any damage to the nerve bundles that ferry these electrical impulses between the brain and the legs turns the neurons that handle walking effectively nonfunctional.

Building on previous research, a team led by neuroscientist Claudia Kathe from the Swiss Federal Institute of Technology Lausanne (EPFL) tested a technique called epidural electrical stimulation, which has shown promise towards reversing such paralysis. They first worked with model animals, and later moved on to nine human volunteers.

As part of this process, the spinal cord was stimulated by a neurotransmitter. This device was permanently implanted through a surgical procedure over the span of the trial. During the trial, the patients also underwent an intensive neurorehabilitation program, which involved them moving in multiple directions with support from a robotic helper system.

Each participant went through five months of stimulation and rehabilitation, with four to five sessions per week. By the end of the trial period, all of the participants were able to take steps on their own, without the aid of the walker.

Surprisingly, however, the patients showed a reduction in neural activity in their lumbar spinal cords while walking, the team reports. This could be due to this activity being concentrated in a subset of neurons that are the most important for walking.

“When you think about it, it should not be a surprise,” Courtine told Dyani Lewis at Nature, “because in the brain, when you learn a task, that’s exactly what you see – there are less and less neurons activated [as you get better at it]”.

Subsequent experiments using mice showed that a single population of previously unknown neurons can learn to take over walking after a lumbar spinal injury. This group of cells is found within the Laminae of the lumbar cord and is made up of cells called SCVsx2::Hoxa10 neurons. Normally, these are not needed for walking, but they play an essential role in recovering after spinal injuries, the team explains; destroying these neurons prevented mice from recovering their walking ability.

That being said, these neurons don’t simply move into the role by themselves. This transition is dependent on activity. SCVsx2::Hoxa10 neurons seem to adopt this role more due to their location than anything else. The team explains that they are “uniquely positioned” to take raw impulses from the brain and transform these into instructions for movement in the muscles.

So we are well on our way to better understanding how paralysis induced by lumbar cord injuries can be treated. That being said, the human nervous system is a hugely complicated system, and these neurons are only one component of the whole. There is still a lot left to uncover — but now we know which direction to go in, at least. In time, these findings could lead to new and better treatment options, massively improving the life quality of people with various kinds of spinal cord injuries.

“These experiments confirmed that the participation of SCVsx2::Hoxa10 neurons is a fundamental requirement for the recovery of walking after paralysis,” the researchers concluded.

The paper “The neurons that restore walking after paralysis” has been published in the journal Nature.