For patients paralyzed by a spinal cord injury, there’s one impossible roadblock to full recovery.
Any movements we execute starts with a series of neurons firing in our brains. The electric signal then travels down our backs to our spinal cords and triggers the specific nerves that control—by contracting and extending—all of our various muscles. When someone is paralyzed due to a spinal cord injury, like a broken neck, the path from brain to limbs is permanently disrupted—kind of like an avalanche that blocks an otherwise accessible mountain trail. Even though both the brain and the limbs work, they can’t communicate with each other, rendering the patient unable to move.
For years, researchers have been working on ways to clear this nerve roadblock. They’ve made progress by creating computers that attach to paralyzed users’ brains and allow them to move a robotic, prosthetic arm, or regain limited movement in one of their own limbs. To date, though, none of these systems have been able to recreate independent movement the way that patients had before. On Nov. 9, researchers from China, Europe, and the US announced the success of a wireless brain-computer interface (BCI) and spinal implant for monkeys with spinal injuries that allows them to walk without assistance, which can theoretically be applied to humans. Their work was published (paywall) in Nature.
Voluntary muscle movement comes from a cascade of signals that originate in our brain’s motor cortex. Previous iterations of BCIs have tracked the patterns of electrical impulses emitted from paralyzed patients’ brains when they thought of moving, and translated these sequences into the movement of an arm—either the patients’ or a robotic one. The trouble is, the brain patterns for movement are so complicated that translating them into smooth movement isn’t always possible.
For example, picking an apple from a fruit basket to eat—a seemingly simple task—involves several steps. First, you have to extend your arm and open your hand in the direction of the apple. Once touching the apple, you have to close your hand to grip the apple and use slight pressure—but not too much—to hold onto it. Then you have to bend your elbow to bring the apple to your mouth, which you can’t see, but the location of which you intuitively know. All of these motions require both generating action and processing sensory feedback, whether sight, touch, or simply knowing where we are in the world, to fine-tune the movements as you go. Our nervous system does it automatically, but it’s an awful lot of information for a computer; the resulting motion from a BCI may be jerky and stiff.
So rather than focusing on decoding more brain impulses, Grégoire Courtine, a neuroscientist at the Swiss Federal Institute of Technology, and his colleagues focused instead on stimulating the spine—the other half of movement signals. “We let the smart spinal cord take care of the details of the muscle activity,” he says.
For a decade, he and his colleagues have been recording spinal activity, first in rodents (paywall), and then in monkeys. They used data from the latter to build a model of the monkey’s spine, and pinpoint the location of the nerves that control leg movement. Then they used these models to make implants for the monkey’s spine. These implants could animate leg muscles, if given a particular signal from a BCI.
In this particular study, the researchers gave two Rhesus monkeys spinal cord injuries that took away movement in one of their hind legs. They gave the monkeys these newly designed spinal implants, and paired them with a simple, wireless BCI. This computer is a tiny array of electrodes that sits on the surface animals’ motor cortices and protrudes just slightly from their heads. This BCI registers basic signals from the brain—such as intent to move—and sends them to the spinal implant with virtually no delay and no cumbersome wires restricting movement. One monkey took six days to regain full walking ability after the initial spinal cord injury; the other could walk after two weeks.
The technology used in the spinal implant, which was developed with researchers at Brown University, has already been approved for clinical trials in humans by the Swiss Agency for Therapeutic Products, which regulates which medical devices are used for humans in Switzerland (it has not been approved for use in the US). Already, two paralyzed patients have been equipped with the spinal device, says Courtine, to see if it can stimulate their own legs; they plan on testing six more volunteers in this initial, proof-of-concept clinical trial to prove that it can work safely in people before engaging in a larger study.
Even if the early work in humans proves safe and effective, it’s still likely be another decade before this spinal technology could be given to hospital patients suffering from loss of limb movement.