Courtine and his colleagues have shown that even with a cut that completely disconnected the brain from the spinal cord, rats were still able to walk, avoid obstacles, and even side-step after training. Although it may seem that some of these movements would need top-down control from the brain, Courtine says they are controlled solely by the spinal cord. “We have smart neuronal networks in the spinal cord that can make a lot of decisions and these networks can even learn,” he says. “What we are seeing is a selection of circuits in the spinal cord that are becoming more efficient at performing the task successfully.”
Unlike the rats with some residual brain input, these have only sensory input and can’t walk toward a treat—they can’t start to move voluntarily. “The movement is completely dependent on input coming from the legs,” says Courtine. They can only start stepping if they are on a treadmill, so that the movement of the hip joints and muscles and the changes in pressure on their paws act as triggers for the spinal programs to start.
Stimulating the Muscles
Patients who have a sensorimotor complete injury, with no ability to move and no sensation, have only a 3 percent chance of being able to move again. So far, locomotor training has not helped them recover the ability take steps. McDonald has been looking into whether a different technique, one that involves electrical stimulation of the muscles, could lead to neuroplasticity—the ability of the nerve cells to grow and change in response to new inputs. He uses cycling functional electrical stimulation (cFES), which allows a completely paralyzed patient to make cycling movements on a standing bicycle. In a study published this year in the Journal of Spinal Cord Medicine, he found that a higher percentage of patients in the cFES group improved their movement abilities compared with the percentage in the control group. For example, some patients were able to move parts of their legs by fully engaging muscles that before they could only get to twitch. Many in the cFES group recovered the ability to feel a pinprick or a light touch. McDonald believes muscle activity leads to growth of new sensory fibers and the changes he saw with these patients.
FES is not new; it has been used to help paralyzed patients since the late seventies. McDonald used cycling FES as part of Christopher Reeve’s therapy about five years after the accident, when Reeves still had absolutely no motor control, and the prognosis was permanent complete paralysis. But McDonald believed that muscle activity could reactivate the nervous system. After about three years of cycling three hours a week, Reeves was able to move some muscles in his arms and legs.
According to Dietz and Harkema, FES does not activate spinal cord circuits in the same way that locomotor training does. For this reason, Harkema thinks it is not the right therapy for patients with incomplete injuries.
While it is not clear whether FES can help patients recover the ability to walk, McDonald says that it does improve people’s quality of life. Previous studies have shown that it prevents muscle atrophy, reduces spasticity and helps improve circulation. His study showed that the treatment reduces cholesterol levels and intramuscular fat, factors that may lead to type-2 diabetes in people with spinal cord injury. Patients also recover bladder control, one of the priorities for those with this problem. One of the most important changes for patients is sensory; the ability to feel a hug again makes a big difference, McDonald says.
Today and the Near Future
Although epidural stimulation is not yet available, FES and locomotor training with the Lokomat and its equivalents are now relatively accessible. Nonprofit rehabilitation centers charge about $120 for a FES session and $175 for a session of locomotor training. In cases where insurance covers the treatment, the patient will pay about $40 per session. For those who do not live near a center, FES bikes are available for home use but cost between 15 and 17 thousand dollars.