When Christopher Reeve became quadriplegic, there was little hope for patients with spinal cord injury. Now researchers are combining what they know about the central nervous system’s ability to rewire and regrow with a new understanding of the hidden smarts of the spinal cord to dramatically improve treatments.
Even the most devastating spinal cord injuries usually do not completely sever the link between the brain, spine and the rest of the body. Scientists are now finding ways to make the most of the remaining connections using a variety of technologies. Studies on electrical stimulation and locomotor training (a treatment that relies on human or robotic assistance during a walking exercise) suggest that it is possible to regrow damaged neuronal circuits in the brain and spine and recover some voluntary control. Some of these studies find that circuits in the spinal cord itself can be coaxed into helping the body move again.
When we walk, two sources of information are processed by the spinal cord. One comes from above: instructions from the brain about where we want to go based on what we see. The other comes from below: sensory information from the muscles, tendons and skin. After a spinal cord injury the communication lines between the brain and spinal cord are cut or dramatically diminished, depending on the severity of the event. Without instructions from the brain, doctors and researchers thought it impossible to regain any type of control over the limbs. But unlike fixed mechanical circuits, the brain and spinal cord are malleable. The axiom in neuroscience is “neurons that fire together wire together,” meaning that connections between neurons grow or atrophy based on activity.
One promising approach is to help paralyzed patients go through the motions of walking with “assistive” technologies supporting their weight. By amplifying the sensory signals that come from the joints as they move and from the soles of the feet as the pressure is rhythmically switched from one foot to the other, researchers think they can compensate for the lack of a strong brain signal. Clinicians use devices such as the Lokomat that support the patient’s weight with a harness and move his legs on a treadmill via robotic leg braces. Susan Harkema, director of the Kentucky Spinal Cord Injury Research Center, notes that when weight-supported on a treadmill, newborns show the right stepping patterns even though they can’t initiate walking on their own. This suggests that some motor “programs” are stored directly in the spinal cord, and can be triggered by sensory input. According to neurologist Volker Dietz, a professor emeritus at the University of Zurich who continues to do research at Balgrist University Hospital in Zurich, scientists have whittled it down to two essential inputs to get stepping patterns in the muscles: contact with the ground and flexion and extension of the hip joint. When researchers measure the activity of the spinal circuits of a paralyzed patient on the Lokomat, they find that the pattern is the same as the one found in healthy volunteers. The difference is that the signal is not strong enough to contract the muscles. Locomotor training is meant to increase the spinal signal.