The spinal cord’s ability to process sensory information after a complete injury could play a key role in the development of new robotic rehabilitative devices.
By Tom Scott
The human body can function under extremely adverse conditions. This is becoming more evident in the field of spinal cord injuries (SCI), where researchers are uncovering the spinal cord’s ability to adapt to injury and the need for advanced robotic-assisted locomotor training devices to aid in recovery.
The Brain-Spinal Cord Connection
During a symposium at the New York Academy of Sciences, Reggie Edgerton, professor of Physiological Sciences at UCLA, presented his team’s research findings, as well as other findings within the field, regarding locomotor training in both human and animal models that demonstrate the spinal cord can not only interpret sensory information after a complete SCI, but also use this information on its own to coordinate physical tasks such as stepping or walking.
“The spinal cord functions as part of the brain; it’s not its servant,” says Edgerton, who is focusing much of his efforts on learning how neural networks in the lumbar spinal cord of mammals, including humans, control stepping and how this stepping pattern becomes modified by chronically imposing specific motor tasks on the limbs after a complete injury. “Plasticity in the spinal cord is pretty remarkable.”
In normal locomotion, the brain and spinal cord communicate with each other and interact with motor pools—which includes all the motor neurons that innervate (stimulate through nerves) one muscle—to control function and generate alternating flexion and extension (i.e., walking). After SCI, communication between the brain and spinal cord is disrupted and the complex circuitry within the spinal cord that interacts with motor pools gets its “wires crossed.” Evidence, however, is suggesting that propriospinal neurons can process sensory information without help from the brain, and with proper training, pharmacological intervention, and simple tonic stimulation, the spinal cord can reorganize itself- -essentially relearning how to coordinate motor pools to conduct locomotion. “Almost all the sensory information is still available after spinal cord injury and what is remarkable is how important this sensory information is used,” says Edgerton.
Using cat and rat models, Edgerton and his colleagues have demonstrated that neural networks in the spinal cord (known as central pattern generators) of these animals are capable of generating numerous rhythmic movements, such as walking and stepping motions after complete spinal cord transection. “Learning is a routine and ongoing process in the intact and injured spinal cord… If the spinal cord never steps, then it learns not to step. If it’s taught to step, it will learn to step,” explains Edgerton. “We can train an adult cat [with complete spinal cord transection] and they will learn to step with full weight bearing. In the rat and the mouse we have not been able to achieve that. So we have used some supplemental ways to try to train the animals and what we basically have to do is enhance the excitability of the cord. One of the ways we can do this is by placing epidural electrodes over the lumbosacral spinal cord and when we do that we can get these animals to step quite well.”
Enabling the Stepper Within
The most interesting of Edgerton’s investigations involved the animals’ responses to sensory information when placed on a treadmill. “We generally think about inducing stepping, but the concept of enabling stepping is an important one with respect to the potential application of these properties” within a rehabilitation setting, Edgerton adds. “When we have the animal’s legs placed on the treadmill and we start to stimulate, nothing happens. If you stimulate and then turn the treadmill on, they start stepping.
So it’s the sensory information that is going into the spinal cord that is telling the spinal cord what to do. Now the peripheral sensory system has taken over generally what we thought would happen supraspinal, but in fact it may be that even in our normal movements, the sensory information may be much more important than we have anticipated. We tend to think that sensory information is a corrective system, but it is much more. My guess is that it is a very important feed-forward system. The spinal circuitry can interpret the sensory information, it can anticipate it, and then respond in an appropriate way to generate effective stepping.”
Reversing the direction of the treadmill belt demonstrates the importance of the peripheral sensory system in generating movement. The animal not only steps backwards, but as it does, there is a change in how the motor pools are coordinated. “It looks like there’s a switch,” says Edgerton. “In backwards stepping, you have reorganization so that the stepping pattern is very different. We can also get an animal to step sideways. So this means the spinal cord knows that the type of sensory information it’s getting, it’s supposed to be stepping sideways.”
One of the most promising implications of Edgerton’s research is that it appears that the human spinal cord reacts similarly to peripheral sensory information after a complete injury; other researchers have used these techniques in human models with encouraging results. Advanced robotic systems will take on an important role in future research, enabling the introduction of new sensory motor training techniques and the successful coordination and recruitment of motor pools in test subjects.
“Even in consistent stepping [after training] there are normal variations within the step,” Edgerton says. “We think this is an important feature that needs to be designed in robotics in training animals to step. It’s important to allow the animals [or humans] legs to move on their own. One scenario is to have the robotic arm move the legs in a fixed pattern, another would be to have the nervous system actually decide where to move the leg instead of being told exactly what to do all the time. In comparing these we found that it’s very important to have this type of software to control the robot in this way. We call this ‘assist as needed’ because we really are trying to copy what a highly skilled therapist would do. They will assist the subject as needed when they are learning to step.”
Tom Scott is staff editor.
