It is well known that neural circuits in the spinal cord control seemingly simple things like the pain reflex in humans, and some motor control functions in animals. Now, a new study published in the journal Nature Neuroscience has shown that the spinal cord is also able to process and control more complex functions, like the positioning of your hand in external space.
“Our research has shown that a least one important function is being done at the level of the spinal cord and it opens up a whole new area of investigation to say: what else is done at the spinal level and what else have we potentially missed in this domain?” said co-lead author Dr. Andrew Pruszynski, a researcher at Western University in London, Ontario, Canada.
“This kind of hand control requires sensory inputs from multiple joints — mainly the elbow and the wrist — and these inputs was previously thought to be processed and converted into motor commands by the brain’s cerebral cortex.”
Using specialized robotic technology, a three degree of freedom exoskeleton, subjects were asked to maintain their hand in a target position and then the robot bumped it away from the target by simultaneously flexing or extending the wrist and elbow.
Dr. Pruszynski and colleagues measured the time that it took for the muscles in the elbow and wrist to respond to the bump from the robot and whether these responses helped bring the hand back to the initial target.
By measuring the latency, or ‘lag,’ in the response, they were able to determine whether the processing was happening in the brain or the spinal cord.
“We found that these responses happen so quickly that the only place that they could be generated from is the spinal circuits themselves,” said co-lead author Dr. Jeff Weiler, a postdoctoral researcher at Western University.
“What we see is that these spinal circuits don’t really care about what’s happening at the individual joints, they care about where the hand is in the external world and generate a response that tries to put the hand back to where it came from.”
This response generated by the spinal cord is called a ‘stretch reflex,’ and has previously been thought to be very limited in terms of how it helps movement.
“Historically it was believed that these spinal reflexes just act to restore the length of the muscle to whatever happened before the stretch occurred,” Dr. Pruszynski said.
“We are showing they can actually do something much more complicated — control the hand in space.”