Spinal Circuits Do More than We Thought

We often think of our brains as being at the center of complex motor function and control, but it turns out the spinal cord is more involved than we think. We know that spinal column circuits control seemingly simple things like the pain reflex in humans and some motor control functions in animals, but research published today in Nature Neuroscience shows that the spinal cord is also able to process more complex functions, like the positioning of your hand in external space.

“This 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?’” says senior author Andrew Pruszynski of Western University.

Using specialized robotic technology, subjects were asked to maintain their hand in a target position, after which a robot bumped it away from the target by simultaneously flexing or extending the wrist and elbow. The researchers measured the time that it took for the muscles in the elbow and wrist to respond to the bump from the robot and whether the hand was brought back to the initial target. By measuring the latency in response, the scientists 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,” says first author Jeff Weiler. “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,” Pruszynski explains. “We are showing they can actually do something much more complicated—control the hand in space.”