'Mini-brain' in spinal cord helps brain keep body balanced

Salk researchers balance with help from 'mini-brain' they helped discover
Credit: Salk Institute

The Human Brain Project and other efforts to build 3D maps showing how the physical surfaces of the brain deliver the power to understand quantum physics or walk across an icy pond without falling will have to tweak their plans. Dinosaurs may not have had a tiny, second brain in their backs to control their hind ends, but humans very well might.

A bundle of neurons within the spinal cord are able to act like a "mini-brain" taking in signals of light touch from the feet and legs, and returning the fine-grained commands that subtly reposition the feet, shift the balance or otherwise helm make the process of maintaining balance on ice or across a narrow walkway seem automatic, according to a paper published in the journal Cell today by researchers from the Salk Institute for Biological Studies and Harvard Medical School

The two teams have been studying the neural pathways of mice to determine how sensations of pain and light touch are gathered, coded and sent to the brain and how commands to respond to those stimuli come back from the brain.

Some of those commands come not from the brain, but from a nerve circuit the team had previously identified as being at least partially responsible for sensations of phantom pain that appears in some suffering maladies such as fibromyalgia or to those who have had a limb amputated.

In a study published by Cell Nov. 20, 2014, the same team of researchers described how they cracked open the "black box" circuitry that allows some people to feel intense pain when subjected only to light touches, changes in temperature or no obvious cause at all, according to Marty Goulding, senior researcher on the Salk team, according to an announcement of the study from Salk.

The study – which was conducted in mice – focused on the "dorsal horn," the point at which sensory neurons from the skin connect to the spinal cord, which contain both pain receptors and touch receptors. Pain receptors activate when a violent touch on the skin sets off a chain reaction of neurochemicals that send a pain signal to the brain. A set of inhibitory neurons can stop the signal with their own chemical response; then those inhibitors function badly or not at all, even a light touch could send a pain signal to the brain.

The study published today follows research that tried to map the circuits responsible for light touch and ended up creating the blueprint for a spinal circuit that combines sensory data that, for example, indicate the body is standing upright, combine them with signals that there is more weight on one side of each foot than the other, and combine those signals into one more concise signal that tells the brain the body is leaning to one side. The brain may send the command to straighten up, which are delivered to a specific type of motor-control neuron in the spinal cord.

Those neurons also appear to be responsible for detecting the edges of a narrow surface across which the mouse walked, a tilting surface that required a change of balance to stay upright, researchers found. Those cells, RORα neurons, also detect millisecond-to-millisecond changes in the way a surface feels by noting changes in the weight, position, pressure, sliding, movement or other indications that the body is standing on unfirm ground.

Rather than just telling the brain about all those subtle changes, it appears the RORα neurons are also able to tell the body how to respond – providing much of the detail of "How" within a command from the brain to "Stand up," for example.

"We think these neurons are responsible for combining all of this information to tell the feet how to move," according to Steeve Bourane, a postdoctoral researcher working in Goulding’s lab, who was lead author of the new paper.

"If you stand on a slippery surface for a long time, you’ll notice your calf muscles get stiff, but you may not have noticed you were using them," Bourane is quoted saying in the Salk release. "Your body is on autopilot, constantly making subtle corrections while freeing you to attend to other higher-level tasks."

That doesn't mean the clump of neurons does some of the brain's thinking for it, only that they're able to order millisecond-by-millisecond adjustments that allow the body to continue balancing once the brain has told it to do so.

Despite decades of study into reflexes, paralysis, growth, coordination, amputation, re-attachment and anything else related to the mechanisms the brain uses to take in information from and issue commands to the body, understanding the specific neurons and data pathways involved in getting the message from the brain to the foot, for example, is still "one of the central questions of neuroscience," Goulding said.

The finding could help advance research into ways to return feeling and control to paralyzed limbs, guidance for the design of humanoid robots and advance understanding of the functions of the brain itself, which seems to use tiny clusters of nerves to gather data from many sensors, encode it into manageable signals and, at least in this case, outsource some of the detail work required to keep the body upright as well.

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