Chuck Bednar for redOrbit.com – Your Universe Online
While it takes a tremendous amount of focus to make it across an icy parking lot in the dead of winter, new research from the Salk Institute for Biological Studies reveals that there are hidden, unconscious things going on in our bodies that help keep us from falling.
In a study published Thursday in the journal Cell, Salk Institute professor Martyn Goulding and his colleagues explain that when we need to keep our balance on slick surfaces such as ice, there is a cluster of neurons in our spinal cords, which serve as a sort of “mini-brain.”
These neurons integrate sensory information and are responsible for making adjustments to our muscles that prevent us from slipping and falling. In their paper, the study authors map the neural circuitry of the spinal cord that processes the sense of light touch, which allows the body to automatically make slight adjustments to foot position and balance that help keep us upright.
First-ever blueprint of the spinal circuit
The experiments conducted by Goulding and his fellow researchers in mice represent the first-ever detailed blueprint of this spinal circuit, which acts like a control center by integrating motor commands from the brain with sensory information from the limbs.
A better understanding of these mechanisms could led to better treatment for spinal cord injuries, and diseases known to affect motor skills and balance. It could also help prevent falls for senior citizens, according to the study authors.
“When we stand and walk, touch sensors on the soles of our feet detect subtle changes in pressure and movement. These sensors send signals to our spinal cord and then to the brain,” said Goulding.
“Our study opens what was essentially a black box, as up until now we didn’t know how these signals are encoded or processed in the spinal cord,” he added. “Moreover, it was unclear how this touch information was merged with other sensory information to control movement and posture.”
Maintaining balance while walking across an icy parking lot requires use of several of our senses. Vision tells us whether we’re on nearly-invisible “black ice” damp asphalt, while the inner ear’s balance sensors help us keep our heads level with the ground. Also, our muscles and joints help monitor our arms and legs as they change position during locomotion.
All of that information, including signals from the light touch transmission pathway detailed in the new study, are transmitted to the brain every millisecond. The brain preprocesses this data in sensory way stations such as the eye or spinal cords. While scientists have long suspected that the neurological processes required for movement required data-processing circuits in the spinal cord, those regions had never been identified or mapped – until now.
Mapping the mini-brain
Using imaging techniques involving a reengineered rabies virus, Goulding and his colleagues traced nerve fibers that carry signals from touch sensors in the feet to their connections in the spinal cord, and found that those fibers connect to a group of neurons known as RORα neurons. RORα neurons, which are named for a specific type of molecular receptor found in those cells, are in turn connected by neurons to the motor region of the brain.
When the RORα neurons in the spinal cord were disabled in genetically modified mice, the researchers found that they were significantly less sensitive to movement across the surface of the skin or to a sticky piece of tape that was placed on their feet. However, the rodents were still able to walk, and could also stand normally on flat ground.
When faced with a more difficult task – walking across a narrow beam elevated off of the ground – the mice struggled and tended to be clumsier than those that had their RORα neurons intact. The study author attributed this to the reduced ability of the animals to sense skin deformation when a foot was slipping off the edge of the beam, and to respond with slight adjustments in the position of their feet required to maintain balance on ice or other slippery surfaces.
The Salk Institute team also found that RORα neurons don’t just receive signals from the brain and the light touch sensors, but also connect directly with neurons located in the ventral spinal cord that control movement. As such, they are essentially at the core of a “mini-brain” which integrates signals from the brain and the senses to ensure that limbs function correctly.
“We think these neurons are responsible for combining all of this information to tell the feet how to move,” explained Steeve Bourane, a postdoctoral researcher in Goulding’s lab and first author of the 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. Your body is on autopilot, constantly making subtle corrections while freeing you to attend to other higher-level tasks.”
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