Many inhabitants of the Northern Hemisphere are currently experiencing the peak of winter weather, and have become quite adept at maneuvering on top of ice. A new study published in Cell from researchers at the Salk Institute for Biological Studies in California suggests that in addition to the normal mechanisms the body uses to remain upright, a group of neurons on the spine are clustered in a “mini-brain” that combines sensory information and motor commands to make small, unconscious movements of the foot in order to provide better balance.
"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," senior author Martyn Goulding said in a press release. "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. Moreover, it was unclear how this touch information was merged with other sensory information to control movement and posture.”
In order to maintain balance while walking upright, the brain processes a great deal of sensory information. A sense of touch can provide information about wind speed or amount of traction, fluid in the ear gives a sense of our orientation relative to the ground, and sight provides clues about obstacles on the ground, such as an upcoming hill or a change in terrain, and even muscles provide information about their positions. However, the neural pathways that interpret this tremendous amount of information to allow the body to make controlled movements has not been well understood.
In the paper, the researchers mapped out the neural circuitry and found that the key to integrating sensory information with the necessary corresponding motor function is a group of sensory neurons on the spinal cord that connect with the RORα neurons that bridge to the brain’s motor region. Together, it serves as a “mini-brain” that facilitates balance under difficult circumstances.
"We think these neurons are responsible for combining all of this information to tell the feet how to move," lead author Steeve Bourane added. "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.”
Disabling this circuitry in mice revealed that while they could still walk under normal conditions, when the terrain got more difficult, they were incapable of making the small foot adjustments in order to maintain balance and walk smoothly. Getting a better understanding of this pathway and other mechanisms the brain uses to process sensory information could lead to improved therapies for those affected by spinal cord disease or trauma.
"How the brain creates a sensory percept and turns it into an action is one of the central questions in neuroscience,” Goulding continued. "Our work is offering a really robust view of neural pathways and processes that underlie the control of movement and how the body senses its environment. We're at the beginning of a real sea change in the field, which is tremendously exciting.”