spinal circuit neurons

This image from Salk Institute shows the neural circuitry in a mouse spinal cord that helps govern balance.

Scientists at the Salk Institute for Biological Studies, La Jolla, Calif, have found that our balance is partly governed by a spinal circuit of neurons that sends sensory information from our ears, eyes, and feet to our brain for processing. In a study article published in the January 29, 2015 edition of Cell, Salk researchers explain how they have mapped the spinal cord’s neural circuitry and found that light touch sensors in our feet inform our bodies of adjustments that must be made in order to maintain balance. This information about touch is integrated with information from our ears and eyes to help us perform fine motor tasks as we move through our environment.

The mouse study from Salk provides the first detailed blueprint for a spinal circuit that serves as the control center for integrating motor commands from the brain with sensory information from the limbs, according to the researchers. The Salk team believes that a better understanding of these circuits may one day aid in developing therapies for spinal cord injuries and diseases that affect motor skills and balance.

“When we stand and walk, touch sensors on the soles of our feet detect subtle changes in pressure and movement,” said Martyn D. Goulding, PhD, a professor at Salk Institute and senior author on the paper. “These sensors send signals to our spinal cord and then to the brain. 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.”

According to Goulding and colleagues, our movements require many of our senses to prevent us from falling. Balance sensors in our inner ear keep our heads level with the ground. Our eyes tell us whether we’re on shiny ice or damp asphalt. Our sense of touch tells us how to adjust our bodies to maintain equilibrium, using sensors in our muscles and joints to track the changing positions of our arms and legs. Multiple streams of sensory data from sound, sight, and touch flow to the brain for processing.

Goulding’s team found that the brain preprocesses this data in sensory “way stations” such as the eye or spinal cord. In their study, the Salk scientists tracked this sensory-motor control system using imaging techniques that rely on a reengineered rabies virus. They traced nerve fibers that carry signals from the touch sensors in the feet to their connections in the spinal cord. They found that these sensory fibers connect in the spinal cord with a group of neurons known as ROR? neurons, named for a specific type of molecular receptor found in the nucleus of these cells. The ROR? neurons are connected by neurons in the motor region of the brain, which suggests they might serve as a link between the brain and the feet.

“We think these neurons are responsible for combining all of this information to tell the feet how to move,” said Steeve Bourane, PhD, a postdoctoral researcher in Goulding’s lab and first author on 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. Your body is on autopilot, constantly making subtle corrections while freeing you to attend to other higher-level tasks.”

Source: Salk Institute for Biological Studies