Inside Salk; Salk Insitute
Home > News & Press > InsideSalk > 08|15 Issue > Walking on ice takes more than brains

Walking on ice takes more than brains

Walking across an icy parking lot in winter—and remaining upright—takes intense concentration. But a new discovery suggests that much of the balancing act that our bodies perform when faced with such a task happens unconsciously, thanks to a cluster of neurons in our spinal cord that function as a “mini-brain” to integrate sensory information and make the necessary adjustments to our muscles so that we don’t slip and fall.

Salk researchers mapped neural circuits in the mouse spinal cord that process light touch signals from the feet, a critical function for fine motor tasks, such as walking on ice. The red cells are RORα neurons, which merge signals from neural fibers coming from the brain and limbs (both colored blue).

In a paper published January 29, 2015 in the journal Cell, Salk Institute scientists map the neural circuitry of the spinal cord that processes the sense of light touch. This circuit allows the body to reflexively make small adjustments to foot position and balance using light touch sensors in the feet.

The study, conducted in mice, provides the first detailed blueprint for a spinal circuit that serves as a control center for integrating motor commands from the brain with sensory information from the limbs. A better understanding of these circuits should eventually aid in developing therapies for spinal cord injury and diseases that affect motor skills and balance, as well as the means to prevent falls for the elderly.

“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,” says Martyn Goulding, senior author and Salk professor. “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.”

Every millisecond, multiple streams of information, including signals from the light touch transmission pathway that Goulding’s team has identified, flow into the brain. One way the brain handles this data is by preprocessing it in sensory way stations such as the eye or spinal cord. In the case of touch, scientists have long thought that the neurological choreography of movement relies on data-crunching circuits in the spinal cord. But until now, it has been exceedingly difficult to precisely identify the types of neurons involved and chart how they are wired together.

In their study, the Salk scientists demystified this fine-tuned, sensory-motor control system. Using cutting-edge 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 RORɑ neurons, named for a specific type of molecular receptor found in the nucleus of these cells. The RORɑ neurons in turn are connected by neurons in the motor region of the brain, suggesting they might serve as a critical link between the brain and the feet.

When Goulding’s team disabled the RORɑ neurons in the spinal cord using genetically modified mice developed at Salk, they found that these mice were substantially less sensitive to movement across the surface of the skin or to a sticky piece of tape placed on their feet. Despite this, the animals were still able to walk and stand normally on flat ground.

However, when the researchers had the animals walk across a narrow, elevated beam, a task that required more effort and skill, the animals struggled, performing more clumsily than animals with intact RORɑ neurons.

The scientists attribute this to the animals’ reduced ability to sense skin deformation when a foot was slipping off the edge and respond accordingly with small adjustments in foot position and balance—motor skills similar to those necessary for balancing on ice or other slippery surfaces.

Another important characteristic of the RORɑ neurons is that they don’t just receive signals from the brain and the light touch sensors, but also directly connect with neurons in the ventral spinal cord that control movement. Thus, they are at the center of a “mini-brain” in the spinal cord that integrates signals from the brain with sensory signals to make sure the limbs move correctly.

The team’s study represents the beginning of a new wave of research that promises to provide precise and comprehensive explanations for how the nervous system encodes and integrates sensory information to generate both conscious and unconscious movement.