Neurons And Why Mice Don't Gallop

A group of special spinal cord nerve cells manages not only running in mice, telling them when to go faster, but also whether to gallop or not.

In most animals, walking and running share common overlapping processes in spinal cord wiring. When an animal's brain wants faster movement, a biological program fires motor neurons in the right sequence and intensity to put one foot in front of the other. To move faster, different neurons join in.

In recent research by Cornell researchers published in Nature Communications, microscopic electrodes were inserted into single nerve cells and electrically stimulated nerves to simulate signals from the brain. A group of neurons called interneurons were discovered that fired only when the brain called for higher speed.

“These neurons don’t play much of a role in moving slowly,” says Ronald Harris-Warrick, professor of neurobiology at Cornell University.

To go faster, mice running on a treadmill simply speed up their left-right motion to go faster. However, University of Chicago researchers recently created mice that switch at higher speeds from left-right running to bounding, with the two front legs and two rear legs moving in synchrony, which is what most four-legged animals do.

The Cornell professor, Harris-Warrick says, however, that for a mouse being chased by many predators, evolution favors left-right running. “Galloping is faster,” he explains, “but if you’re galloping, it’s hard to turn on a dime. You trade speed for dexterity.”

The high-speed neurons apparently activate a neuronal pathway that inhibits bounding. Researchers found they could trigger bounding by chemically treating the nerves in the neuronal pathway.

“What this shows us is that the wiring is all there for a mouse to gallop, but these neurons are preventing the animal from galloping,” Harris-Warrick says.

The two-phase approach to locomotion goes back earlier in evolution to small fish in which the activity of interneurons associated with higher swimming speeds is accompanied by silencing of other neurons active at lower speeds. The high-speed system in small fish allows the fish to make sharp “escape turns.”

This research was funded by the National Institutes of Health (NIH) and the National Science Foundation in the hope that it may eventually lead to better treatment of spinal cord injuries in humans.

Reported in Futurity.org May 16, 2011.