How do you teach someone to Moonwalk? You could watch YouTube videos to learn… or you could just figure out how the brain controls backwards walking. And that’s exactly what scientists have done. They discovered that just two neurons control a fly’s ability to back up , and then they got some fruit flies to back away from a banana.

Being able to put one foot behind the other isn’t uniquely human. Many land animals start walking backwards if they sense an obstacle or danger directly ahead -- but the neural circuitry involved isn’t well understood. After all, it’s not merely a reversal of walking forward: Hip muscles actually move differently in backwards walking. But they do know that the brain plays a commanding role. A change in walking direction is likely triggered by “command” neurons signaling to the motor system, which then alters the timing of leg muscle activation.

team from the Research Institute of Molecular Pathology in Vienna, Austria, started by creating about 3,500 lines of Drosophila fruit flies. To identify neuron combinations that (when activated) can change a fly’s walking direction, each of the lines have a different nervous system “expression pattern.” That is, each fly has a temperature-controlled switch that can turn on different neuron networks. By turning various neurons on, the researchers could narrow down which ones control walking direction.

The team found one batch of flies that would walk backwards when exposed to heat, and they dubbed them “moonwalker” flies. They put these guys in a special chamber and silenced the neurons activated in them. This prevented the flies from walking backwards, even when it would have been really helpful to do so (like when they hit a dead end).

Of the seven neurons uniquely activated in the moonwalker fly, activating just a pair of them -- one in the brain, one along the belly -- was sufficient to make the fly back it up.

Specifically, the descending MDN neurons start in the brain with extensions down to the nerve cord (which runs along the belly); these are required to walk backwards when they encounter an impassable barrier. The ascending MAN neurons start at the bottom of the nerve cord with extensions into the brain; these inhibit forward walking.

Activating the neuron in the brain was enough to induce reverse motion, but activating the second neuron was not. So if the fly was a car, the MDN neurons would be the reverse gear and the MAN neurons would be the brakes – which the fly probably uses as a failsafe that reflexively stops forward motion.

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