Motor neurons are the cells the brain uses to command muscles to act. Scientists typically thought of them as simple connections, much like the cables that link computers with their accessories. Now, in fly studies, researchers at Columbia's Zuckerman Institute have discovered that single motor neurons can each direct an insect's body to move in far more complex ways than previously thought. This is one of the first times scientists have analyzed in 3D what single motor neurons do while the body moves naturally, researchers are only now beginning to uncover the role that single motor neurons play in movement. Measuring the activity of individual neurons in moving animals has proven to be experimentally difficult.

In experiments that began at the Howard Hughes Medical Institute's Janelia Research Campus in Ashburn, Virginia, the first step for the researchers was to activate light-sensitive molecules in the 25 or so motor neurons that control head movements of the sesame-seed-sized fly. This enabled the scientists to use red light to switch on motor neurons one at a time. At the same time, they recorded the resulting head motions while using artificial intelligence techniques to track these movements. the researchers discovered that activating each motor neuron could make the head rotate in a variety of ways, some even in opposite directions from each other, depending on the starting posture of a fly's head.

The research team's thermostat-like model suggests that when the brain wants to move the body a specific way, it cannot simply stimulate the same set of motor neurons each time and expect the same result. Instead, the brain must calculate which motor neurons to activate based on sensory data it receives about the body's current posture. Indeed, deactivating sensory neurons that monitor the fly's head position altered how the insect moved when the scientists stimulated the motor neurons. Next the researchers want to investigate how other kinds of neurons in the fly, such as those in the visual system, interact with motor neurons to control movement.