Body movement-sensing fly neurons are turned off during active motion

In a fruit fly, nerve cells that detect limb movement are silenced when the insect walks or grooms. This on-off switch may help the nervous system to shift between two states: one helps keep the body steady and the other readies it to move. 

UW Medicine neuroscientist John Tuthill explained the difference through a human analogy: “Stabilizing reflexes enable us, for example, to stay upright on a swaying train, while the active mode supports dynamic motions like walking across uneven terrain.”

“All animals possess a sense of their body’s position and motion, known as proprioception, which is used to stabilize their body posture and guide their movements,” he said.

Tuthill is a professor of neurobiology and biophysics at the University of Washington School of Medicine. His lab studies the cells, signals and circuits that govern proprioception and motor control in flies.

Led by his former postdoctoral fellow Chris Dallmann, the research resulting in the new findings was reported Sept. 17 in Nature. 

The researchers showed that proprioceptive nerve cells for sensing leg motion are deactivated during active movement. They also discovered the neural circuit that gives rise to this state-dependent switch, which the brain uses to toggle between maintaining postural reflexes and sustaining voluntary movement. 

The fly’s ability to selectively suppress movement feedback, the researchers surmised, could make the insect more sensitive to sudden external events that would perturb it, and therefore quicker to respond. 

Advancing basic scientific knowledge of the sensory feedback that is flexibly tuned to manage these dual tasks may lay the foundation for future clinical applications. 

“Understanding how proprioception is used to control the body is important for developing treatments for sensorimotor disorders and supporting rehabilitation after injury,” Tuthill said. 

While carrying out this recent study in fruit flies, Tuthill’s lab used cell-type specific calcium imaging to learn that position-detecting nerve cell projections operate across a range of behaviors. The inhibition of movement feedback during walking and grooming occurred via a specific class of nerve cells – interneurons -- that function as a liaison between sensory neurons and motor neurons. 

This selective suppression took place only during active, self-directed movements by the insect, not passive movements of the insect’s limbs. The researchers traced the nerve-signaling pathways that conduct this inhibition in a leg-specific manner.

The researchers indicated that certain findings suggest the inhibition may be carried out predictively when the leg is still at rest, after the interneurons receive signals from the brain and before the onset of movement. 

Dallmann is continuing his research on how neural circuits control movement as a Marie Sklodowska-Curie Fellow at the University of Wuerzburg, Germany.