Fruit flies running on tiny treadmills are assisting scientists in understanding how the nervous system helps animals navigate a complicated and unpredictable environment. The researchers built these compact machines using low-cost components. These treadmills facilitate studies on how fruit flies perceive and adapt to unforeseen changes on the ground as they travel.
Fruit flies walking on miniature treadmills are helping scientists learn how the nervous system enables animals to move in an unpredictable and complex world.
Findings from these fruit fly-sized treadmills were shared on August 30 in Current Biology, a journal published by Cell Press. Videos of the flies in action on the treadmills can be viewed in the online research article. The lead author is Brandon G. Pratt, who recently earned his Ph.D. in physiology and biophysics from the University of Washington School of Medicine in Seattle and is currently a National Science Foundation Graduate Research Fellow.
He designed the small-scale devices using affordable materials, based on a concept developed by Max Mauer, a mechanical engineering graduate from the UW.
Pratt and his research team pointed out that animals, including insects and humans, must quickly recognize and respond to unexpected changes while walking. Without this ability, it would be tremendously difficult for animals to move through their environment, and they would be at a much greater risk of falling.
How does the nervous system identify these surprise occurrences and adjust the body to regain balance while in motion? This crucial question is being examined in John Tuthill’s lab at the UW Department of Physiology and Biophysics, where Tuthill serves as an associate professor and Pratt completed his doctoral research. Lab collaborators Su-Yee J. Lee and Grant M. Chou also played significant roles in this study.
Tuthill’s lab focuses on proprioception: the body’s ability to continuously sense its position and movement. Conditions like illness or injury can disrupt this ability, making it harder for both people and animals to perform straightforward tasks such as reaching for a glass of water or walking a short distance.
Studying how proprioception governs the body when movement becomes unsteady is a primary hurdle for neuroscientists. Experimental interruptions to proprioception can affect animal behavior and complicate efforts to explore its role in natural movements such as walking.
Historically, treadmills have effectively rekindled the walking instinct in animals after disruptions to their nervous systems. They have provided valuable insights into how neural mechanisms control walking and running in both invertebrates, like cockroaches and stick insects, and vertebrates, including rodents, cats, and humans.
Split-belt treadmills have two independent moving belts. These are used by researchers to examine how leg coordination changes when the left legs move at a different pace than the right. Such treadmills have been useful in clinical settings to assess patients who have experienced strokes.
Inspired by both treadmill types, researchers in Tuthill’s lab created small versions to study fruit fly locomotion. Fruit flies serve as an excellent model for examining neural control of movement because they have a compact and fully mapped nervous system. Additionally, there are numerous genetic tools available that allow scientists to manipulate the fly’s nervous system accurately.
The treadmill system in Tuthill’s laboratory encourages flies to walk and facilitates extended 3D tracking. This setup enables the analysis of different walking speeds in flies both with and without impaired proprioception.
While on the treadmill, the flies exhibited bursts of movement, dashing to the front of the chamber and then gliding back on the belt. They spent about half of their time in motion and increased their speed in response to the belt. Similar to humans and cockroaches, their body height increased when they walked faster. Thanks to the treadmill experiments, the researchers recorded the fastest walking speed ever documented for fruit flies.
“They were able to exceed an instantaneous walking speed of 50 millimeters per second,” the researchers observed.
The researchers also suppressed the activity of neurons responsible for proprioception and had the flies move on the linear treadmill. Lacking this sensory input, the flies took fewer but longer strides. Interestingly, their leg coordination appeared unaffected—this could be due to other proprioceptive neurons playing a larger role in maintaining walking coordination, or the nervous system might have adjusted to compensate for the lost feedback.
On the split-belt treadmill, the researchers found minimal impact on leg coordination. However, the middle legs of the flies adjusted their step lengths significantly when the two belts operated at different speeds. The researchers propose that flies alter their stride to walk in a straight line despite rotational disturbances.
“The middle legs are perfectly situated to help stabilize the fly’s body around its center of mass, similar to how one rows a boat from its center,” the researchers clarified.
The scientists emphasized that “these findings highlight how treadmills bridge the gap between free walking and tethered methods for exploring the neural and behavioral processes behind fly locomotion.”
The researchers have made the software and hardware designs for these miniature treadmill systems available as free, open-source resources for other scientists.
This research received funding from a National Science Foundation Graduate Research Fellowship 2018261272, National Institutes of Health grants T32 NS 99578-3, R01NS102333, and U19NS104655, the Searle Scholar Award, Klingenstein-Simons Fellowship, Pew Biomedical Scholar Award, McKnight Scholar Award, Sloan Research Fellowship, the New York Stem Cell Foundation, and a UW Innovation Award. Tuthill is also a Robertson Investigator for the New York Stem Cell Foundation.