Miniature treadmills speed up studies of insect walking

Fruit flies walking on miniature treadmills are helping scientists understand how the nervous system allows animals to move in an unpredictable and complex world.

Results using these fruit fly-sized conveyor belts were reported on August 30 Current Biology. Several videos of flies running on treadmills are available on the online research site. The lead author is Brandon G. Pratt, a recent graduate in physiology and biophysics from the University of Washington School of Medicine in Seattle and a National Science Foundation graduate researcher.

He designed small-scale machines from inexpensive parts, based on a prototype by Max Mauer, a former UW mechanical engineering student.

Pratt and his research colleagues explained that walking animals, including insects and humans, must quickly recognize and deal with unexpected changes beneath their feet. If an animal couldn’t do this, it would be nearly impossible for it to move through the world, and injuries from falls would be likely.

How does the nervous system detect these unexpected events and control the body to regain balance during locomotion? This question is being studied in the lab of John Tuthill in the Department of Physiology and Biophysics at the University of Washington, where Tuthill is an associate professor and Pratt conducted his doctoral research. His lab colleagues Su-Yee J. Lee and Grant M. Chou also contributed to this project.

Tuthill’s lab studies proprioception: the way the body constantly senses its joints and movements. Disease, injury and other factors can interfere with people’s and animals’ ability to coordinate their bodies, making it difficult to do simple tasks like getting a drink of water or walking a few feet.

Studying how proprioception controls the body when it goes awry during locomotion is a fundamental challenge for neuroscientists. Experimental disruptions of proprioception can impede animal behaviors and thus thwart efforts to study the role of proprioception in natural activities such as walking.

Historically, treadmills have indeed revived the urge to walk in animals after nervous system disruptions. Treadmills have provided insight into the neural control of walking and running in invertebrates (animals without backbones) such as cockroaches and stick insects, as well as vertebrates such as rodents, cats, and humans.

Split-belt treadmills have two belts that move independently. Researchers use them to study how coordination between the legs adapts when the legs on the left side of the body move at a different speed than those on the right side. These treadmills have played a clinical role in the evaluation of stroke patients.

Both types of treadmills inspired researchers at the Tuthill lab to design miniature versions to study locomotion in fruit flies. These tiny creatures are a good model for studying the neural control of locomotion because they have compact, fully mapped nervous systems. In addition, a suite of genetic tools allows scientists to perform precise and specific manipulations of the flies’ nervous systems.

The Tuthill lab’s linear treadmill system forces flies to walk and allows for long-term 3D tracking. The researchers were able to analyze walking at different speeds in flies with and without impaired proprioception.

On the treadmill, the flies walked in spurts, sprinting to the front of the treadmill chamber and then back up the belt to the back. They spent about half their time walking. They accelerated as the belt did. Like humans and cockroaches, they grew in size as they walked faster. Using the treadmill in their experiments, the researchers achieved the fastest walking speed ever recorded for fruit flies.

“They were able to exceed an instantaneous walking speed of 50 millimeters per second,” the researchers noted.

The researchers also genetically disabled the neurons responsible for proprioception and had the insects run on a linear treadmill. Without this sensory feedback, the flies took fewer steps, but bigger steps. Surprisingly, their leg coordination did not seem to be affected, perhaps because other proprioceptive neurons are more important for coordinating walking, or because the nervous system may have compensated for the lack of sensory feedback.

The scientists found that the split-belt treadmill had little effect on coordination between the legs. However, the flies significantly altered the step distance of their middle legs when the two belts moved at different speeds. The researchers suggest that the flies alter their steps to continue walking straight in the presence of rotational perturbations.

“The middle legs are ideally positioned to stably rotate the fly’s body around its center of mass, similar to rowing a boat from its center,” the researchers explained.

The scientists noted that “these results illustrate how treadmills fill an important gap between free-running and tethered preparations for studying the neural and behavioral mechanisms of fly locomotion.”

The researchers provided the software and hardware designs for these miniature treadmill systems as free, open source to fellow scientists.