Summary: In the traditional view of the brain, excitatory neurons act like gas pedals (triggering movement), while inhibitory neurons act like brakes (stopping it). However, researchers have flipped this script.
Their study reveals that a specific circuit of inhibitory neurons in fruit flies actually generates and coordinates the rhythmic leg movements used for grooming. By strategically “applying and releasing the brakes” on opposing muscles, these neurons create the back-and-forth motion required for complex, innate behaviors.
Key Facts
- The Connectome Milestone: This research leveraged the landmark 2024 release of the adult Drosophila brain map, which includes 50 million synapses.
- Counterintuitive Discovery: The study proves that rhythmic movement can be driven entirely by inhibitory signals, without needing a dedicated excitatory “start” signal for every individual limb flick.
- Biomimetic Potential: Understanding how simple, local rules of inhibition create complex, fluid movement could revolutionize robotics and the design of coordinated mechanical limbs.
- Undergraduate Contribution: Much of the foundational work involved years of manual neuron tracing and data proofreading by UCSB undergraduates.
- The “Continuous” Mystery: The researchers are now looking into how these circuits allow a fly to transition seamlessly from one complex behavior (like walking) to another (like grooming) without stopping.
Source: UC Santa Barbara
Researchers at UC Santa Barbara are coming ever closer to uncovering the neural circuitry that translates stimulus to action, shining light on previously unseen neural connections and lesser-known functions of neurons that underlie behavior.
Neuroscientists Durafshan Sakeena Syed, Primoz Ravbar and Julie H. Simpson have found that inhibitory neurons — nerve cells known to be responsible for suppressing movement — actively generate and coordinate the rhythmic limb movements required for grooming in fruit flies.
These findings, according to Syed, do not only demonstrate complexities of the animal nervous system that we are only beginning to learn; they also have potential implications for robotics and biomimetic design.
This work, supported by both the National Science Foundation and the National Institutes of Health, are published in the journal eLife.
Going by stopping
The release of the full fruit fly connectome in late 2024 represented a major milestone for neuroscientists, who can now use the complete map of all 139,000 neurons and roughly 50 million synapses in the adult Drosophila brain to find the structural underpinnings of the fly’s complex behaviors.
“We are at a very exciting time right now,” said Syed, a postdoctoral researcher in the Simpson Lab.
Among these behaviors is grooming — the “sweeping” of face, body and feet that rids them of debris. These movements are innate, rhythmic and require coordination between the extension and flexion of opposing muscles and limbs. The question for the researchers was: What happens along the circuit between the fly’s sensory neurons, which sense the dust, and the motor neurons, which tell the limbs to move?
“In between there is a ‘black box’ of neurons and we didn’t know how they receive the information, process it and send it to the motor neurons,” Syed said. Among these “pre-motor” neurons were a lineage of inhibitory neurons that caught the researchers’ attention.
“The thing is, when you think about movement, traditionally you assume that the activated pre-motor neuron would excite the motor neuron,” Syed said, leading to movement. Instead, their optogenetic experiments, which use light to selectively control the activity of specific cells, revealed that these inhibitory pre-motor neurons, which function as a “stop” signal, are capable of driving movement without the need for excitatory signals.
“What they do is put the brakes on one muscle and then the same neuron can remove the brakes from the antagonistic muscle. Since these neuron groups are reciprocally connected, they can induce the alternation between extension and flexion so you can have these repetitive movements,” Syed explained.
While one neuron is busy braking one motion and releasing the brakes for the other, a complementary, connected neuron is doing the opposite on the same set of muscles, braking what was released, and releasing what had been stopped. That results in these alternating moments of extension and flexion.
Coordination is one of the main functions of these neurons, preserving the sequence of braking and releasing, according to Syed, who added that If these inhibitory neurons were continuously activated, or if they were all silenced, in both cases grooming behavior would decrease.
Excitatory neurons are likely still part of the broader motor pathway, she said, but their contribution in this context remains to be tested.
Additionally, the researchers found that there were multiple ways the fly nervous system could control each limb via “specialist” and “generalist” inhibitory neurons. The specialist cells control individual joints and fine movements, while the generalists can function like a switch controlling several movements across multiple joints, which is especially useful and efficient in cases of commonly repeated motions, such as those for grooming, flying, eating and walking.
Generalists can be used to execute these common patterns of motion, while specialists can allow the organism to react to changes in the environment.
This work began before the full fruit fly connectome was completed, and is the result of years of painstaking effort by not just the researchers, but also cohorts of UCSB undergraduates who were trained to proofread the sets of electron microscopy data and trace the individual paths of these inhibitory neurons.
“The undergraduates who worked on this dataset have made important contributions in editing those neurons that laid the foundations for these discoveries,” Syed said. Manual tracing techniques turned into automated reconstructions over the years as the techniques evolved.
In collaboration with Primoz Ravbar, also in the Simpson Lab, they built a computational model that enabled them to test these neural circuits.
Building on this research, future work may involve more interrogations of the neural basis of fruit fly behaviors, such as how the nervous system enables a switch from one complex task to another — investigations that could eventually set the stage for studying more complex organisms in the future.
“We know that flies don’t stop doing what they are doing and then start a new action; it’s continuous.” Syed said. “How does that transition happen? That’s what I’m interested in.”
Key Questions Answered:
A: Think of it like a spring-loaded trap. The muscles are ready to move, but the inhibitory neurons hold them back. By rhythmically “releasing the brake” on one side while “applying” it to the other, the neurons create a high-speed, alternating rhythm that looks like a fluid sweeping motion.
A: Efficiency. If the fly had to think about every single joint for a common task like grooming, its brain would be overwhelmed. Generalists act like a “macro” on a computer, one switch that triggers a complex, pre-programmed sequence across multiple limbs.
A: Very likely. While the fruit fly is a simpler model, the fundamental logic of “reciprocal inhibition” is a core feature of nervous systems. This discovery helps us understand the “hidden math” our own brains use to coordinate walking or typing without us having to consciously fire every muscle.
Editorial Notes:
- This article was edited by a Neuroscience News editor.
- Journal paper reviewed in full.
- Additional context added by our staff.
About this neuroscience research news
Author: Sonia Fernandez
Source: UC Santa Barbara
Contact: Sonia Fernandez – UC Santa Barbara
Image: The image is credited to Neuroscience News
Original Research: Open access.
“Inhibitory circuits control leg movements during Drosophila grooming” by Durafshan Sakeena Syed, Primoz Ravbar, and Julie H. Simpson. eLife
DOI:10.7554/eLife.106446.4
Abstract
Inhibitory circuits control leg movements during Drosophila grooming
Limbs execute diverse actions coordinated by the nervous system through multiple motor programs. The basic architecture of motor neurons that activate muscles which articulate joints for antagonistic flexion and extension movements is conserved from flies to vertebrates.
While excitatory premotor circuits are expected to establish sets of leg motor neurons that work together, our study uncovered an instructive role for inhibitory circuits — including their ability to generate rhythmic leg movements.
Using electron microscopy data in the Drosophila nerve cord, we categorized ~120 GABAergic inhibitory neurons from the 13 A and 13B hemilineages into classes based on similarities in morphology and connectivity.
By mapping their connections, we uncovered pathways for inhibiting specific groups of motor neurons, disinhibiting antagonistic counterparts, and inducing alternation between flexion and extension.
We tested the function of specific inhibitory neurons through optogenetic activation and silencing, using high-resolution quantitative analysis of leg movements during grooming.
We combined findings from anatomical and behavioral analyses to construct a computational model that can reproduce major aspects of the observed behavior, demonstrating that these premotor inhibitory circuits can generate rhythmic leg movements.
