Detailed Wiring Diagram of Fruit Fly Muscles Reveals Unexpected Complexity

Studies on fruit flies are shedding light on the complex neural coordination of movements, enhancing our understanding of motor neuron functionality.

Scientists are developing a wiring diagram of the motor circuits in fruit flies’ central nervous system that control their muscles. This diagram, which is called a connectome, has already provided insights into the complex coordination between the nerves controlling leg and wing movements.

Complexity in Simple Creatures

While fruit flies seem like simple creatures, the researchers said their motor system contains “an unexpected level of complexity.”

“A typical fly motor neuron receives thousands of synapses from hundreds of presynaptic premotor neurons,” the scientists observed. “This number is on par with the scale of synaptic integration in pyramidal cells of the rodent cortex.”

An anatomical reconstruction of motor neurons that control muscles of the fruit fly leg and wing. Credit: Tyler Sloan/Quorometrix Studio

New Studies on Motor Coordination

Two new papers published in the scientific journal Nature have revealed the latest findings in this area, advancing our understanding of how the central nervous system in animals coordinates individual muscles to facilitate a variety of behaviors.

Animation of the anatomical reconstruction of various nervous system structures involved in take-off and flight in a female fruit fly.

Motor Neuron Efficiency and Adaptability

Fruit flies use their legs for numerous activities such as leaping, walking, grooming, fighting, and courtship. They can also adapt their gait to navigate terrains like house plants, walls, damp surfaces, ceilings – and even insect-scale treadmills.

All such movements, from postural reflexes that enable a fly to hold its position steady, to traversing obstacles or changing flight direction, originate through electrical signals from motor neurons. These signals are conducted through threadlike projections from the motor neuron to stimulate muscles.

A fly’s six legs are managed by just 60 to 70 motor neurons, the researchers pointed out. In a cat, they noted, about 600 motor neurons supply a single feline calf muscle. Only 29 motor neurons govern the power and steering muscles of a fruit fly wing. In comparison, a hummingbird’s pectoral muscle is supplied by 2,000 motor neurons.

Although the fly’s motor neurons are few, it performs remarkable aerial and terrestrial feats.

An anatomical reconstruction of the ventral nerve cord of a female fruit fly. Credit: Tyler Sloan/Quorometrix Studio

Wiring Logic of Premotor Circuits

The scientists explained that motor units are composed of a single motor neuron and the muscle fibers that it can excite. Various motor units, activated in different combinations and sequences, collaborate to achieve a myriad of movement behaviors.

The scientists in the two studies were interested in the wiring logic of premotor circuits. They wanted to understand how a fly’s nervous system coordinates motor units to accomplish varied tasks.

Detailed Mapping and Synaptic Architecture

One of the studies employed automated tools, DOI: 10.1038/s41586-024-07389-x

“Synaptic architecture of leg and wing premotor control networks in Drosophila” by Ellen Lesser, Anthony W. Azevedo, Jasper S. Phelps, Leila Elabbady, Andrew Cook, Durafshan Sakeena Syed, Brandon Mark, Sumiya Kuroda, Anne Sustar, Anthony Moussa, Chris J. Dallmann, Sweta Agrawal, Su-Yee J. Lee, Brandon Pratt, Kyobi Skutt-Kakaria, Stephan Gerhard, Ran Lu, Nico Kemnitz, Kisuk Lee, Akhilesh Halageri, Manuel Castro, Dodam Ih, Jay Gager, Marwan Tammam, Sven Dorkenwald, Forrest Collman, Casey Schneider-Mizell, Derrick Brittain, Chris S. Jordan, Thomas Macrina, Michael Dickinson, Wei-Chung Allen Lee and John C. Tuthill, 26 June 2024, Nature.
DOI: 10.1038/s41586-024-07600-z

The research was supported by a Searle Scholar Award, Klingenstein-Simons Fellowship, Pew Biomedical Scholar Award, McKnight Scholar Award, Sloan Research Fellowship, New York Stem Cell Foundation,