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Fruit flies can walk, jump, and dance using the same motor neurons

Research is underway to decipher the motor circuits in the central nervous system that control the movements of a fruit fly. Quite surprisingly, this seemingly simple creature harbors an unexpected level of complexity in its neurons and motor system.

A closer look into their motor neurons has revealed that thousands of synapses are received from hundreds of presynaptic premotor neurons.

Quite astonishingly, this is similar to the scale of synaptic integration in pyramidal cells of the rodent cortex. This observation opens up immense possibilities for advanced studies in neuroscience.

Motor circuits

Motor circuits in the central nervous system are networks of neurons that control muscle movements. These circuits include motor neurons that send signals from the brain and spinal cord to muscles, telling them when to contract or relax.

This network helps us perform actions like walking, talking, and writing by coordinating different muscle groups. Motor circuits ensure our movements are smooth and precise.

The circuits work together with sensory neurons, which provide feedback to adjust and refine our actions. Understanding motor circuits helps scientists learn how our bodies move and how to treat movement disorders.

Complexity of a fruit fly’s motor skills

The findings provide further insights into how animals’ central nervous systems coordinate individual muscles to execute a range of behaviors.

Consider a fruit fly: it uses its legs for a variety of tasks, from leaping and walking to grooming, fighting, and courtship. It can even adjust its gait to handle different terrains like house plants, walls, damp surfaces, and even ceilings.

The remarkable part? All these movements, whether it’s maintaining a steady position or changing flight direction, are controlled via electrical signals from motor neurons. These signals traverse threadlike projections from the motor neuron to stimulate the muscles.

Mighty motor neurons in fruit flies

A fruit fly’s six legs are powered by just 60 to 70 motor neurons. In contrast, a cat’s single calf muscle is served by around 600 motor neurons. It’s even more striking when you realize that a fruit fly’s wing – responsible for power and steering – is governed by a mere 29 motor neurons.

Despite having so few motor neurons, flies demonstrate extraordinary aerial and terrestrial feats. How do they manage this?

The answer lies in motor units, which are made up of a single motor neuron and the muscle fibers it can stimulate.

Various motor units, activated in different combinations and sequences, work together to cover a wide range of movement behaviors.

Premotor circuits

The researchers from the University of Washington School of Medicine in Seattle sought to understand how the fly’s nervous system coordinates these motor units to accomplish different tasks.

Using state-of-the-art tools, machine learning, and cell-type annotation, they identified roughly 14,600 neuronal cell bodies and about 45 million synapses within the ventral nerve cord of a female fruit fly.

They then employed deep learning to reconstruct the anatomy of the neurons and their connections throughout the fly’s body.

The results? A comprehensive map of the muscles targeted by leg and wing motor neurons, and an atlas of the circuits that coordinate flight initiation and mid-air movements.

Mystery of multiple innervations

Interestingly, some muscle fibers in adult flies are innervated by several motor neurons – a feature that appears in some mammals as newborns but usually disappears by adulthood.

The team theorizes that this could offer greater flexibility and explain why insects‘ limbs operate with such precision despite having so few motor neurons.

Wing it like a fly

The researchers also looked into the fruit fly’s wing motor system, divided mainly into three functional sections: powering the wing flapping, steering, and adjusting wing motion.

The study led to a comparison of the organization of premotor circuits for two types of limbs with distinct evolutionary paths and biomechanics.

These studies are crucial for developing connectomes, which are offering groundbreaking insights into neural circuit functionality.

The future looks promising, with the upcoming reconstruction of a male fruit fly’s central nerve cord potentially revealing differences between sexes.

The study is published in the journal Nature.


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