Research in Drosophila shows that descending neurons (DNs) responsible for behavior form complex, behavior-specific networks that challenge the idea that individual neurons independently control actions. This discovery has implications for robotics and the study of movement disorders.

A fundamental question in neuroscience is how animals, including humans, translate brain signals into coordinated movements. Normally, the brain transmits motor commands to the body via “descending neurons” (DNs), which control both simple reflexes and complex behaviors.

But the sheer number of DNs and their complex relationships mean they can be difficult to study in large animals. For example, a mouse has about 70,000 DNs, while the human brain has more than a million.

Fruit fly Drosophila melanogaster is a more manageable model with a relatively simple nervous system. It has about 1300 DNA but can perform complex behaviors such as walking, flying, boxing and courtship. This simplicity combined with advanced genetic tools Drosophila ideal for studying the neural basis of behavior.

Discovery of neuronal behavior in Drosophila

A team of scientists led by Pawan Ramdia from EPFL has now discovered how DN y. Drosophila They focused specifically on “command” DNs, a subset of descending neurons that previous studies have shown are sufficient for full-fledged behavior; in the fruit fly, forward movement, escape, egg-laying, and parts of the insect’s courtship dance.

The study shows that team-like DNs recruit additional DN networks rather than acting alone, providing new insights into how simple brain commands can generate coordinated actions. The research was led by Jonas Brown and Femke Hurtak in Ramdya’s group and Nature.

The researchers used optogenetics, a technique that uses light to control neurons to activate specific command DNs in flies. They focused on three types of DNs that trigger forward walking, antennal clipping, and backward walking, respectively. By recording the activity of other DNs in the brain during these activations, they observed how these initial signals recruited additional neurons.

Neural networks and behavioral features

To better understand the connectivity between these neurons, the team analyzed the fruit fly’s brain connectome (a graph describing the synaptic connections between neurons). By mapping the connections, they identified how the command DNs interact with other DNs.

This approach revealed that the team’s DNs do not act alone, but instead form direct excitatory connections with other DNs, effectively forming networks that work together to produce complex behaviors. For example, a DN responsible for moving forward recruits a larger network of DNs than those controlling simpler activities, such as maintenance. These networks are behaviorally dependent, with different sets of neurons being activated for different actions.

The researchers also conducted experiments on headless flies to highlight the role of these networks. They found that certain behaviors, such as walking backwards, could be performed without the network, while more complex behaviors, such as walking forward and picking oneself up, required the full DN network in the brain.

This research offers a new framework for understanding how brain signals are translated into action: Rather than individual neurons acting as simple command centers, most behaviors may be orchestrated largely through the actions of larger networks. This model could inspire the development of better robot controllers and even help us understand human movement disorders.