Individual movements of animals are controlled by specific neurons. The same can be said about the cessation of action in space, during which movement, nutrition and survival are impossible. Authors of a new paper in the journal Nature used optogenetics (a technology that controls cells using light) to reveal “patterns of neuronal death” in the Drosophila fruit fly. It turns out that the insect uses two different stopping mechanisms. The authors compared one to letting off the gas pedal and the other to pressing the brake.

Animals’ brain and special nerve structures located outside the brain are responsible for complex and diverse movements. The initiation and maintenance of actions are provided by special groups of interconnected nerve cells. But no less important is the stopping of movements, which ensures that behavior is manageable and appropriate to changes in the environment. But scientists don’t yet know much about the neuronal mechanisms of inhibition.

Authors of the article in a leading scientific journal Nature Discovered how motor inhibition is regulated in the Drosophila fruit fly Drosophila melanogaster – one of the fundamental objects of research in biology. Despite the obvious differences in the nervous systems of flies and humans (and other vertebrates), the insect’s movement regulation mechanisms and neural connections are similar.

Previously, the same scientists (a team from the USA and the UK) managed to find out which Drosophila neurons are responsible for crawling forward, walking backward and turning the body. They have now turned to mechanisms of movement termination, using genetic screening to identify movement-related neurons.

Scientists were also helped by optogenetics, a technology to control cells using light-sensitive molecules. Using red light, biologists selectively turned off the necessary neurons, causing the flies to freeze in place. For this, the Drosophila connectome model, that is, the set of all contacts between nerve cells, came in handy.

Three separate neuron groups necessary for the cessation of activity were identified and these were named Foxglove, Bluebell and Brake. The groups distinguish between their mechanisms of action and their targets, that is, the other nerve cells they control. All three are not directly related and may work alternatively depending on the situation. Therefore, neurons of the Foxglove and Bluebell groups are needed in order for the insect to stop walking and turning the body (for example, when searching for food and eating it). Brake neurons stop all motor activity and are also particularly important for grooming operations, such as washing flies with their legs.

The principle of operation of the first two groups is similar to releasing the gas pedal: they “turn off” the neurons involved in movement, specifically due to the inhibitory neurotransmitter – gamma-aminobutyric acid (GABA). Brake cells actively interrupt any movement of the fly by being activated by the mediator acetylcholine and suppressing other neurons. At the same time, the insect freezes in place, but maintains a fixed body position.