A team of neurobiologists from Johannes Gutenberg University Mainz and theoretical biologists from Humboldt-Universität zu Berlin have discovered the nature of electrical activity in the nervous system of insects that controls their flight. In a recent paper published in Nature, they revealed a previously unknown function of electrical synapses used by fruit flies during flight.

Fruit flies beat their wings around 200 times per second to move forward. Other small insects can manage up to 1,000 wingbeats per second, producing the high-pitched buzzing sound we often hear from mosquitoes. To stay aloft, insects must beat their wings at a certain frequency, which they achieve through the reciprocal stretch activation of their antagonistic muscles. This allows them to oscillate at high frequencies and maintain propulsion.

The motor neurons generate an electrical pulse that controls the wing muscles every 20th wingbeat, precisely coordinated with the activity of other neurons. These pulses generate special activity patterns in the motor neurons that regulate the wingbeat frequency. While neural activity patterns of this kind have been observed in fruit flies since the 1970s, the underlying controlling mechanism was not understood until now.

A miniaturized neural circuit regulates insect flight

Researchers at Johannes Gutenberg University Mainz and Humboldt-Universität zu Berlin have found the solution to the puzzle. Professor Carsten Duch of JGU’s Faculty of Biology explained.

“Wing movement in the fruit fly Drosophila melanogaster is governed by a miniaturized circuit solution that comprises only a very few neurons and synapses.”

It is likely that this method of propulsion is not only used by fruit flies, but also by other insect species that number more than 600,000. The researchers focused on Drosophila melanogaster for their neurobiology study because its neural circuit components can be genetically manipulated. Researchers can even turn individual synapses on and off or influence the activity of specific neurons.

For this particular study, the researchers used a combination of genetic tools to measure the neurons’ electrical properties and activity in the circuit. As a result, they identified the cells and synaptic interactions of the neural circuit involved in generating flight patterns. The neural network that regulates flight is composed of only a small number of neurons that communicate through electrical synapses.

New concepts of information processing by the central nervous system

It was previously believed that neurotransmitters were responsible for preventing neurons in the flight network from firing at the same time. However, researchers have discovered that electrical control of neural activity can also create a sequential distribution of pulse generation without the need for neurotransmitters. The neurons generate a specific type of pulse and listen closely to each other, especially if they have just been active.

Mathematical analyses suggested that this would not be possible with regular pulses. Therefore, it seemed unlikely that transmission between neurons in an entirely electrical form would result in this sequenced firing pattern. To test this hypothesis experimentally, certain ion channels in the network’s neurons were manipulated. As predicted, the activity pattern of the flight circuit became synchronized, just as the mathematical model had anticipated. This manipulation caused significant variations in the power generated during flight. Therefore, the desynchronization of the activity pattern determined by the electrical synapses of the neural circuit is necessary to ensure that the flight muscles can consistently generate power output.

The team based in Mainz and Berlin made some surprising findings. Previously, it was believed that electrical synapses resulted in the synchronized activity of neurons. However, the activity pattern that was observed suggests that there may be forms of information processing by the nervous system that are still not fully understood. This mechanism may play a role in thousands of other insect species, as well as in the human brain, where the purpose of electrical synapses is still not fully understood.