Scientists have been perplexed by a mystery for many years, but a team of experimental neurobiologists from Johannes Gutenberg University Mainz (JGU) and theoretical biologists at Humboldt-Universität zu Berlin has managed to unravel it. They have been able to identify the type of electrical activity that regulates insect flight in the nervous system.
They describe a previously unreported role of the electrical synapses used by fruit flies during flight in a research that was recently published in Nature.
Fruit flies Drosophila melanogaster moves forward by beating its wings about 200 times each second. Other tiny insects can even beat their wings 1,000 times each second. This irritating, high-pitched buzzing sound that we typically associate with mosquitoes is caused by the high frequency of wingbeats.
Due to their small bodies, insects must beat their wings at a specific frequency to avoid becoming “stuck” in the air, which behaves as a viscous medium. They adopt a cunning tactic that is common among insects to achieve this. The opposing muscles that raise and depress the wings are flexed back and forth in this manner.
High-frequency oscillation of the system can result in the rapid wingbeat rate needed for propulsion. Each motor neuron produces an electrical pulse that regulates the wing muscles only every 20th wingbeat because the motor neurons are unable to keep up with the speed of the wings. These pulses are precisely coordinated with the activity of other neurons.
The motor neurons that control the frequency of the wingbeat produce unique activity patterns. Although they do not fire simultaneously, all neurons fire at a regular rate. Each of them fires at regular, predetermined intervals.
Wing movement in the fruit fly Drosophila melanogaster is governed by a miniaturized circuit solution that comprises only a very few neurons and synapses.
Professor Carsten Duch
Although fruit fly brain activity patterns of this type have been observed since the 1970s, the underlying regulating mechanism has never been fully understood.
The neural circuit regulates insect flight
Collaborating in the RobustCircuit Research Unit 5289 funded by the German Research Foundation, researchers at Johannes Gutenberg University Mainz and Humboldt-Universität zu Berlin have finally managed to find the solution to the puzzle.
“Wing movement in the fruit fly Drosophila melanogaster is governed by a miniaturized circuit solution that comprises only a very few neurons and synapses,” explained Professor Carsten Duch of JGU’s Faculty of Biology.
And it is extremely probable that this is not just the case in the fruit fly. The scientists hypothesize that over 600,000 known insect species that rely on a comparable form of propulsion use a similar brain circuit.
Because it is easy to genetically modify the many elements of its neural circuit, Drosophila melanogaster is the ideal model organism for research in the field of neurobiology. To cite only two instances, it is possible to directly alter the activity of individual neurons and switch on and off specific synapses.
In this instance, the researchers combined a number of different genetic methods to assess the electrical characteristics and activity of the circuit’s neurons. As a result, they were able to pinpoint each and every brain circuit cell and synaptic contact that contributes to the creation of flight patterns.
They consequently discovered that the neural network controlling flight is made up of just a tiny number of neurons and exclusively uses electrical synapses for communication.
New concepts of information processing by the central nervous system
It was previously believed that when a neuron in the flight network activated, inhibitory neurotransmitter chemicals were released between the neurons, preventing them from firing simultaneously.
The researchers have demonstrated through experiments and mathematical modeling that such a sequential distribution of pulse generation can also take place when brain activity is directly regulated electrically, in the absence of neurotransmitters. The neurons ‘listen’ intently to one another at that point, especially if they have just been active, and produce a unique form of pulse.
Mathematical analyses predicted that this would not be possible with “normal” pulses. Therefore, it would seem implausible that this sequenced firing pattern would result from transmission between neurons in a purely electrical form. A few ion channels in the network’s neurons were altered to conduct an experimental test of this theoretical concept.
As indicated by the mathematical model, the flight circuit’s activity pattern synced as expected. The power produced during flying underwent considerable changes as a result of this experimental adjustment.
It is clear from this that in order to ensure that the flying muscles can provide an even amount of power, the activity pattern controlled by the neural circuit’s electrical synapses must be desynchronized.
The findings of the team based in Mainz and Berlin are particularly surprising given that it was previously thought that interconnections by electrical synapses actually result in a synchronized activity of neurons.
The electrical synapses’ activity pattern found here suggests that the nervous system may interpret information in ways that are still not fully understood. The similar mechanism might also be at work in the human brain, where the function of electrical synapses is still not fully known, in addition to thousands of other insect species.
