Mechanisms and function of asynchronous flight motor pattern generation

Abstract

The vast majority of insect species and therefore overall species uses asynchronous indirect flight as locomotion. Highly specialized muscles generate the power for the up and down stroke of the wing and are the most energy consuming tissues in biology. In Drosophila melanogaster a small network of five motoneurons (MNs) controls the activity of the wing depressor muscle. The firing pattern of these five MNs is well described, as a firing equidistantly splayed-out in time and in a preferred sequence. The approximate firing rate of ~5 Hz is asynchronous, hence the name, to the muscle contraction frequency of around 200 Hz. However, the mechanism that generates this splayed firing pattern and the functional consequences are not yet fully understood. This thesis will describe how the motor pattens are generated by a minimal central pattern generating network (CPG) that consists of five electrically coupled MNs and translates common, unpatterned, cholinergic, excitatory input into splayed-out patterned firing of the MNs. For a given power demand all MNs fire at similar frequencies but in specific sequences, thus desynchronized. Mechanistically, weak electrical coupling together with a specific excitability class is responsible for network desynchronization. Increasing or decreasing the expression of the gap junction protein ShakB through genetic manipulation disrupts the splay state and increases MN firing synchronization, leading to wingbeat frequency fluctuations during flight. Changing the excitability class of the electrically coupled MNs using genetic manipulation of the Shab delayed rectifier potassium channel also shifts network activity to a more synchronized state. The functional consequence of the desynchronized splayed-out motor patterns is to minimize fluctuations in wingbeat frequency. In vivo calcium imaging in single muscle fibers reveals the kinetics of myoplasmic Ca2+-signals, which can be used to link the MN firing pattern and the wingbeat frequency fluctuations: Splayed-out MN firing minimizes fluctuations of average myoplasmic Ca2+-levels across all muscle fibers, ultimately allowing a uniform wingbeat frequency and thus a steady power output over time. The capability of weak electrical coupling together with the right neuronal excitation class to desynchronize network activity has far-reaching implications for neuronal network activity, since gap junction proteins are ubiquitously expressed in neuronal networks throughout different species. It provides a novel mechanism for different synchronization states in all nervous systems, from flies to humans.