Putative power muscle control circuit in flight.
A. Power generation by the power muscles (PM) in flight drives wingbeats. Drosophila wingbeats are over an order of magnitude faster than firing of power MNs. The power MNs within the same motor unit fire in a loose, staggered pattern, likely to smooth out PM calcium changes and wing power output. Wingbeat and power MN traces artistically rendered from prior descriptions (Harcombe and Wyman, 1978, 1977; Tanouye and Wyman, 1981; Wyman, 1966). B. Upstream connectivity of the power MNs (dorsal lateral, DLMns and dorsal ventral, DVMns). The bulk of upstream connectivity of the power MNs is contributed by 42 tectular intrinsic neurons (Tect INs), with many DNs (here subsetted by ≥1% groupwise connectivity with Tect IN groups) and other neuron types with connectivity to Tect INs. Upstream connectivity to the power MNs are also associated with inputs to the ps1 and MNwm36 wing MNs, and the mesVUM-MJ which targets the dorsal lateral muscles. Edges are thresholded at ≥100 synapses. The Tect INs comprise neuron groups 15113, 13060, 14502, 18519, 21307, 15337, 22521, 21381, 22289, 15788, 14625. The Tect IN-like neurons comprise groups 13874, 18143, 22530, 18638. C. Morphology of the Tect IN population, shown together with a single DLMn. D. Connectivity of the Tect INs (individual neurons on x-axis) with upstream DNs and other major inputs, and downstream MNs/efferents (combined by group). Laterality index is the mean of the difference between side-separated connectivity, normalized by per-side cell count and further normalized by sum of side-separated connectivity. The Tect INs show variation in the combinations and strength of their upstream and downstream connectivity, which is partially reflected in their morphological variation (see E). Neuron groups shown on y-axis are a selection from the top input or output neuron types/groups connected to Tect INs, and named by type, with group (5- or 6-digit number) appended if more than 1 group share a type. Dendrogram shows hierarchical clustering of Tect INs by the subset of upstream and downstream connectivity shown here. E. Morphological variation in Tect INs. F. Lateralized connectivity of DNa08 and DNp31 with Tect INs, IN06B066 group 16779 and power MNs, forming a motif where the same DNs appear to excite and inhibit power MNs. Side annotation is neck side for DNs, nerve side for MNs, and soma side for others. G. Connectivity of three example power MN-associated DNs (DNp31, DNg02 and DNp03) with indirect control MNs. H. Schematic of putative power MN control circuit. In flight, DN input controls Tect IN activity, which then excites power MNs through ‘diffuse’ connectivity (individually small synaptic connections, but collectively strong at the population level). DNa08 and DNp31 further excites IN06B066 group 16779, which may form an inhibition-stabilized with Tect INs to limit runaway excitation. Power MNs sum ‘diffuse’ excitation from Tect INs to individually reach their spiking threshold, and weak electrical connectivity between power MNs further enforce an inhibition-like connectivity between them, such that power MNs spike in a slow, staggered manner to smooth out power muscle calcium changes.