The Functional Connectivity between the Locust Leg Pattern Generating Networks and the Subesophageal Ganglion Higher Motor Center

Abstract

30Interactions among different neuronal circuits are essential for adaptable 31 coordinated behavior. Specifically, higher motor centers and central pattern generators 32 (CPGs) induce rhythmic leg movements that act in concert in the control of locomotion. 33Here we explored the relations between the subesophageal ganglion (SEG) and thoracic 34 leg CPGs in the desert locust. Backfill staining revealed about 300 SEG descending 35 interneurons (DINs) and some overlap with the arborization of DINs and leg motor 36 neurons. In accordance, in in-vitro preparations, electrical stimulation applied to the SEG 37 excited these neurons, and in some cases also induced CPGs activity. Additionally, we 38 found that the SEG regulates the coupling pattern among the CPGs: when the CPGs were 39 activated pharmacologically, inputs from the SEG were able to synchronize contralateral 40CPGs. This motor output was correlated to the firing of SEG descending and local 41 interneurons. Altogether, these findings point to a role of the SEG in both activating leg 42CPGs and in coordinating their oscillations, and suggest parallels between the SEG and 43 the brainstem of vertebrates. 44 45 46

The subesophageal ganglion modulates locust inter-leg sensory-motor interactions via contralateral pathways

“…Here we have shown that inputs from the locust SEG modulate interactions between sensory inputs and motor output. Consistent with our previously suggested roles of the insect SEG in leg control (David et al 2016;Knebel et al 2017b), we have shown here that SEG descending signals facilitate intersegmental transfer of sensory information for a functional coordination.…”

“…We have also recently reported that SEG DUM neurons receive an efference copy of the legs' motor output and deliver descending commands (Knebel et al 2017b). The long and highly variable response latencies to the different stimuli observed in the current study, with median delays of up to 480 ms, support the involvement of multi-synaptic 'sensory to neuromodulatory' pathways.…”

“…Our recently reported results (Knebel et al 2017b) suggest a role of the SEG in modulating the coupling between segmental leg CPGs in the absence of leg sensory feedback. Here we sought to uncover the SEG modulation of sensory-motor responses.…”

“…We used a 4 sec time window from the stimulation onset and calculated the Pearson correlation coefficient between the meso-and metathoracic activity vectors, and only statistically significant rho values were taken into account. Analysis of connective recordings was based on spike template recognition using Dataview software (University of St. Andrews), followed by overlay of the events in the different channels to identify descending signals, as further elaborated in Knebel et al (2017b).…”

“…In an attempt to characterize the interplay among the different neuronal elements that control walking in the desert locust, Schistocerca gregaria, we previously mapped the central basis of inter-leg coordination in the thoracic ganglia (Knebel et al 2017a). Following that, we demonstrated the role of the SEG in modulating the coupling between segmental leg CPGs (Knebel et al 2017b). These findings are supported by anatomical descriptions showing that the SEG contains both descending and ascending interneurons and modulatory projections that reach all thoracic ganglia and the brain (Kien and Altman 1984;Bräunig 1991;Roth et al 1994), and thus may be important in translating higher motor commands into direct execution signals.…”