Fig. 6.
A multicompartmental model of the lobula giant movement detector (LGMD) reproduced the subthreshold effects of M conductance (gM). A: illustration of the model morphology from 2 orientations. The LGMD’s 3 dendritic fields (shown in blue) had an average gM density of 26 µS/cm2 at resting membrane potential (Vrest; −65 mV). The LGMD’s primary neurite (shown in black), which connects the dendritic fields to the spike initiation zone (SIZ; red) and continues because the axon (green) had an average conductance density of 93 µS/cm2 at Vrest. The axon extends farther than shown and had a total length of 463 µm. B: the gM value used for simulations had a broad steady-state activation curve with steepness of 12 mV. C: the time constant of gM activation (τm) had a minimum and maximum of 2.5 and 21 ms. D: removal of gM to simulate XE991 application increased the resting membrane potential (resting Vm) by 2.7–3.5 mV. For all simulations, data ranges and variability are from measurements of different model sections. E: measured input resistance (Ri) to step currents increased by 25–42% after gM reduction. F: membrane time constant also increased after simulated XE991 application, by 41–55%. In D–F, central lines are medians, top and bottom box edges are 75th and 25th percentiles, and whiskers denote the extent of data. G: measured summation to simulated excitatory postsynaptic potentials (sEPSPs) increased for all delays after simulated XE991 application; circles and error bars are medians ± mad. Inset shows traces of injected current and membrane potential for sEPSPs with delays of 10 ms. H: median effective Ri to sEPSPs increased from 5.0 to 8.7 MΩ after gM blockade.