Fig. 9

Numerical simulation of the VGCC-SV coupling distance reveals subpools of synaptic vesicles. a EGTA effect on EPSC amplitudes (EGTA/control) was plotted against the number of swellings (circles) and fitted (black) with the equation ± 95% confidence interval (dashed; see Equation (3) in Results). n_Sw is the number of swellings, RRP_St and RRP_Sw are the RRP (readily releasable pool) sizes in entire stalks and per swelling, Inh_St and Inh_Sw are the inhibitory effects of EGTA in stalks and swellings, respectively. The gray zone (right axis) exhibits the occupancy of tightly coupled vesicles. b The least mean squares (LMS) of the fit exhibit the best EGTA effect in swellings (X-ordinate) and stalks (Y-ordinate). c The magnitude of inhibition by EGTA is a function of distance between vesicular Ca²⁺ sensor and voltage-gated Ca²⁺ channel (VGCC) cluster perimeter. Horizontal dashed lines indicate the EGTA inhibition in stalks (0.88) and in swellings (0.44). Vertical shaded regions indicate the range of distances between the sensor location and nearest open VGCC matching EGTA inhibition for different number of VGCCs in cluster. d Estimated coupling distance of loose and tight SVs were plotted against the number of VGCCs in cluster. e We describe a modular model of the heterogeneity in synaptic function. Variable proportions of distinct morphological modules (i.e. swelling and stalk) define functional heterogeneity. The stalk module (black) contains synaptic vesicles (SVs) with tighter coupling (14 nm; tight nanodomain) to larger VGCC clusters present in lower density. The swelling module (red) contains SVs with looser coupling (30–50 nm; loose nanodomain) to smaller clusters present in higher density. Septin 5 (Sept5) differentiates the coupling distances. Simple calyces mostly contain stalk modules while complex calyces are composed of a mixture of stalk and swelling modules. Arrows represent the contribution of the modules