Figure 5. A simple model that uses measured parameters to account for the time course of release.

A, two models of the exocytic limb of the synaptic vesicle cycle where synaptic vesicles are recruited to the readily releasable pool (RRP) from the reserve pool with rate constant k_recr. During AP firing at 20 Hz primed vesicles will be released with a rate constant k_exo defined by the release probability (see text). Aa, all vesicles in the RRP are available for immediate release by an AP. Ab, a subset of the vesicles in the RRP are made available for release with priming rate constant k_prim. B, model predictions at 25°C superimposed on recorded fluorescence trace normalized to a final intensity of 1. The parameter k_exo is 2.0 s⁻¹ when all vesicles in the RRP can be released (Aa, red trace) and 11.5 s⁻¹ when priming is considered to be a factor (Ab, blue trace). k_recr is 0.074 s⁻¹ at both instances while k_prim is 0.8 s⁻¹. Inset shows the initial 5 s at a higher resolution. Note that the model without priming poorly describes how the fluorescence trace develops in the first 5 s. C, model predictions at 35°C with corresponding fluorescence trace. In the model with all RRP vesicles being releasable (Aa, red trace), k_exo is 1.6 s⁻¹ while in the model with priming (Ab, blue trace) k_exo is 8.6 s⁻¹, k_recr is 0.079 s⁻¹ and k_prim is 1.6 s⁻¹. Error bars are s.e.m.