Fig 6. Development of general model described by Eq 1.

A. RRP conceptualized as a collection of autonomous release sites (green squares). B. General model for vesicle recruitment and priming at individual release sites described by Eq 1. Release sites in the F-state (Full) contain a vesicle that is included in the RRP, whereas release sites in E-states (Empty) do not. Recruitment to the RRP at release site i is represented by the transition from E_i to F_i. Subsequent priming that increases the probability of release is represented by increasing the value for β_i over time. The general model can describe the behavior of both sequential and parallel models of vesicle priming as explained in the Results. C. A more traditional sequential model, also consistent with Eq 1, where vesicle priming progresses through discrete states on the way to full maturity.