Results of simple mathematical model. (A) Diagram of N Ca2+ release units (CRUs). During an action potential each unit has a probability of p to be triggered and generate an event with amplitude A. (B) Assuming that triggering from each CRU is an independent event, the kth term of the binomial distribution describes the probability that k events are triggered. (C) Dots show the relationship between Ca2+ transient amplitude (averaged at one SSL of a cell) and PCL. The model can fit the data perfectly (dashed line) assuming either many release units, each with small amplitude (N=300), or fewer release units, each with larger amplitude (N=35). (D) COV data (black dots) from the same cell were compared with model results of N=35 (black solid line), N=300 (black dashed line) and N=35 with a 50% increased probability p (blue dashed line).