Fig. 2.

Various patterns of Ca²⁺ signals can be inferred from properties of single clusters. (A) Configurations of open clusters (red) in the tetrahedral geometry. (B) Left: Waiting time distribution for the opening of a cluster in the rest state (0) and with N_o = 1, 2, or 3 clusters open. The probability to open early increases with N_o (number at lines). Because N_o determines [Ca²⁺], this corresponds to CICR. Right: Probability to open the second cluster at [IP₃] as indicated. It is very unlikely that the second cluster opens before the first one closes with small [IP₃]. (C) Stochastic simulations reveal different types of Ca²⁺ signals at various [IP₃]: only puffs (Upper Left), spiking (Upper Right), bursting (Lower Left), and overstimulation (Lower Right). The colors indicate [IP₃] as in B, p = 3.85 s^-1. (D) The average ISI T_av depends on many different parameters. Here, we show (Left) its dependence on the number of channels per cluster N_ch (black dots) and the cluster distance a (blue triangles), and (Right) its dependence on [IP₃] (black dots) and the channel closing rate γ (blue triangles). Parameter values not mentioned in this legend are given in Table 1.