Fig. 3.

Conditions for spiking. (A) Spiking occurs for values of the coupling and the channel closing rate γ between the red and black symbols. They show the long-T_av criterion for spiking C_l (red) and the short-T_av criterion C_s (black), respectively (see Conditions for Spiking and Bursting). Coupling values smaller than C_l entail essentially only local puffs, and coupling values larger than C_s cause the regime of overstimulation. The spike range increases with γ. We investigated both the four-cluster model (squares) and the eight-cluster model (circles). C_l values are similar for both models, whereas C_s values depend on the number of clusters N_cl. The spike range becomes smaller for a larger number of clusters involved in spike nucleation. Each pair of squares and circles indicates the critical () at the standard parameters (Table 1), whereas coupling was changed by varying cluster distance (values range from 0.5 μm to ∞). The triangles show the critical values for the four-cluster model obtained when we varied by changing [IP₃]. That leads to very similar results. (B) The N_clth root of the short-T_av criterion C_s is shown. It suggests that C_s depends essentially exponentially on N_cl, because the N_clth root of C_s is similar for the four-cluster model (squares) and the eight-cluster model (circles).