Figure 1.

(A) Linear approximation of phase-4 depolarization in a deterministic model of a pacemaker cell. The linear phase can be roughly attributed to the depolarization phase from end-diastolic voltage to v_m = −40 mV. (B–D) Procedure steps for extracting M(t), ∂M(t)/∂t, and σ² from stochastic simulation data. In the first step, M_i(t) during each phase-4 depolarization interval i is approximated by linear curve fitting (B, bold traces). Next, the fitted M_i(t) values are subtracted from the membrane voltage signal v_m(t) during the phase-4 intervals. All other intervals are assigned with an artificial value of 0 (C). Finally, a phase-4 signal is generated by concatenating the subtracted signal segments that temporally correspond to phase-4 depolarization (D).