Figure 1.

Axes labels v and w represent the non-dimensionalized excitation current and recovery current, respectively. (a) Local dynamics described by equations (2.3a,b). The dashed-dotted line represents the null isocline , and the curve represents the null isocline . The dashed section of the curve is the unstable branch. The fixed point of the dynamics is at P, where the null isoclines meet. Parameters are given in table 1. Trajectories (b) and (c) shown as thick, solid lines, vary depending on the initial perturbation from the equilibrium point P. (b) A small perturbation quickly relaxes back to rest. (c) A large perturbation triggers a substantial response. An initial rapid excitation (1) is followed by a period in which the system remains in an excited state (2). After a rapid relaxation (3), the system enters a refractory period in which no further excitation can take place (4), before regaining excitability as it returns to its rest state (5). (d) Local dynamics described by equations (2.3a,b) for a pacemaker cell. Parameters as in table 2. P is an unstable fixed point, causing continuous re-excitation of the cell after a refractory period. (Online version in colour.)