Fig. 4. Mapping of firing properties of the simulated zebrafish muscle onto the V₅₀s of activation and inactivation of the sodium current. In each graph, the V_50,act is plotted against the V_50,inact. This was repeated for stimuli of 0.5, 1, 2 and 4 nA (left to right in each row). The values obtained from in situ recordings (Buckingham and Ali, 2004) are indicated with an x. (A) When the time constant of inactivation is set to those values recorded experimentally (along with those estimated for missing values of membrane potential; see Results), increasing the amplitude of the depolarizing stimulus increases the parameter space over which single spiking is observed but does not greatly increase the area over which repetitive firing is observed between the 1 nA and 4 nA stimuli. (B) If the values of the time constant of inactivation at every membrane potential are doubled (representing a slowing of inactivation), the area of parameter space over which repetitive firing is obtained increases and approaches the parameter set observed in situ, but it is not affected by increasing stimulus amplitude (left to right). Repetitive firing was defined as the occurrence of more than one spike. Black areas, no spikes obtained; red areas, once-only firing; green areas, repetitive firing.