A Threshold Equation for Action Potential Initiation
A, Excitability curve of the model for different values of the ratio gNa/gL (maximum Na conductance over leak conductance), discarding inactivation (h = 1) and other ionic conductances. The resulting threshold is shown with a red dot. B, Excitability curve for different values of half-activation voltage Va. C, Excitability curve for different values of Boltzmann factor ka. D, Threshold as a function of the ratio gNa/gL for the 9 types of voltage-gated sodium channels  with characteristics reported in (Angelino and Brenner, 2007 ). For each channel type, the mean threshold obtained across the dataset is plotted. Nav1., , ,  are expressed in the central nervous system, Nav1.,  are expressed in cardiac and muscle cells and Nav1., ,  are expressed in the peripheral nervous system. Nav1.6 is expressed at the action potential initiation site –.