A Threshold Equation for Action Potential Initiation
A, Voltage trace of the fluctuating conductance-based model (black line) and predicted threshold according to our threshold equation (, red line), calculated continuously as function of h, gK, ge and gi. Black dots represent spike onsets (empirical threshold with the first derivative method). B, Predicted threshold vs. membrane potential for the trace in A. Trajectories lie above theoretical threshold on the right of the dashed line (). C, Zoom on the second spike in A. Colored lines represent increasingly complex threshold predictions: using Na activation characteristics only (blue, ), with Na channel inactivation (green, ), with potassium channel activation (purple, ) and with synaptic conductances (red, ). Here the threshold varies mainly after spike onset.