A Threshold Equation for Action Potential Initiation
Figure 7
Threshold variability and Na channel inactivation in a single-compartment model.
A, We simulated the same model as in Fig. 6, but the half-inactivation voltage Vi was shifted to −62 mV (instead of −42 mV in the original model) to increase threshold variability. As a result, spike height was also more variable. B, The threshold distribution (red) spanned a range of more than 10 mV (standard deviation 2.2 mV) and overlapped with the membrane potential distribution (black). C, According to the threshold equation, most threshold variability was due to Na inactivation. Black dots show the measured threshold vs. the inactivation variable h (in log scale) at spike times. The linear regression (red line) gives a slope of 3.1 mV, close to the value of ka in this model (Fig. 9D).