Spike afterpotentials and initial doublet firing. A, Average V_m trajectory of a typical L2–3 PN after each spike during a current step slightly above rheobase, showing the fAHP, fADP, and mAHP. B, A sharp threshold for initial doublet firing was encountered between steps of 620 pA (black, bottom trace) and 640 pA (gray, top trace). C, The initial doublet elicited by the 640 pA step was followed by a larger mAHP and a longer ISI than the single initial spike caused by the 620 pA step. D, First and last ISIs for a series of 1 s current steps. The first ISI showed a sharp change between 620 and 640 pA, whereas the last ISI did not. E, ISI for the doublet elicited by a current step just above the doublet threshold in 12 L2–3 PNs, plotted against the time from the onset of the first spike to the peak of the fADP during a current step just above rheobase. The dotted line is the line of identity. The solid line is the linear regression for the data points.