A–J show simulations based on our modified version of the Borg-Graham model (). A–C, simulations under control conditions. A, a spike train in response to a 50 ms-long depolarizing current pulse, with a prominent fast AHP (▴) after the 1st spike. B, plots showing the cumulative inactivation of ICT together with IA during the spike train in A. C, the spikes from A, superimposed on an expanded time scale, to illustrate the spike broadening, which is also plotted in J (□). D–F, the BK-current ICT has been omitted (‘blocked’). D, spike train. E, A-current. F, superimposed spikes. Note the reduced spike broadening (plotted in J, ▾) and fAHP. G–I, the inactivation mechanism of the BK-current has been disabled, resulting in a non-inactivating BK-current (IC). Note the uniformly narrow spikes (I), the reduced spike broadening, and the prominent fAHPs throughout the train (▴ in G). J-L, plots of normalized spike duration from 50 ms-long trains of 5 spikes, using three different models. J, our modified version of the ‘Working model’ of Borg-Graham. K, the original ‘Working model’ of Borg-Graham (). L, the Warman model (). With each model, a train of 5 spikes in response to a 50 ms current pulse was simulated, with a transient BK-current (□), without any BK-current (▾), and with a non-inactivating BK-current (○). Action potential trains were elicited by 50 ms somatic current injections of 1·1, 0·6 and 1·5 nA (see , respectively), corresponding to the minimal current needed to elicit 5 spikes. Spike width was measured at 1/3 of total spike amplitude of the 1st spike.