A, superimposed currents (upper panel) produced by a ‘step’ voltage-clamp protocol (lower panel) in a HAC. The protocol consisted of twenty-three 1 s steps from a holding potential of –90 mV, to potentials between –130 and +90 mV. Steps were applied at a rate of 0.2 Hz. For clarity, only currents for steps to –130, –90, –40, –30, –20, +10, +60, +80 and +90 mV are shown; some of these are labelled. Cell capacitance was 7.9 pF. B, mean I–V relation, pooled from data from 50 different HACs. Current for each cell was normalized to cell capacitance (current density) before averaging. Mean capacitance = 8.5 ± 0.42 pF.