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Abstract The properties of the slow inward current, isi in the sino-atrial (s.a.) node of the rabbit have been investigated using two microelectrodes to apply voltage clamp to small, spontaneously beating, preparations. Many of the experimental results can be closely simulated using the computer model of s.a. node electrical activity (Noble DiFrancesco second, by a change in time course of the isi recorded after progressively more negative conditioning hyperpolarizations (a result which is hard to account for using equations for a single gated channel) and, third, by the occasional appearance of a double-peaked isi record, both when isi is recorded after hyperpolarizations and in ‘ staircases ’ during trains of repetitive clamp pulses. In this latter case, the two components of isi show a different pattern of change one from another as the staircase progresses.
Brown et al. (Sat,) studied this question.