Fig. 8. Membrane conductance and the excitability of the cell change concomitantly after a conditioning spike. The change in membrane resistance was evaluated by the slope of the V/I plot constructed with data obtained 1 ms after the onset of a series of test current steps of different intensity (dotted line in inset) applied at different delays after a conditioning spike in four cells (symbol coded). (A) The increase in slope as a function of the delay indicates that after the conditioning spike there is an increase in membrane conductance and a slow return to resting values. (B,C) Change in spike threshold and spike latency as a function of the slope. Change in threshold was defined as the difference between the current intensity required for eliciting a spike when the stimulus was preceded by a conditioning spike at the given delay minus the current intensity required for eliciting a spike without a conditioning spike. As the spike threshold and latency decrease with the slope, the excitability of the cell is reduced by the increase of membrane conductance caused by the conditioning spike.