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Fig. 9. Refractoriness is graded with the conditioning step intensity. (A)
Responses evoked by four intracellular test steps (600 pA, 5 ms) applied at
four different delays after 3 ms conditioning square steps of four different
amplitudes 0, 200, 400 and 600 pA (i–iv, respectively). The increase in
conditioning step amplitude cause: (a) depression of the hump in the
subthreshold responses (compare black traces); (b) increase in spike latency
(compare red spikes with the reference line); and (c) increase in spike
threshold (lack of spikes in black traces in all rows except the control, red
and green traces in row iv). (B) The amplitude of the hump evoked by a
constant test stimulus (600 pA, 5 ms) decreases as the amplitude of the
response evoked by the conditioning stimulus increases (color-coded, 3 ms).
The maximum effect is caused when a conditioning spike is evoked. Note the
undershoot after the sub-threshold stimuli (arrows).
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