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Fig. 5. Typical early subthreshold responses in a spherical cell. (A) Depolarizing
and hyperpolarizing stimuli provoke very asymmetric responses. Note the
hump-and-hold profile for depolarizing steps and a sag depolarization after 63
ms for hyperpolarizing steps (arrows in A, further analyzed in
Fig. 6). (B) Enlarged version
of the first 11 ms of the responses (boxed yellow in A, showing a hump peaking
2.4 ms after the stimulus onset (current steps are equally spaced by 25 pA).
Up to this peak (red line in B) there is a linear relationship between
membrane potential and injected current (red symbols in D). After this moment
the response is asymmetric. For depolarizing steps there is a reduction of the
voltage drop caused by the flow of injected current across the membrane and
consequently an increase in membrane conductance. As shown by the coincidence
of the depolarizing limiting slopes (broken line in D) of the
V/I plots constructed for 10.4 ms (blue line in B; blue
symbols in D) and 63 ms (green line in A; green symbols in D), there are no
further changes in the V/I slope, indicating that the
activated conductance does not inactive with time. For hyperpolarizing steps
the semilogarithmic plot (C) of the voltage derivative vs time of the
bottom voltage trace of B illustrates that there is a simple exponential
relationship (red line) up to 2.4 ms (arrows in B,C). Beyond this time there
is an upward departure from a simple exponential curve, indicating a reduction
of membrane conductance.
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