Figure 2.

Typical S1-S2-induced V_m s

A, image of an adult guinea-pig cell that was stimulated with both positive and negative pairs of S1-S2 pulses. The field pulses were measured to be +5.5/+21.6 V cm⁻¹ and −5.6/-20.9 V cm⁻¹, and are shown in B. C, left and right columns show the V_m responses to the positive and negative fields, respectively, and demonstrate reversal in $V_{\text{m}}^{\text{S}1}$ and $V_{\text{m}}^{\text{S}2}$ at all sites with reversal of field polarity. The last row in C shows the V_m responses superimposed from all sites. The transitions between the uniformly polarized and non-uniformly polarized states of the cell at the onset and cessation of the S1 and S2 pulses are clearly visible. D, illustration of the method to define the S1- and S2-induced V_m responses from the fluorescence signals, using the signal recorded from site 5 (F(5,t)) for the positive field as an example. The action potential amplitude (APA) was computed as the difference between baseline fluorescence (F_b) and plateau fluorescence (F_p), each being averaged over a 3 ms duration (indicated by thick lines) during rest and plateau, respectively. The formulae for computing the S1- and S2-induced responses ($V_{\text{m}}^{\text{S}1}$ and $V_{\text{m}}^{\text{S}2}$) are also shown in D. Cell length: 128 μm.