Figure 2. Line scan imaging during the action potential.

A, fluorescence changes evoked by the mouse action potential. i, the action potential was elicited by a brief current injection. A stimulus artifact is apparent just before the upstroke of the action potential. The peak value for membrane potential (ordinate) is about +50 mV and the duration of the action potential is approximately 130 ms (abscissa). ii, a line scan plot of the change in fluorescence intensity that was activated by the action potential. Position along the line scan is plotted on the ordinate and time on the abscissa. The colour bar on the right indicates the fluorescence ratio (F/F₀). F₀ is the background intensity obtained by measuring fluorescence intensity before the stimulus. iii, the time course of the spatially averaged fluorescence change (F/F₀) during the action potential. B, an expanded view of the relationship between the action potential and the spatially averaged Ca²⁺ transient depicted in A. The panel shows the time course of the action potential (i), the spatially averaged Ca²⁺ transient (ii), and the derivative of the upstroke of the Ca²⁺ transient which reaches a peak approximately 8 ms after the onset of the Ca²⁺ transient (iii). The dashed vertical lines delineate the period over which the majority of Ca²⁺ release takes place. Note the temporal relationship between the action potential and the upstroke of the calcium transient.