Figure 4. Signal-averaged Ca²⁺ spark.

A, 20 sparks signal averaged from a fixed position and confocal plane. Sparks for averaging were selected by assuming that they were associated with a relative fluorescence change of > 1·5. B shows the derivative with respect to time of the fluorescence intensity of the spark depicted in A. Note the temporal similarity of the derivative of the spark to that of the whole-cell transient. C, profiles of relative fluorescence at various times (indicated on the traces) after the beginning of the spark. D, profiles of fluorescence intensity at various distances (indicated on the traces) from the centre of the spark. As the distance from the spark centre increases, the amplitude decreases and the time of rise increases.