The relationship between spark rise time and amplitude. (A) Rise time, T, vs. amplitude, a, of 1,887 sparks collected in 700 images of seven frog fibers with permeabilized membrane, placed in standard glutamate solution. Fibers were imaged at 0.135 μm per pixel and 2 ms per line. Sparks were detected and parameters were measured automatically on an interpolated average of three central pixels. Correlation coefficient r² = 0.015, regression coefficient b = −0.017 ms⁻¹. Inset: top, schematic of the profile F/F₀ vs. t at the spatial center of the spark, indicating a and T; bottom, Ca²⁺ flux during spark is thought to be roughly constant and last for the rise time. (B) Average a vs. average T in “bins” of increasing T for the group of sparks in A. Bars represent ± SEM of a and T. (C) Average a vs. average T, determined as described for B, for a set of 881 sparks detected in 500 images of two frog fibers imaged at 0.14 μm per pixel and 0.5 ms per line. (D) Average a vs. T, for a set of 6,300 sparks in 1,000 images of four permeabilized fibers placed in sulfate-based solution and imaged at 0.23 μm per pixel and 1.875 ms per line.