(A) The final of 5 consecutive Ca²⁺ transients of a voltage-clamped AM (1-Hz pacing) during depolarization from –75 to 0 mV was magnified (corresponding dashed-line boxes) to visualize early Ca²⁺ signal onset locally. Block arrows indicate different Ca²⁺ spark locations at –50 mV, including off-membrane cytosolic sites (CY). Top 2D traces show Ca²⁺ spark events at AT, S, and CY locations, and examples highlighted by color are magnified by corresponding 3D surface plots (bottom panels): macro-spark (pink), ember (green), classic Ca²⁺ sparks (blue), and regenerative macro-sparks. Dot plot correlating Ca²⁺ spark full duration at half maximum (FDHM) and full width at half maximum (FWHM) to segregate classic (blue) from macro-sparks (green) and embers (pink). (B) Box plots comparing Ca²⁺ spark amplitude (Amp), FWHM, and FDHM in different locations. *P < 0.05, by ANOVA. (C) Atrial Ca²⁺ spark model composed of 1 transversal RyR2 cluster row. The indicated AT and S structures were associated with junctional highphos RyR2 clusters. Each cluster contained 35 RyR2 channels stochastically activated at –50 mV; both highphos and lowphos RyR2 clusters initiated SR Ca²⁺ release. Mathematical modeling generated the stochastic open time profiles of individual RyR2 channels for each cluster, which was used to calculate line-scan images of Ca²⁺ sparks with confocal resolution. Ca²⁺ sparks presented as 2D-flattened images (top) are highlighted by color, and the corresponding 3D surface plots are shown (bottom): classic Ca²⁺ sparks (blue), macro-spark (green), ember (pink), and regenerative macro-sparks (gray). N^O_RyR, number of open RyR2 channels. (D) Box and whisker plots comparing Ca²⁺ spark amplitude, FWHM, and FDHM as the average modeling output. (E) Comparing the aggregate properties of calcium sparks show no significant differences between the AM imaging results and spark modeling output. *P < 0.05, by ANOVA (B and D). Box and whisker plots: boxes show lower and upper quartile; whiskers show the 5th and 95th percentiles.