Experimental spark amplitude distribution in cat atrium cannot be accounted for by model. (A) Amplitude distribution of in-focus sparks in skinned cat atrium, replotted from Shkryl et al. (2012). (B) Distributions of peak F/F₀ of simulated sparks with several different couplon sizes. (C) Amplitude (F/F₀-1) distribution of sparks simulated for a 7 × 7 couplon assuming different values of the RyR opening rate constant k_o. For the highest value, sparks are unstable, as shown by the extinction time distribution in the inset.