A, representative spatio‐temporal (ST) map of spontaneous intracellular Ca2+ waves recorded in situ. The ST map is calibrated for amplitude as F/F
0 and colour coded accordingly. B, x–y plot of the Ca2+ activity represented in A. C, 3‐D plot representation of the ST map shown in A. D, 3‐D plot profile of 5 representative Ca2+ waves from the ST map in A. E, summary histograms showing the distribution of values for USMC Ca2+ event frequency (a), amplitude (b), duration (c), spatial spread (d), velocity (e) and number of firing sites (f) (c = 301, n = 31).