Representative Ca sparks generated by our CRU model. Sparks were simulatied and compared at initial JSR Ca loading 1 mM (A) and 0.3 mM (B). Left: Overlapped time series of [Ca] in dyadic cleft (blue line) and [Ca] in JSR (red line). Right: Respective line-scan images of the sparks. The spark at 0.3 mM of JSR Ca loading had a lower amplitude and duration. To better illustrate Ca spread, the color map in line-scan images is saturated at [Ca]_cleft = 1 µM. CRU positions for square lattice configuration are shown by blue lines. The spark was generated by CRU3 and the images show that its outskirt could reach neighboring CRUs (CRU2 and CRU4) at 1 mM JSR Ca loading, but not at 0.3 mM (i.e., at the [Ca]_JSR threshold of spark activation). The membrane potential was −60 mV.