Distinct modes of elemental calcium release. On a terminal cistern, a main RYR cluster consists of two subdomains with an isthmic connection and is surrounded by a few rogue RYR groups (indicated by arrows in A). (A) Quarky calcium release from rogue RYRs only. (B) Calcium spark with two quantal units. Either or both of the subdomains of the main cluster can be activated in a spark. (C) Calcium spark with quarky substructure. During sparks, the neighbor rogue RYRs are also activated in succession, resulting in prolonging calcium release. Cytosol calcium concentration gradients are roughly displayed by colors (top to bottom of the look-up table corresponds to the calcium concentration from resting to sparked). LCC, L-type calcium channel; SR, sarcoplasmic reticulum.