Ca2+ sparks have intrigued researchers since their discovery (Cheng et al., 1993). No one anticipated their existence; therefore, they must be telling us something we did not know. Also appealing are the esthetics of a sharp rise in local Ca2+, extremely limited in space and time but large compared with the recording noise.

These features encouraged from early on a parsimonious interpretation of their origin and significance, including two related aspects: sparks are the result of ryanodine receptor channels (RyRs) opening individually and they are the sole form of Ca2+ release (all release is constituted by sparks). An alternative view gained credence later, that sparks involve several (say, 20) channels opening and closing in concert. This view was fostered by observations of a release that is continuous, or constituted by events smaller than sparks, presumably reflecting the opening of individual channels. Therefore, the...

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