A New View of Ca²+ Sparks in Frog Skeletal Muscle

Calcium sparks are brief, highly localized elevations of [Ca²+], which arise spontaneously or can be evoked by depolarization in many types of muscle cells. The Ca²+ is released into the cytoplasm from intracellular stores (the SR) through the SR Ca²+ release channels (the RYR). Since their discovery (in cardiac muscle; Cheng et al. 1993), Ca²+ sparks have attracted substantial attention. The reasons for this are clear: first, Ca²+ sparks almost certainly serve as “building blocks” for larger cellular Ca²+ signals, such as Ca²+ waves or the Ca²+ transients that trigger muscle contraction. In some cases, modulation of the whole-cell Ca²+ transient occurs by variation in the number of building blocks. Second, the highly localized nature of the changes in [Ca...