Sparks and embers of skeletal muscle: the exciting events of contractile activation.

Intracellular calcium concentration ([Ca(2+)](i)) is a key player in a wide range of cellular functions from long-term effects that determine the fate of the cell to immediate responses as secretion and motility. To initiate contraction, calcium ions in skeletal muscle are released into the myoplasm through the calcium channels, the ryanodine receptors, of the sarcoplasmic reticulum. The opening of these channels give rise to localised increases in [Ca(2+)](i), originally termed calcium sparks, that fuse and generate the global calcium transient. Whereas calcium sparks in amphibians are abundant and stereotyped, events in mammalian skeletal muscle are scarce and morphologically diverse. This review compares the different forms of calcium release events, occurring spontaneously or evoked by a depolarising pulse, observed in the different classes of vertebrates. It then addresses the questions whether or not these events can be considered as elementary and how the global calcium transient can be reconstructed from them.