A guide to sparkology: The taxonomy of elementary cellular Ca²⁺ signaling events

Since the discovery of the Ca²⁺ spark as an elementary event of cellular Ca²⁺ signaling almost 15 years ago, the family of newly described Ca²⁺ signal entities has been ever growing. While scientists working in Ca²⁺ signaling may have maintained an overview over the specifics of this nomenclature, those outside the field often make the complaint that they feel hopelessly lost. With the present review we collect and summarize systematic information on the many Ca²⁺ signaling events described in a variety of tissues and cells, and we emphasize why and how each of them has its own importance. Most of these signals are taking place in the cytosol of the respective cells, but several events have been recorded from intracellular organelles as well, where they may serve their own specific functions. Finally, we also try to convey an integrated view as to why cellular microdomain signaling is of fundamental biological importance.