Module 2: Figure Ca2+ signalling dynamics
The dynamics of Ca2+ signalling.
The dynamics of Ca2+ signalling are governed by an interplay between the ON and OFF reactions that control the fluxes of Ca2+ across both the plasma membrane and the internal organelles such as the endoplasmic reticulum (ER) and mitochondria. External stimuli activate the ON reactions, which introduce Ca2+ into the cytoplasm either through channels in the plasma membrane or from internal stores such as the ER. Most cells make use of both sources, but there are examples of cells using either external or internal sources to control specific processes. Most of the Ca2+ that enters the cytoplasm is adsorbed on to buffers, while a much smaller proportion activates the effectors to stimulate cellular processes. The OFF reactions remove Ca2+ from the cytoplasm using a combination of mitochondria and different pumping mechanisms. When cells are at rest, these OFF reactions keep the concentration low, but these are temporarily overwhelmed when external stimuli activate the ON reactions. Sequential activation of the ON and OFF reactions gives rise to the Ca2+ transients (Module 2: Figure Ca2+ transient mechanisms), which are such a characteristic feature of Ca2+ signalling systems. PMCA, plasma membrane Ca2+-ATPase; SERCA, sarco/endo-plasmic reticulum Ca2+-ATPase.