Calcium concentration microdomains
Calcium concentration microdomains (CCMs) are sites in a cell's cytoplasm with a localised high calcium ion (Ca2+) concentration. They are found immediately around the intracellular opening of calcium channels; when a calcium channel opens, the Ca2+ concentration in the adjacent CCM increases up to several hundred micromolar (μM).
Source: Wikipedia — Calcium concentration microdomains (CC BY-SA 4.0)