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How does calcium act as a signal in living cells?

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A labeled diagram of the periodic table entry for calcium, showing its atomic number 20, symbol Ca, atomic weight 40.078, electron configuration, and classification as an alkaline-earth metal. Icons indicate calcium is a solid with a face-centered cubic crystal structure and strongly basic oxides at room temperature. — calcium The chemical element calcium is the most abundant metallic element in the human body.

In living cells, the calcium ion ( ${Ca}^{2 +}$ ) functions as a second messenger, a molecule that transmits signals from a cell surface receptor to internal targets to change cellular behavior.12

At rest, cells maintain a very low concentration of cytoplasmic calcium by expending energy to pump the ions out of the cell or into storage, creating a steep concentration gradient. When a cell is stimulated, specific cell membrane channels open, allowing calcium to flow rapidly into the cytoplasm from the extracellular space.132

This transient increase in cytoplasmic calcium concentration acts as a signal to trigger rapid responses, such as muscle contraction or neuronal activity. Once the signal is transmitted, the action is brief; the cell quickly restores the low resting concentration by pumping the calcium back out or binding it to intracellular molecules. This precise regulation of calcium levels is essential for maintaining normal neuromuscular function, membrane integrity, and other vital biological processes.1324