Calcium signaling is a universal cellular communication system that uses changes in intracellular calcium ion concentration to regulate diverse physiological processes. At rest, cells maintain very low cytosolic calcium, with much higher levels stored in the endoplasmic or sarcoplasmic reticulum and in mitochondria. External stimuli such as hormones, neurotransmitters, mechanical stress or changes in membrane potential trigger rapid, localized calcium entry through channels in the plasma membrane or release from internal stores.

These calcium elevations vary in amplitude, duration, frequency and spatial pattern. Cells interpret this “calcium code” to control gene expression, metabolism, secretion, contraction and cell survival or death. Key molecular players include voltage gated calcium channels, receptor operated channels, store operated channels such as those formed by Orai proteins, and intracellular release channels like IP3 receptors and ryanodine receptors. Pumps and exchangers, particularly SERCA in the endoplasmic reticulum and plasma membrane calcium ATPases, restore low resting levels and shape signaling dynamics.

Mitochondria take up calcium through specialized transporters, coupling calcium signals to ATP production while also influencing cell death pathways when overloaded. In neurons and muscle cells, highly organized calcium microdomains and repeated oscillations support fast, precise control of synaptic transmission and contraction. In immune cells and many others, store operated entry links depletion of internal stores to sustained calcium influx required for activation.

Disruption of calcium signaling contributes to cardiovascular disease, neurodegeneration, muscular disorders, immune dysfunction and cancer, making its components important therapeutic targets. Understanding how cells generate and decode calcium signals remains central to explaining how complex behaviors emerge from molecular events.