What is the role of calcium ions (Ca2+) as a second messenger?

Calcium ions play an important role in stimulus-response reactions of cells as a second messenger. They achieve this by keeping cytoplasmic Ca2+ concentration low at rest and readily transporting Ca2+ in response to stimuli, which activates the cellular reaction. All cells use calcium ions to regulate their activity in response to intrinsic and extrinsic stimuli. Alterations in cellular Ca2+ signaling or Ca2+ homeostasis may weaken the organism’s physiological processes. In skeletal muscle, calcium regulates the function of muscle, controlling the supply of ATP when it is required. In addition to muscle contraction, calcium ions as a second messenger play a role in neuronal transmission (excitatory synapse), cellular motility, neurogenesis, and fertilization.