Calcium ion (Ca²⁺) is a positively charged ion of the element calcium that plays a crucial role in many biological processes in the human body. It is involved in muscle contraction, neurotransmitter release, enzyme activation, and many other signaling pathways.

In muscle cells, calcium ions bind to regulatory proteins, causing a change in the shape of the protein that enables the muscle to contract. In neurons, calcium ions play a key role in the release of neurotransmitters from synaptic vesicles, allowing the transmission of signals from one neuron to another.

Calcium ions are also important in many cellular signaling pathways, where they act as second messengers, triggering a cascade of cellular responses. They are involved in regulating gene expression, cell division, and apoptosis (programmed cell death).

The concentration of calcium ions in cells is tightly regulated by various transporters and channels, and disruptions to calcium homeostasis can lead to a range of diseases and disorders, including muscle spasms, seizures, and cardiac arrhythmias.