Any of a family of voltage-gated ion channels that selectively allow the passage of calcium ions into cells when triggered by a change in the membrane potential of the cell. The resulting influx of calcium ions can have various physiological effects, notably the contraction of muscle fibres in muscle tissues, including skeletal muscles and heart muscle. Calcium ion channels are also crucial in regulating the tone of smooth muscle, for instance in blood vessels, and generating the action potentials of cardiac pacemaker cells in the sinoatrial node of the heart. Their activity can be modified by drugs—calcium channel blockers—to treat conditions such as angina pectoris, hypertension, and cardiac arrhythmias.