1. Calcium is a universal second messenger. It is involved in a large variety of processes, among them: Fertilization Cell death Neuronal communication, excitability and memory storageNeuronal communication, excitability and memory storage Muscle contraction and heart beat

2. It is believed that information is “stored” in the spatio- temporal distribution of the calcium concentration. Thus, cells need to control very tightly its internal Ca 2+ concentration. In several instances and various cell types, calcium oscillations may be observed. Apparently oscillations would be used to relay information over prolonged time intervals (calcium_oscillations).(calcium_oscillations). Intracellular calcium waves. Intracellular calcium waves. Fontanilla and Nuccitelli Since the cytosolic calcium concentration cannot be too large for a prolonged period of time, cells use transient elevations of calcium to send messages. Intercellular calcium waves Intercellular calcium waves Propagation of the leading edge of a Ca 2+ wave in the rat retina. Video shown at normal speed. Width of image, 240  m. Waves initiated with mechanical stimulus (10 msec, 15 -25  m). Newman and Zahs, Science, 275 844 1997

3. Calcium is not free in the cytosol Calcium is contained in the endoplasmic (ER) or sarcoplasmic reticulum (SR). (ver) (ver) It is also contained in the extracellular medium. Calcium enters the cytosol through specialized channels. Calcium channels can be voltage or ligand gated.voltageligand Calcium is removed from the cytosol by calcium pumps or ion exchangers that pump it out to the extracellular medium or into intracellular stores (SR or ER and mitochondria). It is also buffered by proteins.