Ryanodine Receptors (RyRs), named so due to their direct modulation by the plant alkaloid ryanodine, are the large conductance Ca²⁺ channel in the endoplasmic reticulum (ER) membrane of excitable cells such as muscles and neurons. There are three isoforms which form homotetramers: RyR1 expressed mainly in skeletal muscle; RyR2 is particular to myocardium and RyR3 is mostly expressed in the brain.