sentences:  Excitation-contraction coupling in myocardium relies on sarcolemma depolarization and subsequent Ca2+ entry to trigger Ca2+ release from the sarcoplasmic reticulum.When an action potential depolarizes the cell membrane, voltage-gated Ca2+ channels (e.g., L-type calcium channels) are activated.CICR occurs when the resulting Ca2+ influx activates ryanodine receptors on the SR membrane, which causes more Ca2+ to be released into the cytosol.