Final answer:
Myocardial action potential in cardiac contractile cells involves a stable resting potential, depolarization due to Na+ influx, a plateau phase due to Ca2+ influx, and repolarization due to K+ efflux.
Step-by-step explanation:
The myocardial action potential process primarily involves depolarization and repolarization phases in cardiac contractile cells. Contractile cells maintain a stable resting potential around -80 mV for atrial cells and -90 mV for ventricular cells. Upon excitation by an action potential, voltage-gated Na+ channels open, prompting depolarization with a swift Na+ influx, increasing the membrane potential towards +30 mV. This leads to the closing of Na+ channels and the commencement of the plateau phase, characterized by the opening of slow Ca2+ channels and a limited K+ efflux, which maintains the membrane potential for about 175 ms. Repolarization follows when Ca2+ channels close and K+ channels open, allowing K+ to leave the cell, restoring the potential to its resting state over approximately 75 ms. This entire action potential event spans 250 to 300 ms.