What happens during each of the three phases of an action potential in ventricular contractile fibers?Anatomy-Heart
Your question is in regards to stages/phases of action potential of the myocardium. The first stage is called the resting membrane potential phase. This phase is when the muscle is at rest or the myocardium is between beats. Three major electrolytes are involved/must be present to have a resting membrane potential, sodium, potassium, and calcium. The muscle must also be innervated. When a motor impulse is received from the vagus nerve this causes changes in the plasma membranes permeability which allows sodium to exit the cell or become extracellular and allows potassium to enter the cell or become intracellular. Serum calcium must be present in sufficient quantities for this process to occur.
The second phase is when the proteins actin and myosin with the help of calcium cause the myofilaments to slide past each other. This is called the sliding filament theory.
The third phase is the refractory period in which everything returns to normal but very quickly the process starts again. Keep in mind that adequate stores of energy supplied by ATP must be present.