<prompt> Answer this question truthfullyWhat causes Phase 3 of the myocardial action potential and what happens to the voltage-gated channels involved in this phase?  <response>: <reason> Phase 3 of the myocardial action potential is caused by massive efflux of K ions out of the cell and closure of voltage-gated Ca2 channels. This repolarization phase allows the cardiac muscle cells to relax and prepare for the next contraction by restoring the negative membrane potential. 