<prompt> Answer this question truthfullyWhat causes Phase 3 of the SA node action potential and what happens to the channels involved in this phase?  <response>: <reason> Phase 3 of the SA node action potential is caused by activation of K channels and inactivation of Ca2 channels. During this phase, the membrane potential rapidly repolarizes as K ions move out of the cell, while Ca2 channels close to prevent further influx of calcium ions. This allows the cardiac muscle cells to relax and prepare for the next heartbeat. 