<prompt> Repolarization is due to opening of which channels-NaHC03KCl  <response>: Ans. is c i.e., K Phases of Action Potentialo The action potential, which is recorded using an intracellular electrode, has following phases : -Resting stage : - This is the resting membrane potential before the action potential. At rest, membrane is said to be polarized because - 70 mV negative membrane potential RMP that is present. Strictly speaking, it is not a part of the action potential.Depolarization phase : - The potential shoots up to 35 mV in less than a millisecond. It is due to opening of voltage gated Na channels which causes influx of sodium, so inside negative charge becomes positive. From - 70 mV to -55 mV firing Ivel or threshold point the voltage gated sodium channles open partially. This portion ofAP- 70 mV to -55 mV is called prepotentialor footof AP. Once the threshold level -55 mV is reached voltage gated Na channels open completely and the potential shoots up suddenly. It is called depolarization, since it represents loss of the original polarity of cell membrane.Repolarization phase : - The potential drops to near resting level, i.e., reversal to original polarity repolarization. At the start of repolarization end of depolarization, Voltage - gated Na channels close and Voltage - gated K channels open which cause efflux of K out of the ceil. So, inside positive charge onceagain becomes negative due to efflux of positive charge K.After depolarization: - The repolarization is very fast till 70 of repolarization has been acheived. After that it is slows down. This slower phase of repolarization is called after depolarizationAfter-hyperpolarization : - The intracellular negativity overshoots the normal RMP of - 70 mV to reach a more negative value of about - 75 mV and then slowly retruns to the normal RMP of - 70 mV. 