<prompt> Threshold level for neuronal action potential--70 mv70 mv-55 mv55 mv  <response>: Ans. is c i.e., -55 mv The action potential, which is recorded using an intracellular electrode, has following phases : -1 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.2 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 of AP -70 mV to -55 mV is called prepotential or foot ofAP. 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.3 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 cell. So, inside positive charge once again becomes negative due to efflux of positive charge K.4 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 depolarization5 After-hyperpolarization: During repolarization phase K channels open and there is K efflux. The membrane voltage falls back repolarizes to resting potential, i.e., -70 mV. At this voltage, K channels close, but quite slowly. Therefore, more K ions leak out, causing a brief period in which the voltage falls below -70 mV, i.e. hyperplorization. Then, KNa pump works to re-establish resting potential by pumping Na ions back out and K ions back into the cells. 