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Phases of the Action Potential


The action potential has key phases including the rising phase, overshoot, falling phase, undershoot, and refractory period.

Read the following statements closely, and select ALL accurate descriptions.


The rising phase is a rapid depolarization of membrane caused by the influx of ${ Na }^{ + }$ through voltage-gated sodium channels.


The overshoot phase occurs following the rising phase as the opened voltage-gated sodium channels make the membrane highly permeable to sodium ions.

Sodium ions rush into the cell and increase the membrane potential value, ${ V }_{ m }$, making it more positive than 0 mV, but ${ V }_{ m }$ can never reach the equilibrium potential of the sodium ion, ${ E }_{ Na }$.


The falling phase is caused by sodium channel inactivation and the efflux of ${ K }^{ + }$ through newly opened voltage-gated potassium channels.


The undershoot phase is a brief hyperpolarization of the membrane potential. In other words, at this phase, ${ V }_{ m }$ is more negative than the resting potential.


The refractory period is the “downtime” when a subsequent action potential cannot be generated.

To initiate a second action potential, the voltage-gated sodium channels have to be reset, so they are de-inactivated with a sufficiently negative membrane potential.