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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.

A

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

B

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, ${ E }_{ Na }$.

C

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

D

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.

E

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.

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