As more channels open, Na+ inflow increases, and the membrane depolarizes further, and more Na+ open.
Repolarization
The membrane potential is restored to the resting state of -70mV.
Mechanism:
At the same time as voltage-gated Na+ are open, voltage-gated K+ channels become opened. The opening of K+ channels and the eventual closing of Na+ channels produces the repolarization phase of the AP as K+ leaves the cell. This changes the membrane potential from +30 mV to -70 mV.
Hyperpolarization
Following repolarization, there may be an after-hyperpolarizing phase during which the membrane potential temporarily becomes more negative then the resting level.
Mechanism:
With voltage-gated K+ channels open, the outflow of K+ can be large enough to cause a hyperpolarization. The K+ channels remain open and the membrane potential becomes even more negative than the normal RMP of -70 mV approaching ~ -90 mV.
Repolarization to RMP
K+ voltage gated channels close and the Na+/K+ pump continues to restore the RMP to -70mV.
Refractory period
The period of time after an action potential begins that an excitable cell cannot generate another AP in response to a normal threshold stimulus. During the Absolute Refractory Period even a very strong stimulus cannot initiate a second action potential. A Relative Refractory Period is the period of time during which a second AP can be initiated, but only by a larger-than-normal stimulus.
**Graded potentials do not exhibit a refractory period**
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