When the stimulus voltage is increased, . When the stimulus voltage is increased, . sodium permeability into the cell decreases sodium permeability into the cell increases to overcome the potassium exiting a greater-than-threshold depolarization results a greater-than-threshold depolarization results and sodium permeability into the cell increases to overcome the potassium exiting.

a greater-than-threshold depolarization results and sodium permeability into the cell increases to overcome the potassium exiting.

Explanation:

The occurrence of action potential requires the existence of active voltage-dependent channels to open and close in response to the membrane depolarization.  

When depolarization reaches and supers the threshold point, voltage-dependent Na+ channels open and the ion gets into the cell. The entrance of Na+ decreases the electric charge difference between the cellular interior and exterior. Membrane depolarizes and produces the action potential that tends to reach a potential equilibrium for Na+ = +60mV. The higher the Na+ input is, the greater is the depolarization and the greater is the Na+ permeability.

Once Na+ channels are closed, voltage-dependent K+ channels open, increasing the membrane permeability to the ion, letting K+ get out of the cell, following the electrochemical gradient.  

The closure of Na+ channels and opening of K+ channels are responsible for membrane repolarization.

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