The sodium ion permeability begins to decrease initially during the peak of depolarization.
Action potentials are produced when various ions pass through the neuron membrane. Sodium channels open in response to a stimulus. Because sodium ions are much more abundant on the outside and the interior of the cell is negative in comparison to the outside, sodium ions rush in.
The sodium (Na+) permeability of most axons increases quickly and transiently, followed by a slower but more sustained increase in potassium (K+) permeability. Both permeabilities are voltage dependent and increase as the membrane potential decreases. The kinetics and voltage dependence of Na+ and K+ permeabilities completely explain action potential generation.
Depolarizing the membrane potential to the threshold level causes a rapid, self-sustaining increase in Na+ permeability, which causes the action potential to rise; however, the Na+ permeability increase is brief and is followed by a slower increase in K+ permeability, which returns the membrane potential to its normal negative resting level.
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