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Sodium channels possess an inherent inactivation mechanism that prompts rapid reclosure, even as the membrane remains depolarized. During this equilibrium, the sodium channels enter an inactivated state, temporarily halting the influx of sodium ions until the membrane potential becomes negatively charged again.
The rapid depolarization of the cell, during phase 0, causes the membrane potential to approach sodium's equilibrium potential (i.e. the membrane potential at which sodium is no longer drawn into or out of the cell). As the membrane potential becomes more positive, the sodium channels then close and lock, this is known as the "inactivated" state.
The inward flow of sodium ions increases the concentration of positively charged cations in the cell and causes depolarization, where the potential of the cell is higher than the cell's resting potential. The sodium channels close at the peak of the action potential, while potassium continues to leave the cell.
Persistent sodium current generation is hypothesized to occur by the incomplete inactivation of the voltage-gated sodium channel current (INa), where the channel becomes constitutively active and conducts sodium, creating a "persistently active" inward sodium current. Upon depolarization, the four identical motifs of the sodium channel (which ...
Once the cell has been depolarized, voltage-gated sodium channels close, causing potassium channels to open; K+ ions then proceed to move against their concentration gradient out of the cell. [ 3 ] However, if the voltage is below the threshold, the neuron does not fire, but the membrane potential still fluctuates due to postsynaptic potentials ...
The depolarization of the membrane allows calcium channels to open as well. As sodium channels close calcium provides current to maintain the potential around 20 mV. The plateau lasts on the order of 100 ms. At the time that calcium channels are getting activated, channels that mediate the transient outward potassium current open as well.
There are five phases of an action potential: threshold, depolarization, peak, repolarization, and hyperpolarization. Threshold is when the summation of MEPPs reaches a certain potential and induces the opening of the voltage-gated ion channels. The rapid influx of sodium ions causes the membrane potential to reach a positive charge.
Additional depolarization activates additional Na + channels. This cycle leads to a very rapid rise in Na + conductance (g Na), which moves the membrane potential close to V Na. The cycle is broken when the membrane potential reaches to the sodium equilibrium potential and potassium channels open to re-polarize the