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Potassium ions continue to move out of the axon so much that the resting potential is exceeded and the new cell potential becomes more negative than the resting potential. The resting potential is ultimately re-established by the closing of all voltage-gated ion channels and the activity of the sodium potassium ion pump. [10]
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.
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 ...
This is followed by the opening of potassium ion channels that permit the exit of potassium ions from the cell. 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 ...
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
The open sodium channels allow more sodium ions to flow into the cell and resulting in further depolarisation, which will subsequently open even more sodium channels. At a certain moment this process becomes regenerative (vicious cycle) and results in the rapid ascending phase of action potential. In parallel with the depolarisation and sodium ...
The current underlying the action potential depolarization is selective for sodium. Repolarization depends on a selective potassium current. These currents have very specific responses to voltage changes which quantitatively explain the action potential. Substitution of non-permeable ions for sodium abolishes the action potential.
The sodium–potassium pump, a critical enzyme for regulating sodium and potassium levels in cells. Sodium ions (Na +) are necessary in small amounts for some types of plants, [1] but sodium as a nutrient is more generally needed in larger amounts [1] by animals, due to their use of it for generation of nerve impulses and for maintenance of electrolyte balance and fluid balance.