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Introduction: an action potential is an electrical impulse that is used in the body to transmit information (in the nerves and the brain) or to initiate an action (such as contraction of a muscle or secretion from a gland).
An action potential is a sudden change in the membrane potential. In the diagram, in red, the potential suddenly changes from the resting potential (approx. -90 mV; blue) to + 30 mV (red). After a few milliseconds, the membrane potential returns to the resting potential.
There is also a sodium concentration gradient, induced by the same sodium-potassium pump. But this sodium concentration gradient is opposite to the potassium gradient; there is much more sodium outside then inside the cell. However, at rest, the sodium channels are closed.
Why is there a rapid sudden influx of sodium ions?
Two reasons:
Because of this sudden and rapid influx of sodium ions into the cell, a lot of positive charges (=potential) also flow into the cell, thereby reducing the resting negative potential. This is the depolarization and the inside of the cell now becomes positive (+ 30 mV).
The potassium ions will flow out of the cell, according to their concentration gradient, and they will take positive charges with them, thereby inducing negative potential inside the cell; this process is called the repolarization. This process continues until the potential is back to the resting potential.
Fortunately, it is of course the sodium-potassium pump that will restore this concentration gradient. As this pump works continuously, as it were in the background, the concentration gradients will be kept at their required levels.
The word ‘depolarization’ means ‘decrease in polarization’. This is because, in the old days, physiologists believed the potential, during the action potential, decreased to zero.
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However, we still call the change of potential from -90 to +30 mV ‘depolarization’, although this is technically not fully correct.
Another important concept in the context of creating an action potential is the concept of the ‘threshold’.
If such a fluctuation is strong enough, then this will cause enough sodium channels to open, which will cause a massive influx of sodium ions into the cell and induce a fully-fledged depolarization.
In other words, it is not possible to have a small action potential or half an action potential or something like that; it is either a full action potential or not at all. This is called the ‘all-or-none’ law.
One more important concept: the refractory period! This is the period, during and after the action potential, during which, another action potential cannot be generated.
During the absolute refractory period, which starts immediately at the depolarization of the action potential, it is absolutely not possible to induce a new action potential (illustrated as the blue dashed action potential in the figure).
During the relative refractory period, it is possible to induce a new action potential but it takes more strength (more energy or electricity), as shown by the green action potential in the figure. The action potential is then often somewhat smaller (as not all channels are fully excitable again).
This happens when the potassium channels become even more open than before. After some time, the potassium channels go back to their normal state and the membrane potential stabilizes at the normal resting level.
However, it is important to know that the sodium channels are sensitive to the potential level across the cell membrane potential. As the resting potential decreases (=closer to zero), more sodium channels will open.
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