A.3.3. The Action Potential


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).

A. What is an Action Potential?
1.
An action potential is a sudden change in the 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 potential returns to the resting potential.

One ActionPotential
2.
The initial phase is called the depolarization (because the potential goes from very negative to zero; 'de' = means less potential difference).

3.
The potential actually crosses the zero level and reaches values up to + 30 mV (inside positive!). The restoration of the potential back to the resting potential is called the repolarization.

B. How is an Action Potential created?

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1.
Remember, in the resting state, that the inside of the cell is negative (-90 mV) because of the potassium concentration gradient. The potassium channels are open and therefore, there is an efflux of potassium ions and their charges, making inside negative and outside positive.
step1: ActionPotential
2.
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.
3.
If the cell becomes excited (by an external stimulus), then the sodium channels will open. This will cause a massive and rapid influx of sodium ions into the cell!

step2: ActionPotential
4.
Why is there a rapid sudden influx of sodium ions?
Two reasons:
a. Concentration gradient (there is less sodium ions inside then outside).
b. Potential gradient (inside is negative at rest and this will attract the positive sodium ions).
5.
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).

step3: ActionPotential
6.
After a little time (millisecond), the sodium channels close (automatically; they always do; it is a property of these channels). This will stop the influx of sodium ions and stop the depolarization.
7.
But the potassium channels are still open! And now the potential gradient has disappeared! Therefore, the potassium ions, once again, will flow out of the cell (efflux).

step4: ActionPotential
8.
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.
9.
So, at the end of the action potential, the potential inside the cell is back to -90 mV (= resting potential).

step5: ActionPotential
10.
In summary, the action potential was actually performed by the influx of sodium ions followed by the efflux of some potassium ions.

C. Some additional notes:

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1.
It is important to realize that only a small amount of sodium and potassium ions have to flow in and out of the cell to depolarize and repolarize the cell. But it does mean that the concentration gradient for Na+ and K+ has decreased a little.
2.
Still, a cell could generate thousands of action potentials before the concentration gradients have fallen to such low levels that action potential generation has become impossible.


3.
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.
4.
Of course, the pump will only work if it gets ATP (=energy). This is typical of a living cell. If the cell dies, ATP formation stops, the pump stops and the concentration and electrical gradients will gradually disappear.


D. Overshoot of an Action Potential:

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1.
As we said above, the initial phase of the action potential is called ‘depolarization’ and the second phase is called the ‘repolarization’.




Overshoot of an ActionPotential
2.
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.
3.
Later, it was discovered that the depolarization did not stop at zero but continued into a positive potential, usually about +30 mV. We now call this positive potential, an overshoot!
4.
However, we still call the change of potential from -90 to +30 mV ‘depolarization’, although this is technically not fully correct.
5.
The same reasoning applies of course to the ‘repolarization’.


E. Threshold of an Action Potential:

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1.
Another important concept in the context of creating an action potential is the concept of the ‘threshold’.


Threshold of an ActionPotential
2.
In this diagram, you can see at first that there is a small fluctuation, a small depolarization, which does not reach the value of the threshold (in this case about -70 mV).
3.
These small depolarizations are caused by local potentials, hormones, or chemical transmitters flowing from neighboring cells or areas.
4.
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.
5.
In other words, if a depolarization is strong enough to reach threshold, this will generate an action potential. If the depolarization is not strong enough, there will not be an action potential.
6.
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.


All or None 'law' for an ActionPotential

F. The Refractory Period of an Action Potential:

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1.
One more important concept: the refractory period! This is the period, during and after the action potential, during which, another action potential can not be generated.

2.
This is caused by the fact that the sodium channels, after they have opened and (automatically) closed, are for a short period, not able to open again. During that period, they are inexcitable.
The RefractoryPeriod of an ActionPotential
The RefractoryPeriod of an ActionPotential
3.
In classic physiology, there are two types of refractory periods;
a) the absolute refractory period and
b) the relative refractory period.

4.
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).
5.
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).
6.
After the relative refractory period, all sodium channels are excitable again and a full action potential can again be generated (second red action potential).




(G: Hyperpolarization, H: Threshold more in depth)

G. Hyperpolarization

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1.
In some (nerve) cells, after the repolarization, the membrane potential ‘undershoots’ below the resting potential. This is called ‘hyperpolarization' (= more negative than the resting potential) or ‘after potential’.
Hyperpolarization after an ActionPotential
2.
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.
3.
Important is that during this brief period; the potential is further ‘away’ from the threshold making it more difficult to initiate an action potential during this period.


H. Threshold; positive feedback loop!

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1.
There is something interesting about the threshold, the one that a membrane potential has to reach before generating a full-blown action potential.
2.
Remember that an action potential is generated by an (electrical) impulse that depolarizes the membrane at a location.

3.
This local depolarization is due to the opening of some of the sodium-channels in that location, which therefore allows some sodium-ions to flow into the cell.

4.
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.
threshold Advanced of an ActionPotential
5.
If only a few sodium-channels open, the resulting depolarization will not reach threshold and no action potential will be initiated.

6.
But if the initiating impulse is strong enough, and threshold is reached, then the opening of the sodium channels, and the resulting sodium-influx, will induce more sodium-channels to open!
7.
Such a cycle is called a positive feedback loop (remember the negative and positive feed back loops?). In other words, more and more sodium channels will now open until they are ALL open!
8.
This is what actually happens during the depolarization phase of the action potential. It is really an explosion of sodium channels that all open in a very brief period of time.
9.
Of course, the end of this ‘explosion’ occurs when all the sodium channels are open, and after a few microseconds, automatically start to close!
10.
This marks the end of the depolarization (and high time to start the repolarization!)



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A.3.3. The Action Potential

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