A.3.4. Action Potential Propagation


Introduction:
An action potential is propagated along the cell membrane of a nerve or a muscle cell.

Structural and physiological components required:
1. A cell membrane.
2. Na+ and K+ ion channels in this membrane.
3. The membrane has a resting potential (approx. -80 to -90 mV).

A: Propagation of an Action Potential:

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1.
An action potential has been initiated (by whatever mechanism) at one location in the cellular membrane (indicated in panel 1 in red).


2.
This means that at that location, the inside of the cell is then briefly positive and the outside is negative (because the Na+ channels are open and the positive Na+ ions have flown into the cell). This occurs at the height of the action potential, during the overshoot.
3.
But in the neighborhood of that activated membrane, left and right, the membrane is still resting and displays a resting potential: i.e. inside negative and outside positive. This membrane is still passive (blue), and its trans-membrane potential is approximately at –90 mV.
4.
Because both the activated and the resting membrane are adjacent to each other and bathed in extra- and intracellular fluids that contain ions, several ions will start to flow from one area to the neighboring area according to their charges, as shown in panel 2 with the arrows.
5.
For example, inside the cell, positive ions (such as K+) will flow from the activated region to the resting region, as they are attracted by the negative polarity in those regions. Similarly, outside the cell, positive ions (such as Na+) will flow from the resting area to the central activated area.



Electrical Propagation
6.
These current fluxes or current circuits (as these are called) will affect the resting potential in the resting membrane. The flux of positive K+ ions inside the membrane will decrease the negative resting potential.
7.
At the same time, the removal of positive Na+ ions outside the membrane adjacent to the activated area will decrease the positive potential; hence the difference between the inside and outside becomes less.
8.
In this example, the potential difference across the neighboring membranes is decreased from –90 mV to –80 mV. This is a local depolarization, and the potential comes closer to the threshold.




9.
When the depolarization in the resting membrane reaches threshold, a new action potential is generated at that new location. This is indicated in panel 3; that part of the membrane now shows a full action potential with a value of +30 mV. In other words, the action potential has now ‘moved’ or propagated (or ‘jumped’ or ‘conducted’; they all mean the same thing) to this new location.
10.
Because in this example the action potential started in the middle of the membrane, the action potential will depolarize the membrane both to its left and to its right; both will reach threshold and both will show a new propagation. In other words, the action potential is propagating in both directions.
11.
These new action potentials will then influence the next piece of resting membrane and the whole story starts all over again.




12.
So, step-by-step, the action potential generates every time a new action potential in 'front' of itself. This is called ‘propagation’!

B. Some important Notes

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1.
This propagation is quite fast as it depends on the speed of opening of the Na+ channels and the flux of ions in- and outside the membrane.


2.
Propagation can occur in any direction, forward or backward, left or right, because the structure of the membrane, with its potassium and sodium ion channels, is the same in all directions.
3.
Once an action potential is propagating in one direction, it cannot turn back. That is because the part of the membrane that has been activated with an action potential is now refractory. This means that the membrane cannot be re-activated again for a (short) time period. This period is called the refractory period.
RefractoryPeriod & Propagation
Collision of two action potentials
4.
If two separate action potentials propagate towards each other in the same membrane they will eventually collide. Because each action potential cannot activate the channels that were already activated by the other action potential (= because they are refractory/inactivated) both action potentials will stop propagating. They therefore cannot ‘cross’ each other such as waves in the sea can.
5.
Propagation of the action potential, once initiated, continues ‘automatically’ as described here. But where does it stop? In general, the action potential stops propagating when it reaches the end of the membrane, at the end of the cell.

6.
In small cells, this is quickly achieved but some cells can be quite long such as for example the axons of nerve cells; these can reach many centimeters up to a meter in length (the axons in the leg for example; these run from the lower part of the spinal cord all the way to the big toe!).

C. But, we have a problem here!

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1.
In the previous paragraphs, we have described the propagation of an action potential along the membrane of a cell. This propagation is not very fast; about 0.25 cm/sec.
2.
In most cases, this is absolutely fine. In small cells, the speed of this propagation is good enough to pass on a message.


3.
However, some nerve cells are very long. This is the case for example in the arms and the legs when the cell body is located in the spinal cord and the synapses are located in the hand or the feet, several feet (or meters) away.
4.
At a speed of 0.25 cm/sec, it will take 4-6 seconds to pass the message to the muscles in your feet and your big toe! This takes too much time!


5.
How can we speed up the propagation of the action potential? Fortunately, nature, during the evolution of the body, has provided for a solution!
6.
The solution is to speed up the propagation speed of the action potential by a system that is called “saltatory propagation”. This is discussed in the next page!

D. We also have a second problem:

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1.
On this page, we have described the propagation of an action potential, along the cell membrane, in one cell.

2.
In many cases, the action potential has to propagate into a next, adjacent, cell. How is this done?

3.
Propagation from one cell to another is a very different issue and is dealt with in two different ways:

4.
a) in the electrical synapse
b) in the chemical synapse.



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A.3.4. Action Potential Propagation

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