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Structural and physiological components required:
An action potential has been initiated (by whatever mechanism) at one location in the cellular membrane (indicated in panel 1 in red).
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’ (see previous page).
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.
Because both the activated and the resting membranes are adjacent to each other and bathed in extra- and intracellular fluids that contain ions, several of those ions will start to flow from one area to the neighboring area according to their charges, as shown in panel 2 with the arrows.
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.
These current fluxes or current circuits (as these are called) will affect the resting potential in the neighboring resting membranes. The flux of positive K+ ions inside the membrane will decrease locally the negative resting potential.
At the same time, the removal of positive Na+ ions outside the membrane adjacent to the activated area will decrease the local positive potential; hence the difference between the inside and outside becomes less.
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.
When the depolarization in the neighboring 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.
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, left and right.
These new action potentials will then influence the next piece of resting membrane, initiate a new action potential and the whole story starts all over again.
So, step-by-step, the action potential generates every time a new action potential in ‘front’ of itself. This is called ‘propagation’!
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.
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 (see previous page).
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!).
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.
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.
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!
On this page, we have described the propagation of an action potential, along the cell membrane, in one cell.
Propagation from one cell to another is a very different issue and is dealt with in two different ways:
