The Cardiac Action Potentials


A. A few major points about the Cardiac Action Potential:

As with other muscles (skeletal, smooth muscles), cardiac cells are excitable and show, upon stimulation, an action potential. The cardiac action potential has some similarities and some differences with the action potential in other muscles.


Similarity 1:

The upstroke of the action potential (= depolarization) is also caused by opening of sodium channels and by the influx of Na+ into the cell.

Similarity 2:

The repolarization of the action potential is also caused by the opening of potassium channels and the efflux of K+ out of the cell.

Difference 1:

The plateau. In contrast to the skeletal muscles, the potential after the upstroke and before the repolarization remains more or less stable at about 0 mV level. This is called the plateau (plateau is a French word for a flat surface on top of a mountain). This is the case for the cells in the ventricles.

Difference 2:

The pacemaker potential. In nodal cells and in Purkinje cells, the resting potential is not stable but gradually depolarizes (see panel C).



B. Local Cardiac Action Potential:

In the nerves and the skeletal muscle cells, the action potentials more or less look the same. This is not the case at all in the heart. Depending on the tissue and the location in the heart, the action potentials vary a lot in their shape:

1. Sinus Node:

These cells show a gradual depolarization and a gradual repolarization. The resting potential is not stable but shows a slow depolarization.

2. Atrium:

These cells show a fast depolarization and a slow repolarization. The resting potential is flat. There is no plateau.

3. AV-node:

These nodal cells, as in the sinus node, show slow depolarization and repolarization while the resting potential is not stable.

4. Purkinje cells:

These cells show a fast depolarization, a long plateau and a slower repolarization. The resting potential is not stable.

5. Ventricles:

These ventricular action potentials are very similar to Purkinje cells: fast depolarization and a long plateau. However, their resting potential is stable.



C. Pacemaker Potentials:


As indicated in the previous paragraph, the resting potentials in three types of cardiac cells are not stable; in the sinus node, the AV-node and the Purkinje cells.


In this diagram, the (diastolic) potential between two successive action potentials is plotted for these three cells.


In all three cases, the potential is not stable (= not “resting”) but slowly depolarizes.




However, the sinus node depolarizes faster (= steeper) than the cells in the AV-node while the Purkinje cells show the slowest depolarization.


This slow depolarisation during diastole (=diastolic depolarization) is crucial as these cells can therefore make new action potentials. These are therefore the pacemaker cells.


Although all three types make action potentials, the sinus node cell makes them faster than the other cells. This brings us to an important rule of the heart; the fastest pacemaker is the pacemaker for the heart.


But if, for some reason, the sinus node is not able to excite the heart, then the AV-node can become the new, but slower, pacemaker. If the AV-node also cannot excite the heart, then the Purkinje cells can take over, albeit at an even slower rate.


D. Topography of the Cardiac Action Potentials.


This diagram was created to show you the location of the different types of action potentials in the heart.


The nodal potentials are very similar (sinus node and AV-node).


The left and right atrium shows the same type of atrial action potential.



The ventricular action potential is also similar in the right and the left ventricle.


In fact, all the action potentials in the ventricles are similar in that they all show a long plateau. The only difference in the ventricles is that the Purkinje cells also show diastolic depolarization while the myocardium does not.




E. Plateau and calcium ions.


The plateau is required in the ventricles because this is when calcium ions flow into the cells. This calcium is used for the contraction.



Although the atrial myocardium does not have a real plateau, its action potential still lasts longer than a nerve or skeletal muscle action potential.


This is because calcium channels do open during the repolarization and calcium ions do flow into the atrial cell to be used for the subsequent contraction