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The heart is special for many reasons but also because it has a unique propagation system in which the electrical impulse (= the action potential) propagates through this organ. This system of propagation is called the Cardiac Conduction System.
There are three types of structures/tissues involved in this Cardiac Conduction System:
The nodal tissue is located in two areas:
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The myocardium – these are all the muscle cells in the wall of the atria and the ventricles. These cells are connected to each other through special channels called “gap junctions” (also called ‘connexion‘). Through these channels, an action potential can propagate from one cell to another, a similar mechanism as in the electrical synapse (see A.3.6. The Electrical Synapse).
Microscopic image of a myocardium:
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The Purkinje tissue – cells that are specialized in fast propagation. Named after Dr. Purkinje who first discovered these special cells:see Wikipedia
The Purkinje cells are arranged in several bundles and are only located in the ventricles – not in the atria!
The first Purkinje bundle is the His-bundle that starts at the bottom of the AV-node, and passes through the Annulus Fibrosus (= AV-fibrous barrier).
In the ventricles, the Purkinje tissue runs along the ventricular septum in two separate bundles; the right and the leftbundle branches.
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Unfortunately (for the students!), the shape of the cardiac action potential is not the same at different places in the heart but depends on the location in the heart.
(Remember what is an action potential? See: A.3.3. The Action Potential)
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In the atria, the action potential is triangular with a fast depolarization and a slower repolarization. In the ventricles, the potential between the depolarization and the repolarization is stable for some 100-300 msec and close to 0 mV. This is called the ‘plateau’. (footnote: what is a plateau?)
Please note that in both the atrial and in the ventricular myocardium, the potential between the action potential and the next, is stable, at about -90 mV.
However, in some cardiac tissues, the resting potential is not stable. This is the case in the nodal tissues (sinus node and AV-node) and in the Purkinje tissues.
In those tissues, the resting potential, after the repolarization of the previous action potential, slowly depolarizes (blue in the diagram).
So, in summary, action potentials in the heart have different shapes. The most important elements are:
Diastolic potentials from the nodal tissue (sinus and AV-node) and from the Purkinje fibers show a slow depolarization(green in the diagram).
The action potentials from the ventricles (myocardium and Purkinje tissue) show a plateau, located between the depolarization and the repolarization (red in the diagram).
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You may remember that calcium ion plays an important role in inducing contraction.
In the skeletal muscles, calcium ions are stored inside the cells, in the sarcoplasmic reticulum, from where they are released to flow to the sarcomere to induce contraction.
In the heart, calcium ions are not stored inside the cells, but flow from outside into the cell. The trigger for calcium ions to flow into the cells is the action potential! Specifically, between the depolarization and the repolarization; in fact, during the plateau!
To understand the cardiac conduction system and how the heart behaves electrically, it is important to understand the location (= the topography) of the different type of action potentials in the heart.
This diagram therefore summarizes the different shapes of the cardiac action potentials and their distribution in the heart.
Note that the action potentials in the ventricles (myocardium and Purkinje) all have a plateau whereas the action potentials in the atria, both myocardium and nodal tissues, don’t have a plateau!
Footnote:
What is a plateau? In the mountains, at the top, there is sometimes not a peak but a flat piece of land. Such a flat land, high on top of a hill or a mountain, is called a plateau (from the French, meaning a ‘plate’).
