B.3.1. The Cardiac Conduction System

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A. Components of the Cardiac Conduction System

B. The shape of the Cardiac Action Potentials

C. Diastolic Depolarization

Advanced - C1. The many shapes of the cardiac action potentials

C2. Advanced – Calcium influx during the action potential

D. Topography of the Cardiac Action Potential

A. Components of the Cardiac Conduction System:

1.
The heart is special for many reasons but also because it has a unique system in which the electrical impulse (= the action potential) propagates through this organ. This system of propagation is called the Cardiac Conduction System.

2.
This propagation occurs through special structures that are responsible for the initiation and the propagation of the electrical impulse (= the action potential).

3.
There are three types of structures/tissues involved in this Cardiac Conduction System:
a. the nodal tissue
b. the myocardium
c. the Purkinje tissue

4.
The nodal tissue is located in two areas:
a. the sinus node – located in the wall of the right atrium close to the inlet of the superior and inferior Vena Cava.
b. the atrio-ventricular node (= AV-node) located between the atria and the ventricles

5.
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”. Through these channels, an action potential can propagate from one cell to another.

6.
The Purkinje tissue – cells that are specialized in fast propagation.
Named after Dr. Purkinje who first discovered these special cells: wikipedia.org/Jan_Evangelista_Purkyně.

7.
The Purkinje cells are arranged in several bundles and are only located in the ventricles – not in the atria!

8.
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).

9.
In the ventricles, the Purkinje tissue runs along the ventricular septum in two separate bundles; the right and the left bundle branches.

The Cardiac Conduction
System

B. The shape of the Cardiac Action Potentials:

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1.
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)

2.
In the atria, the action potential is triangular with a fast depolarization and a slow 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’.

What is a plateau? In mountains, sometimes, at the top, there is 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’).

The shapes of the cardiac action potentials

C. Diastolic Depolarization:

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1.
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.

2.
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.

3.
In those tissues, the resting potential, after the repolarization of the previous action potential, slowly depolarizes.

Pacemaker Potentials

4.
Since this depolarization occurs between one action potential and the next, it occurs in the diastolic period and is therefore called the ‘diastolic depolarization’.

5.
This gradual depolarization, at a certain moment, reaches threshold and then initiates a new action potential. Therefore, this is the basis of the pacemakers in the heart.

C1. Advanced – The many shapes of the cardiac action potentials:

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1.
So, in summary, action potentials in the heart have different shapes. The most important elements are: - a diastolic depolarization - a plateau - a fast or slow depolarization

2.
Action potentials from the nodal tissue (sinus and AV-node) show a slow depolarization (green in the diagram)

3.
Action potentials from nodal tissue and Purkinje tissue show a diastolic depolarization (blue in the diagram).

4.
The action potentials from the ventricles (myocardium and Purkinje tissue) show a plateau, located between the depolarization and the repolarization (red in the diagram).

The different shapes of the Cardiac Action Potentials

C2. Advanced – Calcium influx during the action potential:

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1.
You may remember that calcium ion plays an important role in inducing contraction. (See A.4.3. The Sarcomere:)

2.
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.

3.
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!

D. Topography of the Cardiac Action Potential:

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1.
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.

2.
This diagram therefore summarizes the different shapes of the cardiac action potentials and their distribution in the heart.

3.
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!

Cardiac Action Potentials