Cardiac Action Potential - Propagation

Overview

The ordered propagation of the Cardiac Action Potential is the basis for coordinated contraction of the heart. Spread of the cardiac action potential can be thought of as coordinated on two different scales: Locally between adjacent cardiomyocytes, and globally throughout the entire heart. These different scales of coordinated propagation operate using different mechanisms and are discussed separately below

Local Spread

As discussed in Cardiac Action Potential - Cellular Basis the molecular mechanism by which the cardiac action potential spreads from cell to cell is the free flow of positive ions between cardiomyocytes through gap junction-rich intercalated discs. This allows an action potential event in one cardiomyocyte to induce action potential events in adjacent cells with a slight time delay. Macroscopically, this phenomenon manifests as a wave of depolarization which spreads across sections of myocardium from their initiation point. An important feature of this action potential wave is that it does not bounce back and forth throughout the myocardium once initiated. This is a consequence of both the anatomy of the heart and the cardiomyocyte Refractory Period discussed in Cardiac Action Potential - Cellular Basis. The cardiomyocyte refractory period prevents immediate restimulation of a cardiomyocyte which has undergone an action potential and thus disallows an action potential from regressing to a cardiomyocyte from neighboring cells. Consequently, depolarization waves tend to progress unidirectionally from their initiation point to the remainder of unstimulated areas of myocardium

Global Spread

On an organ-level scale, the cardiac action potential is initiated by the SA Node in the right atrium, subsequently encompasses both atria, and after a delay propagates to both ventricles which are rapidly encompassed. On average, this entire process takes only 0.2 seconds. The speed by which normal, contractile cardiomyocytes propagate action potentials from one to another is not nearly fast enough to allow organ-level spread of the cardiac action potential within 0.2 seconds. Instead, specialized, non-contractile cardiomyocytes, termed "Conducting Cardiomyocytes", exist which propagate action potentials to one another much more rapidly. Consequently, global spread of the cardiac action potential is largely dependent on the anatomical organization of these specialized, conducting cardiomyocytes which are usually bundled in specialized conducting fibers. Below we detail the order in which the cardiac action potential is spread through the organ and which fibers of conducting cardiomyocytes are responsible for the spread.

Initiation of the cardiac action potential is performed by the SA Node located in the superior, posterolateral wall of the right atrium. The basis of rhythmic activation of the SA Node is discussed in Cardiac Action Potential - Rhythmicity

The SA Node is directly connected to contractile atrial cardiomyocytes and certain conducting fibers known as the "Interatrial Band" which rapidly spread the action potential from the right to the left atrium. Thus, following initiation by the SA Node, the cardiac action potential first spreads throughout both atria. During this process the cardiac action potential reaches the AV Node, located just above a small canal through the cardiac fibrous skeleton which connects the atria and the ventricular myocardial tissue

The tough, cardiac fibrous skeleton which separates the atria and the ventricles and provides the framework for the heart valves is electrically inert and thus cannot conduct the cardiac action potential. The only location through which the action potential can propagate from atria to ventricles is through a small canal roughly located between the atrioventricular valves and the aortic valve. The conducting cardiomyocytes located just above this canal are known as the AV Node and fibers which emanate from this node and travel through the canal are solely responsible for conducting the cardiac action potential from the atria to the ventricles. Uniquely, the AV Nodal cells conduct cardiac action potentials relatively slowly and thus significantly delay the spread of the cardiac action potential from the atria to the ventricles. This AV delay is critical for ensuring that the atria have sufficient time to contract and fill the ventricles prior to ventricular contraction

The AV Node is attached inferiorly to a set of highly specialized conducting cardiomyocytes known as Purkinje Fibers which transmit the cardiac action potential with incredible speed throughout both ventricles. The first set of purkinje fibers activated are those in the interventricular septum which are organized in bundles known as the bundles of His. These branch just as they emerge from the fibrous canal into Right and Left bundles which travel down the interventricular septum to the apex of the heart and then continue throughout the outer walls of both ventricles.

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