13.4 : Conduction System of the Heart

The cardiac conduction system produces and transmits electrical impulses that prompt myocardial contraction, ensuring efficient heart function. This intricate system ensures that the heart beats in a coordinated and efficient manner, beginning with the atria and then the ventricles. The conduction system optimizes cardiac output by maintaining this precise sequence, which is crucial for adequate blood circulation.

This system relies on the unique properties of nodal and Purkinje cells: automaticity, excitability, and conductivity. These properties allow the heart to maintain a rhythmic and coordinated contraction sequence, optimizing cardiac output.

The Sinoatrial (SA) Node

At the core of this system is the sinoatrial (SA) node, the heart's primary pacemaker, located where the superior vena cava connects the right atrium. In a resting adult, the SA node typically fires 60 to 100 impulses per minute, though this rate adjusts according to the body's metabolic needs.

The electrical impulses created by the SA node propagate through the atria via both the atrial myocardium and internodal pathways, leading to atrial contraction. The impulses then reach the atrioventricular (AV) node, situated in the right atrial wall near the tricuspid valve.

The Atrioventricular (AV) Node

The Atrioventricular node plays a crucial role in delaying the transfer of impulses to the ventricles. This allows sufficient time for the atria to complete their contraction and for the ventricles to fill with blood. After the delay, the impulses travel down the bundle of His, which splits into the right and left bundle branches.

The left bundle branch further divides into anterior and posterior branches, facilitating impulse transmission to the left ventricle.

Finally, the impulses reach the Purkinje fibers, distributing the electrical signal throughout the ventricles and triggering a synchronized contraction.