Cardiac conduction system
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Conducting system of heart
The conducting system of the heart, is a specialized tissue located on the inner aspect of the myocardium, plays a crucial role in the heart’s functionality by conducting electrical impulses. This system is key to the heart’s ability to contract spontaneously. It is grouped in nodes and networks of specialized cardiac muscle cells, which are responsible for initiating and coordinating the complex process of cardiac contraction. Through this coordinated effort, the cardiac conduction system conducts the heart beats in a synchronized manner, enabling it to effectively pump blood throughout the body.
Sinuatrial node
An essential component of the conducting system is the Sinuatrial (SA) node, which, in a healthy heart, is responsible for generating excitation signals, acting as the heart’s pacemaker. Positioned at the upper end of the crista terminalis, the SA node is strategically located to spread signals across the atria (Left atrium and Right atrium), triggering their contraction and ensuring they contract in a synchronized manner. Shaped like a tadpole, the SA node is connected to the myocardium through multiple interdigitations. This close connection significantly enhances the efficiency with which the SA node initiates and coordinates the heart’s electrical impulses. From the atria, the initiated depolarization travels towards the ventricles.
Atrioventricular node
Before reaching the ventricles, the electrical impulse is funnelled towards the ventricles and slowed by another important node, the Atrioventricular (AV) node. This pivotal structure consists of specialised cells located near the heart’s center, within a region known as the Triangle of Koch. This triangular area is positioned near the opening of the coronary sinus and close to the septal cusp of the tricuspid valve. After passing through the AV node, the impulse moves towards the atrioventricular bundle, where it begins its crucial task of rapidly extending the impulse into the ventricular musculature.
Atrioventricular bundle
Atrioventricular bundle of is composed of a set of fibers that conduct electrical impulses from the AV node to the ventricles. Originating from the AV node, the atrioventricular bundle divides into right bundle branch and left bundle branch, each directing impulses to the respective ventricles.
Right bundle branch
The first bundle that we will look at is the Right Bundle Branch (RBB). It guides electrical impulses toward the apex of the right ventricle and subsequently to the anterior papillary muscle via the septomarginal trabecula. As it advances, the RBB follows the septal wall of the right ventricle, at one point it crosses the ventricular cavity through the moderator band. This passage allows the branch to extend its reach, branching out to both the base and the apex of the right ventricle. This way it ensures that the electrical impulses are evenly distributed across the right ventricular musculature, facilitating synchronized contraction for efficient blood ejection.
Left bundle branch
The left bundle branch descends towards the apex of the left ventricle. As it advances, it subdivides into three distinct fascicles: the superior (left anterior branch of atrioventricular bundle), the intermediate (left intermediate branch of atrioventricular bundle), and the inferior (left posterior branch of atrioventricular bundle). These branches play a crucial role in distributing electrical impulses evenly throughout the left ventricle, facilitating coordinated ventricular contractions essential for effective blood circulation. Let’s examine each distinct fascicle separately.
The the superior (left anterior branch of atrioventricular bundle) of the left bundle branch, travels along the top of the left ventricle towards the superior papillary muscle.
The intermediate (left intermediate branch of atrioventricular bundle) of the left bundle branch, courses superficially through the subendocardium along the septum of the left ventricle towards the apex.
The inferior (left posterior branch of atrioventricular bundle) of the left bundle branch, runs superficially through the subendocardium along the lower aspect of the left ventricle, heading towards the inferior papillary muscle of left ventricle.
Subendocardial branches (Purkinje Fibers)
From the left ventricle bundle branches and the right ventricle bundle branch, a mesh-like network of specialized cells, known as the Purkinje fibers, spreads along the inner layer of the myocardium. These specialized cells interface directly with the cardiac muscle cells. Their role is to distribute impulses throughout the ventricular musculature, ensuring synchronized contractions. Furthermore, this network enhances the functional contact between the conduction pathway and the cardiac musculature.