How do conducting fibres ensure the coordinated contraction of the entire ventricle wall?

Sourav PanNovember 9, 2024

Answered

The conducting fibers of the heart, particularly the Purkinje fibers, play a crucial role in ensuring the coordinated contraction of the entire ventricle wall. Here’s how they achieve this:

1. Structure of the Conducting System

• Sinoatrial (SA) Node: The electrical impulse originates in the SA node, which is the natural pacemaker of the heart. This impulse spreads through the atria, causing them to contract.
• Atrioventricular (AV) Node: After a brief delay at the AV node, where it allows the atria to finish contracting and emptying blood into the ventricles, the impulse is transmitted down through the bundle of His.
• Bundle of His and Bundle Branches: The bundle of His splits into right and left bundle branches that run along the interventricular septum toward the apex of the heart.
• Purkinje Fibers: These fibers extend from the bundle branches and spread throughout the ventricular myocardium, reaching all parts of the ventricles.

2. Rapid Conduction of Electrical Signals

• Speed of Conduction: Purkinje fibers have a fast conduction rate, allowing electrical impulses to be transmitted quickly across the ventricular walls. The action potential reaches all ventricular muscle cells in about 75 milliseconds . This rapid conduction ensures that all parts of the ventricles receive the electrical signal almost simultaneously.
• Simultaneous Contraction: Because Purkinje fibers distribute impulses efficiently throughout the ventricular myocardium, this leads to nearly simultaneous contraction of all ventricular muscle cells. This coordinated contraction is essential for effectively pumping blood from both ventricles into the pulmonary artery and aorta .

3. Directional Contraction

• Contraction Sequence: The contraction begins at the apex of the heart and progresses upward toward the base. This is similar to squeezing a tube from one end, ensuring that blood is efficiently ejected into the great arteries .
• Mechanism: The structure of Purkinje fibers allows for a wave-like propagation of depolarization that ensures that contraction starts at the bottom (apex) and moves upward, facilitating effective blood expulsion from both ventricles.

4. Prevention of Tetany

• Sustained Action Potentials: The action potentials in cardiac muscle cells are sustained due to their unique ionic properties, preventing tetany (a state of prolonged contraction). This ensures that all myocardial cells have adequate time to contract before relaxation begins