2.5 Conducting System of the Heart
The conducting system of the heart is a specialized network that enables the rhythmic contraction of cardiac muscle. This system ensures the heart beats in a coordinated manner, pumping blood efficiently throughout the body.
Myogenic Contraction: The heart's rhythm is myogenic, meaning the impulse for contraction originates from within the cardiac muscle itself, not from external nerve stimulation. The heart can continue to beat even when removed from the body, demonstrating its intrinsic ability to generate electrical impulses.
Cardiac Conduction System: This is a network of specialized, interconnected cardiac muscle tissue responsible for initiating and coordinating the heart's contractions.
Components of the Conduction System
Sinoatrial (SA) Node
Location: Embedded in the upper wall of the right atrium, near the entrance of the vena cavae.
Function: Acts as the heart's natural pacemaker. It generates spontaneous action potentials at the highest frequency, setting the rhythm for the entire heart.
Origin: Developed from the sinus venosus.
Atrioventricular (AV) Node
Location: Found near the junction of the right atrium and right ventricle.
Function: Receives the electrical impulse from the SA node. It deliberately slows down the signal's propagation, creating a crucial delay.
Atrioventricular (AV) Bundle (Bundle of His)
Location: A strand of specialized muscle fibers that extends from the AV node into the interventricular septum (the wall between the ventricles).
Function: It is the only electrical connection between the atria and the ventricles. It splits into two main branches:
- Right and Left Bundle Branches: These travel down the interventricular septum towards the apex (bottom tip) of the heart.
Purkinje Fibers
Location: The terminal branches of the bundle branches that spread throughout the ventricular walls.
Characteristics:
- Large-diameter cardiac muscle fibers
- Fewer myofibrils (they do not contract forcefully)
- Well-developed intercalated disks with numerous gap junctions
Function: These features allow for very rapid transmission of action potentials, ensuring a coordinated and powerful contraction of the ventricles from the apex upwards.
The Pathway and Timing of Conduction
The sequence of electrical activation is as follows:
- SA Node Initiates: An action potential originates in the SA node and spreads across the atrial walls, causing them to contract.
- Signal Reaches AV Node: The impulse travels from the SA node to the AV node (takes approximately 0.04 seconds).
- AV Node Delay: The action potential is slowed at the AV node for approximately 0.11 seconds. This delay is critical because it allows the atria to finish contracting and empty their blood into the ventricles before the ventricles begin to contract.
- Signal to Ventricles: The impulse passes from the AV node to the AV bundle and down the right and left bundle branches to the apex of the heart.
- Ventricular Contraction: Purkinje fibers rapidly distribute the action potential throughout the ventricular walls, causing a synchronized, powerful contraction that pumps blood out of the heart.
The timing of this pathway correlates with the Phases of Heartbeat→, where atrial contraction precedes ventricular contraction.
Science, Technology, and Society: Cardiac Arrhythmias and Pacemakers
Cardiac Arrhythmia: A disturbance or irregularity in the heart's electrical rhythm.
- Bradycardia: A slow heart rate (less than 40-60 beats per minute)
- Tachycardia: A fast heart rate (more than 100 beats per minute)
Artificial Pacemaker: A medical device surgically implanted under the skin. It generates electrical impulses to initiate myocardial contraction at a set rate, thereby correcting arrhythmias and restoring a normal heart rhythm.
Possible Questions and Answers
Q: What does it mean for the heart to be myogenic?
A: It means the heart generates its own contractile rhythm from within the cardiac muscle, specifically the SA node, without needing external stimulation from the nervous system.
Q: Why is the SA node called the pacemaker of the heart?
A: Because it has the fastest rate of spontaneous depolarization and generates action potentials more frequently than any other part of the cardiac muscle, setting the pace for the entire heart.
Q: What is the biological significance of the delay at the AV node?
A: The approximately 0.11 second delay allows the atria to fully contract and empty their blood into the ventricles before the ventricles are stimulated to contract. This ensures efficient, one-way blood flow through the heart chambers.
Q: How do Purkinje fibers facilitate a coordinated ventricular contraction?
A: They are large-diameter fibers with numerous gap junctions, allowing them to conduct the electrical impulse much more rapidly than other cardiac tissue. This ensures the entire ventricular myocardium contracts in a synchronized, powerful wave.