Cardiovascular Physiology: The Conduction System, Electrophysiology, The Cardiac Cycle, and Regulation of Cardiac Output

Cardiac Conduction System

Overview of the Cardiac Conduction System

The cardiac conduction system is responsible for initiating and coordinating the electrical impulses that trigger heart contractions. This system ensures the heart beats in a coordinated and rhythmic manner, allowing efficient blood flow throughout the body.

• Sinoatrial (SA) Node: The primary pacemaker of the heart, located in the right atrium. It generates action potentials that set the heart rate.

• Atrioventricular (AV) Node: Receives impulses from the SA node and introduces a delay, allowing the atria to contract before the ventricles.

• AV Bundle (Bundle of His): Conducts impulses from the AV node to the bundle branches.

• Right and Left Bundle Branches: Carry impulses through the interventricular septum toward the apex of the heart.

• Purkinje Fiber System: Rapidly conducts impulses to ventricular myocardium, ensuring coordinated ventricular contraction.

Example: The SA node initiates an impulse, which travels through the atria, pauses at the AV node, then rapidly descends through the bundle branches and Purkinje fibers to stimulate ventricular contraction.

Pacemaker Cells and Heart Rhythm

Each population of pacemaker cells in the heart can potentially set the heart rate, but the one with the fastest intrinsic rate (usually the SA node) dominates and sets the overall rhythm.

• Sinus Rhythm: The normal rhythm of the heart, generated by the SA node.

• Latent Pacemakers: Other cells (e.g., AV node, Purkinje fibers) can act as pacemakers if the SA node fails, but at a slower rate.

Additional info: If the SA node is damaged, the AV node or Purkinje fibers can take over, but this often results in bradycardia (slower heart rate).

Propagation of Action Potentials

Under normal conditions, the SA node generates an action potential that spreads through the atria via gap junctions. The impulse is then conducted to the AV node by specialized fibers, introducing a brief delay before reaching the ventricles.

• Gap Junctions: Allow direct electrical communication between cardiac muscle cells, enabling rapid spread of depolarization.

• AV Node Delay: The delay at the AV node (about 0.13 seconds) is due to fewer gap junctions and the presence of the fibrous skeleton, allowing the atria to contract before the ventricles.

Action Potential Pathway

Action potentials begin in the SA node and are carried by atrial cardiomyocytes and intermodal pathways to the AV node. After a slight delay, the impulse travels into the AV bundle, the only electrical connection between the atria and ventricles, and then down the bundle branches and Purkinje fibers to the ventricular myocardium.

• Sequence: SA node → Atrial muscle → AV node (delay) → AV bundle → Bundle branches → Purkinje fibers → Ventricular muscle

• Ventricular Syncytium: The coordinated contraction of ventricular muscle fibers ensures efficient ejection of blood.

Example: The delay at the AV node allows the atria to finish contracting and empty their blood into the ventricles before ventricular contraction begins.

Summary Table: Components of the Cardiac Conduction System

Component	Location	Function
SA Node	Right atrium	Primary pacemaker; initiates action potentials
AV Node	Interatrial septum	Delays impulse; allows atrial contraction
AV Bundle (Bundle of His)	Interventricular septum	Conducts impulses to bundle branches
Bundle Branches	Interventricular septum	Carry impulses to Purkinje fibers
Purkinje Fibers	Ventricular walls	Distribute impulse to ventricular myocardium

Additional info: The fibrous skeleton of the heart electrically isolates the atria from the ventricles, ensuring that impulses only pass through the AV node to reach the ventricles.