The heart functions as a dual pump, rhythmically contracting to maintain effective circulation. This rhythmic contraction is orchestrated by a specialized conduction system composed of nodes and conductive cardiac muscle fibers that generate and propagate electrical impulses , ensuring synchronized contraction of atrial and ventricular chambers.
This intrinsic conduction system enables the heart to beat automatically and rhythmically without external neural stimulation. Efficient cardiac pumping depends on precise timing: the atria contract slightly before the ventricles , allowing ventricular filling, and the ventricles contract simultaneously to maximize cardiac output.
Overview of Cardiac Conduction System Components
- Sinoatrial (SA) Node
- Internodal and Intra-atrial Pathways (including Bachmann's bundle)
- Atrioventricular (AV) Node
- Bundle of His (AV Bundle)
- Right and Left Bundle Branches
- Purkinje Fibers
Each component ensures the stepwise and time-optimized conduction of action potentials across cardiac tissue.
Sequential Events in the Cardiac Cycle
- Impulse Generation : The SA node initiates an action potential.
- Atrial Conduction : Impulse spreads across the atria → Atrial systole .
- AV Node Delay : Conduction slows at the AV node, allowing full ventricular filling.
- Ventricular Conduction : Impulse travels via the Bundle of His → bundle branches → Purkinje fibers → Ventricular systole .
Detailed Components of the Conduction System
1. Sinoatrial (SA) Node
- Location : Upper wall of right atrium, near the entrance of the superior vena cava .
- Structure : Elliptical strip of specialized pacemaker cells (~2–3 mm wide, ~10–15 mm long).
- Function :
- Primary pacemaker of the heart ( 60–100 bpm ).
- Generates spontaneous depolarizations due to unstable resting membrane potential (phase 4 automaticity).
- Impulses spread through gap junctions across atrial muscle fibers.
- Autonomic Regulation :
- Sympathetic stimulation (β₁ receptors) → ↑ heart rate.
- Parasympathetic stimulation (M₂ receptors via vagus nerve) → ↓ heart rate.
- Blood Supply :
- Typically from the right coronary artery (RCA) ; in some individuals, from the left circumflex artery (LCx) .
- Special Feature : Rich in β₁ and M₂ receptors , giving it high sensitivity to autonomic input.
2. Internodal and Intra-atrial Pathways
- Function : Transmit impulses from SA node to AV node.
- Major Tracts :
- Anterior internodal pathway : Includes Bachmann’s bundle , which transmits impulse from right to left atrium .
- Middle (Wenckebach) and Posterior (Thorel) internodal tracts .
- Conduction Velocity : ~0.5–1.0 m/s, faster than surrounding atrial myocardium (~0.3 m/s).
- Timing : Takes ~ 0.03 seconds for impulse to travel from SA node to AV node.
3. Atrioventricular (AV) Node
- Location : Posteroinferior part of interatrial septum, near opening of coronary sinus , at the base of the right atrium .
- Function :
- Delays conduction ( ~0.09 sec ) to allow full ventricular filling before contraction.
- Acts as a secondary pacemaker (intrinsic rate: 40–60 bpm ).
- Conduction Delay Mechanism :
- Fewer gap junctions and narrow fibers → slower impulse propagation.
- Additional delay (~0.04 sec) occurs as impulse enters the Bundle of His .
- Blood Supply : Typically from a branch of the right coronary artery or left circumflex artery .
- Clinical Relevance :
- A common site for heart blocks and re-entrant arrhythmias .
- AV nodal delay ensures atrial contraction precedes ventricular contraction .
4. Bundle of His (AV Bundle)
- Anatomy : Continuation of AV node, penetrating the fibrous skeleton of the heart into the interventricular septum .
- Function :
- The only electrical connection between atria and ventricles.
- Allows unidirectional impulse propagation (atria → ventricles).
- Structure :
- Divides into right and left bundle branches .
- Cells exhibit fast conduction properties and some pacemaker activity.
- Clinical Note :
- Preserved conduction in this region is critical. Damage here causes bundle branch blocks or complete heart block .
5. Right and Left Bundle Branches
- Course :
- Right bundle branch (RBB): Travels along the right side of the septum to the apex of the right ventricle.
- Left bundle branch (LBB): Divides into anterior and posterior fascicles supplying the left ventricle.
- Location : Beneath the endocardium on respective sides of the septum.
- Function : Conduct impulses rapidly to Purkinje fibers.
- Blood Supply :
- Supplied by septal branches of left anterior descending (LAD) artery.
6. Purkinje Fibers
- Structure :
- Large, specialized cardiac fibers with abundant glycogen and few myofibrils.
- High density of gap junctions → rapid conduction velocity (~4 m/s) .
- Location : Subendocardial layer of both ventricles.
- Function :
- Final conduit of the conduction system.
- Ensures simultaneous depolarization of both ventricles for powerful ejection of blood.
- Intrinsic Rate : 15–40 bpm if upstream pacemakers fail.
Summary of Conduction Times
| Pathway | Approximate Time (sec) |
|---|---|
| SA node to AV node | 0.03 |
| AV node delay | 0.09 |
| AV bundle delay | 0.04 |
| Total atrial to ventricular delay | ~0.16 |
| Purkinje system → Myocardium | ~0.03–0.06 |
Clinical Correlation
- Arrhythmias arise when there's disruption in impulse generation or conduction.
- SA node dysfunction → sinus bradycardia or arrest.
- AV block → first-, second-, or third-degree heart blocks.
- Bundle branch blocks affect synchronization of ventricular contraction.
- Pacemakers may be required for sustained bradyarrhythmias.
- Electrocardiogram (ECG) findings correlate with specific conduction defects (e.g., prolonged PR interval in AV block).
High-Yield Points
- SA node = primary pacemaker (60–100 bpm).
- AV node = gatekeeper ; delays conduction (40–60 bpm).
- Only Bundle of His bridges atria and ventricles.
- Purkinje fibers = fastest conduction velocity (~4 m/s).
- Total delay between atrial and ventricular contraction ≈ 0.16 seconds .
- Autonomic regulation alters rate of SA node firing.
- Ischemia of the AV node or Bundle of His → conduction blocks .
- Bachmann’s bundle = primary path to the left atrium .
