Conduction System of the Heart

Components of the Cardiac Conduction System

The cardiac conduction system facilitates the coordinated contraction of the heart through specialized nodal tissue and conducting fibers:

  1. Sinoatrial (SA) node

  2. Interatrial tract (Bachman’s bundle) and internodal pathways

  3. Atrioventricular (AV) node

  4. AV bundle (Bundle of His)

  5. Right and left bundle branches (RBB and LBB)

  6. Subendocardial plexus of Purkinje fibres

Impulse Generation and Pacemaker Rates

The SA node acts as the primary pacemaker due to its high rate of spontaneous impulse generation. If another site generates the rhythm, it is termed an ectopic pacemaker.

  • SA node: 70−80/min70-80/min

  • AV node: 40−60/min40-60/min

  • Atrial muscle: 40−60/min40-60/min

  • Ventricular muscles: 20−40/min20-40/min

Internodal Pathways

Three specialized pathways conduct impulses from the SA node to the AV node:

  • Anterior internodal pathway of Bachman: Passes anterior to the superior vena cava.

  • Middle internodal pathway of Wenckebach: Passes posterior to the superior vena cava.

  • Posterior internodal pathway of Thorel: Descends through the crista terminalis.

Conduction Timing and Velocities

Conduction velocity varies by tissue to ensure efficient cardiac cycles. The AV nodal delay (0.1 sec0.1\,sec) is critical for allowing ventricles to fill before contraction.

Tissue

Conduction Rate (m/sm/s)

Purkinje system

4 m/s4\,m/s

Atrial pathways / Bundle of His / Ventricular muscle

1 m/s1\,m/s

SA node / AV node

0.05 m/s0.05\,m/s

Total Conduction Time Summary:

  • SA node to AV node: 0.03 sec0.03\,sec

  • AV node delay: 0.13 sec0.13\,sec (total sequence time)

  • Bundle branches: 0.03 sec0.03\,sec

  • Purkinje fibres: 0.03 sec0.03\,sec

Neural Innervation and Autonomic Effects

The heart is regulated by sympathetic and parasympathetic systems. At rest, parasympathetic tone predominates.

Sympathetic Influence (T1−T5T1-T5)

  • Neurotransmitter: Norepinephrine acting on β1\beta_1 receptors.

  • Positive Chronotropic effect: Increased heart rate.

  • Positive Dromotropic effect: Increased conduction velocity (decreased PR interval).

  • Positive Bathmotropic effect: Increased excitability.

  • Positive Inotropic effect: Increased force of contraction.

Parasympathetic Influence (Vagus Nerve)

  • Neurotransmitter: Acetylcholine acting on M2M_2 receptors.

  • Innervation: Right vagus to SA node; left vagus to AV node.

  • Negative Chronotropic and Dromotropic effects: Decreased heart rate and conduction velocity.

  • Negative Bathmotropic and Inotropic effects: Decreased excitability and force (affects atria only).