Learning Objective
Describe the specialized conduction properties of the AV node and Purkinje fibers, including differences in conduction velocity, fiber diameter, gap junction density, and phase 0 depolarization characteristics.
While all cardiac tissue can conduct electrical impulses, certain regions are uniquely adapted to ensure the timed, directional, and efficient propagation of the cardiac action potential. Conduction velocity varies across the heart due to structural and functional differences in these specialized cells.
Atrioventricular (AV) Node – Slow Conduction
The AV node acts as the physiological delay point between atrial and ventricular depolarization. Its slow conduction protects the ventricles from excessively rapid atrial rates and ensures coordinated filling before contraction.
Key features:
- Small fiber diameter → reduces conduction velocity
- Low density of gap junctions → fewer electrical connections between cells
- Slow phase 0 depolarization (due to Ca²-dependent upstroke instead of Na⁺-dependent)
- Produces the critical AV nodal delay (~0.1 sec)
Concept: The AV node slows conduction to allow time for ventricular filling before contraction.
Purkinje Fibers – Rapid Conduction
Purkinje fibers are the fastest-conducting cells in the heart and ensure synchronous depolarization of the ventricles, enabling efficient ejection of blood.
Key features:
- Large fiber diameter → promotes rapid signal transmission
- High density of gap junctions → extensive electrical connectivity
- Rapid phase 0 depolarization (due to abundant voltage-gated Na⁺ channels)
- Constitutes the His–Purkinje system, with the highest conduction velocity (~2–4 m/s) in the heart
Concept: Purkinje fibers rapidly distribute depolarization throughout the ventricles for coordinated contraction.
