Overview
This lecture covers the conduction system of the heart, the functional syncytium of cardiac muscle, conduction pathways, depolarization sequence, conduction velocities, and pacemaker hierarchy.
Cardiac Muscle as a Functional Syncytium
- Cardiac muscle cells are tightly connected by gap junctions, enabling coordinated contraction as a unit (functional syncytium).
- Gap junctions allow bidirectional electrical current flow between cardiomyocytes.
- Electrical synapses, unlike chemical synapses, permit current to move in both directions across the heart.
Cardiac Conduction Pathway
- Electrical impulses begin at the sinoatrial (SA) node.
- Impulse travels via internodal pathways to the atrioventricular (AV) node.
- Bachmann's bundle (interatrial tract) transmits the impulse from the right to the left atrium.
- After the AV node, impulses cross fibrous tissue via the bundle of His.
- The bundle of His divides into left and right bundle branches, reaching the apex before turning upward.
- Purkinje fibers distribute the impulse throughout ventricular muscle.
Sequence of Depolarization
- Depolarization starts at the SA node, proceeds through atria, AV node, septum, apex, and then up the ventricular walls.
- Ventricular depolarization spreads from endocardium (inner) to epicardium (outer).
- Electrical activity always precedes mechanical contraction in cardiac tissue.
Conduction Velocities
- Atrial muscle: ~0.3 m/s; atrial internodal pathways: ~1 m/s.
- AV node: slowest conduction at 0.05–0.1 m/s.
- Bundle of His and Purkinje fibers: fastest conduction at 2–4 m/s.
- Conduction velocity depends on action potential spread, not on action potential duration.
AV Node Delay and Functional Significance
- Delay at AV node (~0.12–0.16 seconds total from SA node to bundle branches) is due to fewer gap junctions and slow response action potentials.
- AV node delay allows atria to fully contract and empty before ventricular contraction.
- AV node acts as a gatekeeper, preventing excessively rapid atrial impulses from reaching the ventricles (e.g., during atrial fibrillation).
Pacemaker Hierarchy and Overdrive Suppression
- SA node is the primary pacemaker (70–80 bpm intrinsic rate).
- AV node (40–60 bpm), bundle of His (~40 bpm), and Purkinje fibers (15–20 bpm) can serve as backup pacemakers if higher nodes fail.
- Overdrive suppression means the fastest pacemaker site sets the heart rate.
- Latent pacemakers take over rhythm if the leading pacemaker fails or is suppressed.
Key Terms & Definitions
- Functional Syncytium — A network of connected cardiac cells that contract together as one unit.
- Gap Junctions — Specialized connections allowing direct electrical communication between cardiac cells.
- SA Node (Sinoatrial Node) — The heart's primary pacemaker; initiates electrical impulses.
- AV Node (Atrioventricular Node) — Conducts impulses from atria to ventricles with a delay.
- Bundle of His — A conduction pathway from AV node to bundle branches.
- Purkinje Fibers — Terminal fibers distributing impulse throughout ventricles.
- Overdrive Suppression — The pacemaker with the highest rate controls the heart rhythm.
Action Items / Next Steps
- Review conduction pathway and sequence of depolarization.
- Memorize the hierarchy and intrinsic rates of pacemakers.
- Understand the functional significance of AV node delay.