Introduction
Conduction of Excitation in Cardiac Muscle and Electrocardiography
Myocardial Cell Types: Contractile vs. Conducting
The heart consists of two primary types of muscle cells: contractile cells and conducting cells. Contractile cells generate the force necessary to pump blood, while conducting cells are specialized for rapidly transmitting action potentials throughout the myocardium, ensuring synchronized contractions.
Sequence of Excitation
The excitation sequence in the heart is highly coordinated:
SA Node: Located in the right atrium, the sinoatrial (SA) node is the heart's natural pacemaker, capable of generating action potentials spontaneously, thus setting the heart rate.
Atrial Internodal Tracts and Atria: The action potential travels from the SA node through the atrial internodal tracts to both atria, ensuring simultaneous depolarization and contraction.
AV Node: Positioned at the atrioventricular junction, the AV node slows down the conduction of the action potential (AV delay), allowing the ventricles to fill with blood before contraction.
Bundle of His, Purkinje System, and Ventricles: The action potential moves through the bundle of His, bundle branches, and Purkinje fibers, ensuring rapid and synchronized ventricular depolarization from apex to base.
Conduction Velocity and Its Determinants
Conduction velocity is crucial for heart function coordination and varies among myocardial tissues:
The AV node has the slowest conduction velocity (0.01–0.05 m/s), contributing to the AV delay.
Purkinje fibers have the fastest conduction velocity (2–4 m/s), facilitating quick ventricular activation.
Differences in conduction velocity are due to structural variations. The AV node has thinner myocytes and fewer gap junctions, leading to higher resistance and slower conduction. In contrast, Purkinje fibers have abundant gap junctions and larger diameters, promoting rapid conduction. Factors such as cell membrane resistance (Rm) and internal resistance (Ri) also affect conduction velocity.
Refractory Periods: Safeguarding Against Uncontrolled Contractions
Refractory periods are critical for preventing uncontrolled contractions:
Absolute Refractory Period (ARP): The cell is completely unresponsive to stimulation, preventing new action potentials during this phase.
Relative Refractory Period (RRP): The cell is less excitable but can be stimulated by a stronger-than-usual stimulus.
Supranormal Period (SNP): The cell shows increased excitability, making it easier to fire an action potential.
Autonomic Nervous System Modulation
The autonomic nervous system regulates heart rate and conduction velocity:
Sympathetic Stimulation: Increases heart rate (positive chronotropic effect) and accelerates conduction through the AV node (positive dromotropic effect) via β1 receptor activation, enhancing Ca2+ influx.