Cardiac Electrical Conduction and Electrolyte Movements During Myocardial Contraction
Overview of the Cardiac Conduction System
The cardiac conduction system consists of specialized cardiomyocytes that initiate and propagate electrical impulses through a defined pathway: the sinoatrial (SA) node → atrial myocardium → atrioventricular (AV) node → His bundle → right and left bundle branches → Purkinje fibers → ventricular myocardium. 1, 2, 3
The Electrical Pathway
The electrical impulse originates in the SA node (the natural pacemaker), where specialized cells spontaneously depolarize. This impulse spreads through the atrial myocardium within approximately 200 milliseconds 1. The signal then reaches the AV node, which provides a critical delay allowing atrial contraction to complete before ventricular activation begins.
From the AV node, the impulse travels through the His-Purkinje system at roughly twice the speed of atrial conduction 1. The His bundle divides into right and left bundle branches, which further subdivide into Purkinje fibers that rapidly distribute the electrical signal throughout both ventricles. Ventricular activation proceeds from endocardium to epicardium 1, 3.
Electrolyte Movements During the Cardiac Action Potential
Depolarization Phase
During depolarization, the primary electrolyte movement involves:
- Rapid sodium (Na+) influx through fast sodium channels, causing the sharp upstroke of the action potential
- This generates the QRS complex on the ECG, representing ventricular depolarization 4, 5
Plateau Phase (Phase 2)
The plateau phase is characterized by:
- Calcium (Ca2+) influx through L-type calcium channels
- This calcium entry triggers additional calcium release from the sarcoplasmic reticulum (calcium-induced calcium release)
- The calcium binds to troponin, initiating myofilament contraction 6
- This phase corresponds to the ST segment on the ECG 7, 8
Repolarization Phase (Phase 3)
During repolarization:
- Potassium (K+) efflux through various potassium channels
- The transmembrane potential returns from approximately +10 mV to -85 mV (resting potential) 7
- Repolarization proceeds from epicardium to endocardium—opposite to the direction of depolarization 7, 8
- This generates the T wave on the ECG 7, 8
Clinical Significance of Electrolyte Abnormalities
Abnormalities in serum calcium and potassium can cause primary repolarization abnormalities, manifesting as ST segment and T wave changes on the ECG 7, 8, 7, 8. These electrolyte disturbances alter the shape and duration of the action potential plateau and rapid repolarization phases without necessarily affecting depolarization.
Key Electrophysiological Principles
The action potential duration varies across the ventricular wall:
- Epicardial cells have shorter action potential duration than endocardial and midmyocardial cells 7, 8
- This creates an inverse relationship between activation time and action potential duration 8
- These inherent differences result from electrotonic interactions during repolarization 7, 8
Ion Channel Expression
The cardiac conduction system expresses ion channels fundamentally different from working myocardium 3. This specialized expression pattern is critical for:
- Automaticity in the SA node
- Controlled conduction delay in the AV node
- Rapid impulse propagation through the His-Purkinje system
The unique ion channel profile includes specific sodium, calcium, and potassium channel subtypes that determine the electrophysiological properties of each conduction system component 2, 3.
Note: I cannot provide a photograph or diagram as I am a text-based system. However, standard medical textbooks and educational resources contain excellent illustrations showing:
- The anatomical pathway of the conduction system
- The phases of the cardiac action potential with corresponding ion movements
- The correlation between action potentials and the surface ECG
For visual learning, I recommend consulting cardiology textbooks or reputable medical education websites that provide labeled diagrams of these concepts.