phase 4 depolarization
Gradual spontaneous depolarization during diastole in pacemaker cells, leading to automatic impulse generation.
Full Definition
Phase 4 depolarization refers to the gradual, spontaneous depolarization that occurs during diastole in cardiac pacemaker cells, particularly those in the sinoatrial node, atrioventricular node, and His-Purkinje system. Unlike working myocardial cells that maintain a stable resting potential, pacemaker cells exhibit this slow upward drift in membrane potential due to the interplay of ionic currents, including the 'funny current' (If), calcium currents, and decreasing potassium conductance. When phase 4 depolarization reaches threshold potential, it triggers an action potential and subsequent cardiac contraction. Enhanced phase 4 depolarization in abnormal locations can lead to ectopic rhythms and arrhythmias. Understanding this mechanism is crucial for comprehending automaticity disorders and the effects of various antiarrhythmic drugs.
Usage
Usage note: Always include 'phase' when referring to this concept to distinguish from general depolarization.
In Context
- "Enhanced phase 4 depolarization in the His-Purkinje system resulted in accelerated idioventricular rhythm." — Electrophysiology textbook
- "Antiarrhythmic therapy aimed to suppress abnormal phase 4 depolarization in ectopic foci." — Cardiology treatment plan