Cardiac Myocyte Action Potential 

Overview: Cardiac myocytes have a unique action potential characterized by a prolonged plateau phase, which is essential for effective contraction of the heart. The ventricular action potential lasts approximately 200–400 ms. 

Phases of Cardiac Myocyte Action Potential: 

Phase 0 – Rapid Depolarization: 
 – Event: Fast upstroke of the action potential. 
 – Ion channels: Voltage-gated Na+ channels open.  
– Ion movement: Na+ flows into the cell.  
– Membrane potential: Rises from ~-90 mV to about +20 mV.  
– Significance: Initiates contraction. 
 
Phase 1 – Initial Repolarization:  
– Event: Slight repolarization after the peak.  
– Ion channels: Transient K+ channels (Ito) open briefly.  
– Ion movement: K+ flows out of the cell.  
– Membrane potential: Drops slightly from +20 mV to around 0 mV.  
– Significance: Prepares for plateau phase. 
 
Phase 2 – Plateau:  
– Event: Prolonged depolarization.  
– Ion channels: L-type Ca2+ channels open; some K+ channels remain open.  
– Ion movement: Ca2+ in, K+ out. 
 – Membrane potential: Remains around 0 mV for ~200 ms.  
– Significance: Sustains contraction and triggers calcium-induced calcium release. 
 
Phase 3 – Rapid Repolarization:  
– Event: Membrane potential returns to resting. 
– Ion channels: L-type Ca2+ channels close; delayed rectifier K+ channels open fully.  
– Ion movement: K+ flows out.  
– Membrane potential: Returns to ~-90 mV.  
– Significance: Ends contraction and prepares cell for next action potential. 
 
Phase 4 – Resting Membrane Potential: 
– Event: Cell is at rest.  
– Ion channels: Inward rectifier K+ channels (IK1) maintain resting potential; Na+/K+ ATPase maintains gradients.  
– Membrane potential: ~-90 mV.  
– Significance: Stable resting state, ready for next action potential. 

Key Features:  
– Long-duration action potential prevents tetany.  
– Calcium influx during plateau is crucial for excitation–contraction coupling.  
– Effective refractory period lasts nearly the entire action potential, preventing premature re-excitation. 
 
Diagram of Cardiac Action Potential: