In cardiac muscles (heart muscles), ion movement is essential for generating the electrical impulses that trigger heartbeats. This is known as the cardiac action potential and it ensures that the heart contracts and relaxes rhythmically.

⚡ Key Ions Involved:

📈 Phases of Cardiac Action Potential (Ventricular Myocytes)

🟦 Phase 4: Resting Potential

• High K⁺ inside, high Na⁺ & Ca²⁺ outside

• Membrane potential ~ –90 mV

• Cell is ready to fire

🟩 Phase 0: Rapid Depolarization

• Na⁺ channels open

• Na⁺ rushes in

• Membrane becomes positive inside

• Trigger for contraction begins

🟨 Phase 1: Initial Repolarization

• Na⁺ channels close

• K⁺ briefly exits

• Slight drop in membrane potential

🟥 Phase 2: Plateau Phase (Unique to Cardiac Muscle)

• Ca²⁺ channels open

• Ca²⁺ enters slowly

• K⁺ leaves slowly

• Maintains long plateau to prevent early repolarization

• This prolongs contraction for effective blood pumping

🟧 Phase 3: Repolarization

• Ca²⁺ channels close

• K⁺ exits more

• Cell returns to –90 mV

💥 Role of Each Ion in Function

🫀 Clinical Relevance

• Too much K⁺ (Hyperkalemia): Slows heart, can cause arrhythmia

• Too little K⁺ (Hypokalemia): Increases excitability, risk of fibrillation

• Calcium blockers: Reduce heart contractility, used in hypertension