The Electrical Energy Behind Each Heartbeat

The heartbeat is triggered and coordinated by electrical impulses generated and conducted by the heart's specialized conduction system.

Conduction Pathway

1. SA node (sinoatrial) — right atrium; primary pacemaker (60–100 bpm intrinsic rate).
2. Atrial pathways (Bachmann's bundle, internodal tracts) → atrial depolarization (P wave).
3. AV node (atrioventricular) — delays impulse ~120 ms (allows atrial contraction/ventricular filling).
4. Bundle of His → right & left bundle branches → Purkinje fibers → rapid ventricular depolarization (QRS complex).

Electromechanical Coupling

Action potential → Ca²⁺ release from sarcoplasmic reticulum → myofilament sliding → contraction. The electrical event precedes and triggers the mechanical squeeze.

Energy Source

ATP hydrolysis by myosin ATPase powers contraction. Mitochondria (35% of cardiomyocyte volume) generate ATP from fatty acids, glucose, lactate, ketones.

Clinical Relevance

• ECG records electrical activity, not mechanical.
• Arrhythmias = electrical disorders (e.g., AFib, VT, heart block).
• Pacemakers/ICDs deliver electrical therapy.