The Heart's Own Electrical Wiring
Think of the heart as a pump that must beat in a precise, coordinated rhythm — top chambers (atria) first, then bottom chambers (ventricles). This rhythm isn't controlled by your brain; the heart has its own built-in electrical system that generates and spreads the signal to contract. That system is the cardiac conduction system.
The key idea: a small group of cells spontaneously fires an electrical impulse, which then travels along specialised pathways, forcing each part of the heart to contract in the correct order. No external trigger needed — the heart is self-starting.
The Components and Their Roles
The system consists of four main structures, arranged in sequence:
| Structure | Location | Function |
|---|
| SA node (sinoatrial node) | Right atrium, near the opening of the superior vena cava | Pacemaker — generates the impulse spontaneously (~70–80 times/min) |
| AV node (atrioventricular node) | Lower part of the right atrium, near the interatrial septum | Delays the impulse (~0.1 s) so atria finish contracting before ventricles start |
| Bundle of His (atrioventricular bundle) | Interventricular septum | Conducts the impulse from the AV node into the ventricles |
| Purkinje fibres | Spread throughout the ventricular walls | Distribute the impulse rapidly to the ventricular muscle |
The SA node is called the natural pacemaker because it sets the heart's rhythm. If it fails, the AV node can take over at a slower rate (~40–60/min).
How the Impulse Travels
-
SA node fires — the impulse spreads across both atria via gap junctions in the atrial muscle cells. This causes the atria to contract (atrial systole).
-
Impulse reaches the AV node — here it is delayed by about 0.1 second. This delay is critical: it gives the atria time to empty their blood into the ventricles before the ventricles contract.
-
Impulse passes through the Bundle of His — this bundle splits into right and left branches that run down the interventricular septum.
-
Impulse spreads via Purkinje fibres — these fibres fan out from the bundle branches into the ventricular walls. They conduct the impulse extremely fast, so the ventricles contract almost simultaneously from the apex upward, squeezing blood out efficiently.
The sequence is SA node → AV node → Bundle of His → Purkinje fibres. This ensures atria contract first, then ventricles — a one-way, timed wave.
Why This Matters …