The Cardiac Conduction System: Pathway & Order

Every heartbeat starts as an electrical impulse that travels a fixed route through the heart. Trace that pathway and the whole ECG — and every arrhythmia — starts to make sense.

What is the cardiac conduction system?

The cardiac conduction system is the network of specialised cells that generates and carries the heart's electrical impulse, coordinating each heartbeat. The signal follows one route: SA node → AV node → bundle of His → right and left bundle branches → Purkinje fibers. That sequence makes the atria contract first, then the ventricles.

The conduction pathway, in order

1. SA (sinoatrial) node — the natural pacemaker in the upper right atrium near the SVC. It fires the impulse.
2. AV (atrioventricular) node — near the atrial septum; it briefly delays the impulse so the atria finish emptying before the ventricles fire.
3. Bundle of His — carries the impulse into the interventricular septum.
4. Right and left bundle branches — run down either side of the septum.
5. Purkinje fibers — spread the impulse through the ventricular muscle for a coordinated contraction.

Intrinsic pacemaker rates

Each part of the system can pace the heart, but at progressively slower rates — a built-in backup hierarchy:

Pacemaker	Intrinsic rate
SA node	60–100 bpm
AV node / junction	40–60 bpm
Ventricles (Purkinje)	20–40 bpm

Normally the fastest pacemaker — the SA node — sets the pace and suppresses the others.

How it maps to the ECG

The conduction sequence is exactly what the ECG records: the P wave is atrial depolarization from the SA node, the PR interval includes the AV-node delay, and the QRS complex is ventricular depolarization through the bundles and Purkinje fibers. When conduction fails at the AV node, you see heart block.

Autonomic control

The system runs on its own but is tuned by the autonomic nervous system: sympathetic stimulation speeds the SA node and conduction (raising heart rate), while parasympathetic (vagal) tone slows it. This is why atropine, which blocks vagal tone, treats symptomatic bradycardia.

Key takeaways

• The conduction pathway is SA node → AV node → bundle of His → bundle branches → Purkinje fibers.
• The AV node delays the impulse so atria empty before ventricles fire.
• Intrinsic rates fall down the chain: 60–100, 40–60, then 20–40 bpm.
• The pathway maps directly onto the P wave, PR interval, and QRS.
• Sympathetic tone speeds it; vagal tone slows it.

Frequently asked questions

What is the cardiac conduction system?

The network of specialised cells that generates and conducts the heart's electrical impulse to coordinate each heartbeat, running from the SA node to the Purkinje fibers.

What is the order of the cardiac conduction system?

SA node → AV node → bundle of His → right and left bundle branches → Purkinje fibers.

What is the natural pacemaker of the heart?

The SA (sinoatrial) node, in the upper right atrium, which normally fires at 60–100 beats per minute.

Why does the AV node delay the impulse?

So the atria have time to finish contracting and filling the ventricles before the ventricles are triggered to contract.

What are the intrinsic pacemaker rates?

SA node 60–100 bpm, AV node/junction 40–60 bpm, and the ventricles (Purkinje) 20–40 bpm.

How does the conduction system relate to the ECG?

The P wave is SA-node-driven atrial depolarization, the PR interval reflects the AV-node delay, and the QRS is ventricular depolarization through the bundles and Purkinje fibers.