Cardiac conduction cells are specialised modified cardiomyocytes with reduced contractile capacity and enhanced electrical automaticity and conduction velocity. The SAN pacemaker cells have the fastest spontaneous depolarisation rate (60-100 bpm), suppressing subsidiary pacemakers. The His-Purkinje system conducts at 2-4 m/s compared to 0.5 m/s for working myocardium, ensuring rapid synchronous ventricular activation.
| Location | Specialised muscle cells in the heart wall, from the sinoatrial node at the top of the right atrium to the Purkinje network under the ventricular endocardium |
|---|---|
| Parts |
|
| Nerve Supply | Vagus nerves (slow the sinoatrial node and atrioventricular conduction); Sympathetic cardiac nerves (speed them) |
| Blood Supply | Sinoatrial nodal artery (from the right coronary artery in about 60%); Atrioventricular nodal artery (from the right coronary artery in most people); Septal branches of the left anterior descending artery (bundle branches) |
Starts each heartbeat and spreads the impulse in order: atria first, a short delay at the atrioventricular node, then rapid spread through the ventricles so they contract together.
SAN dysfunction (sick sinus syndrome) produces bradycardia, sinus pauses, and tachy-brady syndrome from fibrosis of the SAN and surrounding atrial conduction tissue, requiring pacemaker implantation. AV nodal block (first, second Mobitz I/II, and complete) interrupts conduction at the AV node, requiring pacing for complete block. Bundle branch block (right BBB, left BBB) delays ventricular activation in the affected ventricle, producing QRS widening on ECG. Left bundle branch block is pathological in 98% of cases. Catheter ablation of arrhythmia substrates targets specific areas of the conduction system.
Fibrosis, ischaemia, or iatrogenic injury (cardiac surgery, catheter ablation, valve replacement) of the AV node produces complete heart block with independent atrial and ventricular rhythms; the escape rhythm from the His-Purkinje system is typically 20-40 bpm insufficient to maintain cardiac output; emergency temporary pacing and permanent pacemaker implantation are the standard treatments.
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