Sinoatrial node, atrioventricular node, bundle of His, bundle branches, Purkinje fibres
Mnemonic
The order of the conducting system, with the intrinsic rates falling as you descend:
- Sinoatrial node, 60 to 100 per minute, in the right atrium at the opening of the superior vena cava
- Atrioventricular node, 40 to 60, in the interatrial septum near the coronary sinus
- Bundle of His, then the left and right bundle branches
- Purkinje fibres, 20 to 40
“The pacemaker is whichever tissue is fastest”, which is why the sinoatrial node normally leads and why an escape rhythm from lower down is always slower and broader.
The atrioventricular node is the only normal electrical connection between atria and ventricles, because the fibrous skeleton insulates the rest, and its deliberate delay allows atrial contraction to complete ventricular filling.
Expansion
- Sinoatrial node - in the right atrium at the upper end of the crista terminalis
- Internodal pathways through atrial myocardium
- Atrioventricular node - in the triangle of Koch, in the interatrial septum
- Bundle of His - pierces the fibrous skeleton
- Right and left bundle branches - down the interventricular septum
- Purkinje fibres - into the ventricular myocardium
The deliberate delay at the atrioventricular node is what allows atrial contraction to complete ventricular filling before systole begins, and it is represented by the PR interval on the electrocardiogram.