Internodal pathways and Bachmann bundle
Tractus internodales
Also known as: internodal tracts, Bachmann bundle, interatrial band
The internodal pathways are routes of faster conduction through the atrial walls. The anterior, middle and posterior internodal tracts carry the impulse from the SA node to the AV node, and Bachmann bundle carries it across to the left atrium.
Overview
Three internodal pathways are classically described. The anterior pathway gives off Bachmann bundle, which crosses to the left atrium. The middle pathway is named after Wenckebach and the posterior pathway, which runs along the crista terminalis, after Thorel.
Impulses travel along these routes at about 1 m/s, compared with about 0.3 m/s in ordinary atrial muscle. They reach the AV node roughly 0.03 s after leaving the SA node.
Whether these are true insulated tracts, like the bundle branches, is debated. They are better understood as preferential pathways created by the way the muscle bundles are oriented, for example in the crista terminalis and the limbus of the fossa ovalis. The P wave on the ECG represents the spread of activation across both atria along these routes.
- Location
In the walls of the right atrium and the interatrial septum. Bachmann bundle runs across the roof of both atria.
Function
- Conduct the impulse rapidly from the SA node to the AV node.
- Synchronise left atrial activation with the right atrium (Bachmann bundle).
Clinical significance
Interatrial block
When conduction through Bachmann bundle is delayed, the P wave becomes broad (≥120 ms) and notched. This interatrial block predisposes to atrial fibrillation and stroke.
P-wave morphology
A tall, peaked P wave of at least 2.5 mm in lead II (P pulmonale) suggests right atrial enlargement. A broad, bifid P wave (P mitrale) suggests left atrial enlargement.
Compiled from standard anatomy and physiology references — see the Cardiovascular System sources. For education only; not medical advice.