bundle of His

The bundle of His is the cable that carries the heartbeat's signal from the atrioventricular node into the ventricles. Think of the atrioventricular node as a slow checkpoint and the bundle of His as the on-ramp to the high-speed conduction highway that follows it. It is named after Wilhelm His Jr., the anatomist who described it.

Anatomically the bundle runs from the lower part of the atrioventricular node, penetrates the fibrous ring that separates atria from ventricles, and travels along the top of the interventricular septum. There it splits into the right and left bundle branches. Crucially, the bundle of His is normally the only conducting tissue that crosses the atrioventricular insulating barrier, so it is a single point of failure: damage here can disconnect the ventricles from the atria's rhythm.

Because it is such a strategic relay, the bundle of His has become a target for modern device therapy. In His-bundle pacing, a lead is placed directly on the bundle so that pacing recruits the heart's own fast conduction network, producing a more natural, synchronized ventricular contraction than pacing the ventricular muscle directly.