Purkinje fibers

The Purkinje fibers are the final, fastest leg of the heart's wiring — the spray of fine cables that distributes the electrical signal into the ventricular muscle. If the bundle branches are the highways, the Purkinje fibers are the dense web of local streets that reaches every neighborhood almost at once. They are named after the physiologist Jan Evangelista Purkinje.

These fibers run just beneath the inner lining (endocardium) of both ventricles and conduct impulses faster than any other cardiac tissue — several times faster than ordinary muscle. This speed is the whole point: it lets the wave of excitation reach the entire ventricular wall in a near-instantaneous, coordinated sweep, so the ventricles contract as a powerful, unified squeeze rather than a sluggish ripple.

Purkinje cells also have latent automaticity, meaning they can fire on their own if the higher pacemakers fail — but only at a slow escape rate of roughly 20 to 40 beats per minute. That makes them a last-resort backup. The same cells, when injured or stretched, can also become a source of dangerous extra beats and ventricular arrhythmias.