Purkinje fibers

Purkinje fibers are the terminal component of the cardiac conduction system, following the sequence: SA node → AV node → bundle of His → left and right bundle branches → Purkinje fibers → ventricular myocardium. Histologically, they are large, pale-staining cells with abundant glycogen and few myofibrils, located just beneath the endocardium. They contain numerous gap junctions (connexins), which contribute to their rapid conduction velocity (~2-4 m/s), the fastest of any cardiac tissue, allowing near-simultaneous depolarization of the ventricles and efficient, coordinated contraction. This rapid, synchronized activation is essential for effective systolic ejection. Functionally, Purkinje fibers have automaticity and can serve as a subsidiary or 'escape' pacemaker if the SA and AV nodes fail, with an intrinsic firing rate of 15-40 bpm (slower than SA node's 60-100 bpm and AV node's 40-60 bpm). Clinically, Purkinje fiber dysfunction or abnormal automaticity is implicated in various arrhythmias, including bundle branch blocks (when conduction through the bundle branches leading to Purkinje network is impaired, causing widened QRS complexes), ventricular escape rhythms, and some forms of ventricular tachycardia/fibrillation (triggered activity or reentry involving Purkinje-myocardial junctions). Purkinje fibers are also relevant in the pathophysiology of idiopathic ventricular fibrillation and Brugada syndrome, where Purkinje-related triggered activity may initiate malignant arrhythmias. On EKG, the QRS complex represents ventricular depolarization, which depends on intact Purkinje fiber conduction; a narrow QRS indicates normal conduction through this system, while bundle branch blocks cause QRS widening (>120 ms) due to asynchronous ventricular activation.