Electrical Conduction System of The Heart - Conduction Pathway

Conduction Pathway

Action potentials arising in the SA node (and propagating to the left atrium via Bachmann's bundle) cause the atria to contract. Simultaneously, action potentials travel to the AV node via three internodal pathways. After a delay, the stimulus is conducted through the bundle of His to the bundle branches and then to the purkinje fibers and the endocardium at the apex of the heart, then finally to the ventricular myocardium. The pathway can be summarized as: SA node → anterior, middle, and posterior internodal tracts → transitional fibers → AV node → penetrating fibers → distal fibers → Bundle of His (AV bundle) → right and left bundle branches → Purkinje fibers → myocardium. The total time taken by the nerve impulse to travel from the SA node to the ventricular myocardium is 0.19 seconds. Microscopically, the wave of depolarization propagates to adjacent cells via gap junctions located on the intercalated disk. The heart is a functional syncytium (not to be confused with a true "syncytium" in which all the cells are fused together, sharing the same plasma membrane as in skeletal muscle). In a functional syncytium, electrical impulses propagate freely between communicating cells via gap junctions, so that the myocardium functions as a single contractile unit. This property allows rapid, synchronous depolarization of the myocardium. While normally advantageous, this property can be detrimental as it potentially allows the propagation of incorrect electrical signals (e.g., via an ectopic pacemaker). Gap junctions can close, e.g., after a cardiac ischemic event such as myocardial infarction, thus isolating damaged or dying tissue in the myocardium, which then no longer participate in synchronous myocardial contractility.