Purkinje fibers are specialized cells located within the heart that are part of its electrical conduction system. First described in 1839 by Czech scientist Jan Evangelista Purkyně, these fibers are not nerves but are a unique type of heart muscle cell. Their primary purpose is to carry electrical signals throughout the heart’s lower chambers, or ventricles. This rapid signal transmission ensures that the ventricles contract in a uniform and efficient manner.
Role in the Heart’s Conduction System
The heart’s ability to pump blood effectively relies on a precisely timed sequence of electrical events, much like an electrical wiring system. This sequence begins in the sinoatrial (SA) node, the heart’s natural pacemaker, located in the right atrium. The impulse generated by the SA node spreads across the atria, causing them to contract, and then travels to the atrioventricular (AV) node, which briefly delays the signal before relaying it onward.
From the AV node, the electrical impulse moves down a path called the bundle of His, which divides into right and left bundle branches. These branches extend down the muscular wall separating the ventricles. The Purkinje fibers represent the final and most extensive part of this pathway, branching out from the bundle branches and spreading throughout the inner walls of both ventricles.
This extensive network ensures that the electrical signal is delivered almost simultaneously to all parts of the ventricular muscle. The impulse travels very quickly through these fibers, triggering a contraction of the ventricles from the bottom up. This specific motion is efficient for ejecting blood from the right ventricle into the pulmonary artery and from the left ventricle into the aorta.
Specialized Cellular Structure
Purkinje fibers conduct electrical signals rapidly because their cellular structure is highly adapted for this function, unlike ordinary cardiac muscle cells. A primary difference is their size; Purkinje cells are larger in diameter than typical heart muscle cells. This increased size reduces internal resistance, allowing impulses to travel at 2–3 meters per second compared to 0.3–0.4 meters per second in ventricular muscle.
These cells also have a different internal composition. They contain fewer myofibrils, the protein strands responsible for muscle contraction. Since the main job of Purkinje fibers is conduction rather than generating force, they do not need a dense arrangement of contractile elements.
To fuel their high-speed activity, Purkinje fibers are rich in glycogen, a stored form of glucose that provides a ready energy supply. This abundance of glycogen makes them more resistant to conditions of low oxygen than other heart cells. They also possess a high density of gap junctions, which are specialized protein channels that directly connect adjacent cells. These junctions allow electrical ions to pass freely and quickly from one cell to the next, ensuring the signal propagates rapidly.
Impact on Heart Rhythm
The proper function of Purkinje fibers is directly linked to maintaining a stable heart rhythm. When these fibers are damaged or become dysfunctional, they can disrupt the heart’s normal electrical activity, leading to arrhythmias. Damage can result from various conditions, including a myocardial infarction (heart attack), heart failure, or genetic factors.
One common issue is premature ventricular contractions (PVCs), which are extra, abnormal heartbeats that originate in the ventricles. An irritable focus within the Purkinje network can fire an impulse on its own, independent of the normal signal from the SA node, causing an early and less effective contraction. While occasional PVCs can be harmless, frequent occurrences may indicate an underlying problem.
More serious conditions can also arise from Purkinje fiber malfunction. Ventricular tachycardia is a rapid, regular heart rhythm originating from the ventricles, often due to a faulty electrical circuit involving these fibers. This condition can impair the heart’s ability to pump blood and may escalate to ventricular fibrillation, a chaotic and life-threatening state. These rhythm disturbances can be identified on an electrocardiogram (ECG), where abnormal ventricular activity points to an origin within this specialized network.