The heart is an independent electrical generator that powers its own mechanical contractions. This process, known as cardiac conduction, is the highly organized system of electrical impulses that coordinates the heartbeat. By generating and transmitting electrical signals, the conduction system ensures that the heart’s four chambers contract in a precise sequence, allowing for efficient blood circulation.
The Heart’s Intrinsic Electrical Pathway
The process of cardiac conduction begins in the Sinoatrial (SA) node, a specialized cluster of cells located in the upper wall of the right atrium. Functioning as the heart’s natural pacemaker, the SA node spontaneously generates the electrical impulse that sets the heart rate. This initial signal quickly spreads across the walls of the right and left atria, causing them to contract and push blood into the ventricles.
The signal next converges at the Atrioventricular (AV) node, situated in the lower part of the interatrial septum. Here, the impulse is delayed briefly. This pause allows the atria to fully empty their blood into the ventricles before the next contraction begins, maximizing the heart’s pumping efficiency.
After the pause, the impulse moves rapidly into the Bundle of His, a tract of specialized fibers running down the wall separating the ventricles. The Bundle of His divides into the left and right bundle branches, which deliver the signal to the respective sides of the heart. These branches terminate in the Purkinje fibers, a network that fans out across the ventricular muscle walls. The Purkinje fibers transmit the signal almost instantaneously throughout the ventricles, triggering a synchronized contraction that pushes blood out to the lungs and the rest of the body.
Cellular Basis of the Electrical Impulse
The electrical signal is fundamentally a rapid, controlled shift of charged particles, or ions, across the membrane of a heart cell. This shift is called an action potential and involves depolarization and repolarization. At rest, heart muscle cells maintain a negative charge inside relative to the outside, known as the resting membrane potential.
The impulse begins with depolarization, where channels open, allowing a rapid influx of positively charged Sodium (\(\text{Na}^{+}\)) ions into the cell. This reverses the electrical potential, triggering the muscle contraction. An inflow of Calcium (\(\text{Ca}^{2+}\)) ions maintains the contraction state, followed by repolarization as Potassium (\(\text{K}^{+}\)) ions flow out. This restores the negative charge, causing the muscle to relax. This cyclical exchange creates the electrical wave that propagates from cell to cell to sustain the rhythmic heartbeat.
Measuring Conduction: The Electrocardiogram (EKG)
The electrical activity generated by the heart’s conduction system can be recorded and visualized using an Electrocardiogram (EKG). Electrodes placed on the skin detect the small electrical changes that occur with each heartbeat and translate them into a graphic tracing of waves.
A normal EKG tracing displays three main components correlating with the heart’s electrical events. The P wave represents the depolarization of the atria. The QRS complex signifies the depolarization of the ventricles, leading to their contraction. The T wave represents the repolarization, or electrical recovery, of the ventricles, allowing the muscle to relax.
Intervals between these waves, such as the PR interval, provide information about the speed of conduction, particularly the delay occurring at the AV node. Analyzing the shape and timing of these waves allows healthcare professionals to assess the integrity of the underlying electrical pathway.
When the Conduction System Malfunctions
Disruptions to the normal flow of electricity lead to arrhythmias, which are abnormalities in the heart’s rate or rhythm. These malfunctions can arise from problems in impulse generation at the SA node or issues with signal propagation along the pathway.
One common category is a heart block, involving a delay or complete interruption of the signal traveling from the atria to the ventricles, usually at the AV node. A first-degree AV block slows the signal, resulting in a prolonged PR interval on an EKG. A third-degree, or complete, heart block means the signal is entirely blocked, causing the atria and ventricles to beat independently, which results in a slow ventricular rate.
Another malfunction involves disorganized electrical activity, such as Atrial Fibrillation. Here, the atria generate chaotic, uncoordinated electrical signals instead of a single, uniform impulse. The ventricles receive rapid, irregular impulses, leading to a fast and erratic heartbeat that reduces pumping efficiency. Other issues, like a bundle branch block, delay the signal in one of the bundle branches, causing the two ventricles to contract out of sync.