Depolarization is the electrical event that initiates every heartbeat, serving as the fundamental trigger for the heart’s pumping action. It is a rapid shift in the electrical charge of heart muscle cells, causing the usually negative internal charge to become momentarily positive. This change in electrical potential travels quickly across the heart tissue, acting like a wave of electricity that signals the muscle cells to contract. Without this precisely timed electrical firing, the coordinated squeeze required to pump blood could not occur. Depolarization translates the heart’s electrical rhythm into mechanical force.
The Cellular Basis of Depolarization
Heart muscle cells maintain an electrical difference across their membranes, known as the resting potential, typically around -90 millivolts (mV). This resting state is achieved by specialized pumps that maintain a higher concentration of positive sodium ions outside the cell and positive potassium ions inside the cell. Depolarization begins when an electrical stimulus reaches the cell and causes the membrane potential to reach a critical threshold.
Once the threshold is met, the cell rapidly opens voltage-gated sodium channels, allowing a massive influx of positively charged sodium ions into the cell. This sudden rush of positive charge reverses the cell’s internal electrical polarity, making the inside briefly positive, defining depolarization. Following the sodium influx, slower-acting L-type calcium channels open, allowing calcium ions to enter and sustain the positive charge in a phase known as the plateau. This sustained electrical shift is the action potential, which propagates the signal to adjacent cells.
The Heart’s Electrical Conduction Pathway
The electrical signal for the heartbeat is initiated in the Sinoatrial (SA) Node, located in the upper wall of the right atrium, which functions as the heart’s natural pacemaker. The wave of depolarization spreads from the SA Node across both atria, causing them to contract and push blood into the ventricles. The impulse then converges at the Atrioventricular (AV) Node, which momentarily delays the electrical signal for approximately 100 milliseconds.
This brief delay allows the atria to fully empty their blood into the ventricles before the lower chambers begin to contract. From the AV Node, the impulse travels rapidly down the Bundle of His, which divides into the right and left bundle branches. The signal is then distributed through the Purkinje fibers, which reach deep into the ventricular muscle tissue, ensuring a nearly simultaneous depolarization and powerful contraction of both ventricles.
The Mechanical Result: Heart Contraction
Depolarization is the electrical “switch” that immediately leads to the mechanical “squeeze” of the heart muscle through excitation-contraction coupling. The electrical current travels along the cell membrane and into internal structures (T-tubules), triggering the opening of calcium channels. This initial influx of calcium stimulates the release of a much larger quantity of stored calcium from the sarcoplasmic reticulum.
The resulting high concentration of calcium within the muscle cell cytoplasm is the direct trigger for contraction. Calcium ions bind to the regulatory protein Troponin-C, causing a change in the contractile filaments, actin and myosin. This interaction allows the myosin filaments to bind to and pull the actin filaments, which shortens the muscle cell and results in the physical contraction.
How Depolarization is Measured on an EKG
An Electrocardiogram (EKG or ECG) is a non-invasive test that measures the overall electrical activity of the heart from the body’s surface. The tracing records the collective depolarization and repolarization of millions of heart cells. The first small upward deflection on the EKG tracing is the P wave, which corresponds to the wave of depolarization spreading across the atria.
The next prominent feature is the QRS complex, a sharp, large deflection that represents the massive depolarization of the ventricles. Because the ventricles contain significantly more muscle mass than the atria, the QRS complex is much larger than the P wave. After the QRS complex, the T wave represents repolarization, the electrical return to the resting state, completing the electrical cycle for the ventricles.