The cardiac action potential is the rapid, precisely orchestrated change in electrical voltage across a heart muscle cell’s membrane that triggers each heartbeat. Unlike the quick electrical spike in a nerve cell, which lasts only a millisecond or two, the cardiac version stretches out over several hundred milliseconds, creating the sustained contraction needed to pump blood. This extended shape comes from a carefully timed relay of ion channels opening and closing in sequence, and disruptions to any step in that relay can produce life-threatening rhythm disturbances.
How a Single Heartbeat’s Electrical Signal Unfolds
A cardiac action potential moves through five conventionally numbered phases (0 through 4), though it helps to think of them not as separate events but as overlapping waves of different ions flowing across the cell membrane.
At rest (phase 4), a ventricular muscle cell sits at a negative internal voltage, roughly −90 millivolts. That resting state is maintained largely by an inward rectifier potassium current that holds the membrane steady by allowing potassium to flow in the direction that keeps the voltage negative.1PubMed. The inward rectifier current (IK1) controls cardiac excitability and is involved in arrhythmogenesis When a neighboring cell’s electrical impulse arrives, the voltage nudges positive just enough to open sodium channels. Nav1.5, the main voltage-gated sodium channel in the heart, then drives a massive, rapid influx of positively charged sodium ions, shooting the membrane voltage from −90 mV to around +20 or +30 mV in about a millisecond.2PubMed Central. When the Gates Swing Open Only: Arrhythmia Mutations That Target the Fast Inactivation Gate of Nav1.5 This sharp upstroke is phase 0, and its speed determines how fast the electrical wave spreads through the muscle.
Almost immediately after the peak, a brief dip called phase 1 occurs. Transient outward potassium channels open quickly, letting potassium rush out and partially pulling the voltage back down.3PubMed Central. Inducing I(to,f) and phase 1 repolarization of the cardiac action potential with a Kv4.3/KChIP2.1 bicistronic transgene This dip sets the stage for the feature that makes the cardiac action potential so distinctive: the plateau.
During phase 2, the membrane voltage hovers near zero for an unusually long time. This plateau exists because two opposing forces roughly balance each other. Calcium flows in through L-type calcium channels, providing a sustained inward positive current, while potassium channels continue letting potassium leak out.4PubMed Central. Control of L-type calcium current during the action potential of guinea-pig ventricular myocytes The L-type calcium current dominates the early plateau and then slowly fades, controlled mainly by calcium-dependent inactivation, meaning the very calcium that enters the cell gradually shuts those channels off.5PubMed Central. Profile of L-type Ca2+ current and Na+/Ca2+ exchange current during cardiac action potential in ventricular myocytes
As calcium channels wind down, potassium currents take over in phase 3. The rapid and slow delayed rectifier potassium currents, along with the inward rectifier current, drive the membrane voltage back toward its resting level.6PubMed Central. Cardiac Potassium Channels: Physiological Insights for Targeted Therapy Repolarization accelerates as it progresses because more potassium channels activate at these voltages, creating a snowball effect that brings the cell back to rest. The sodium-calcium exchanger also contributes to inward current during this late phase, helping to remove excess calcium from the cell.7PubMed Central. Profile of L-type Ca2+ current and Na+/Ca2+ exchange current during cardiac action potential in ventricular myocytes
Why the Plateau Matters So Much
That long, flat plateau is not just an interesting shape on a graph. It serves two critical purposes. First, it gives the heart muscle cell time to contract fully. Skeletal muscle fires and relaxes so fast that rapid re-stimulation can cause a sustained cramp (tetanus), but cardiac muscle’s prolonged action potential means the cell is electrically unresponsive (refractory) for almost as long as it takes to contract and relax. This built-in refractory period prevents tetanus in the heart, which would be fatal since a tetanized heart cannot pump.
Second, the plateau’s calcium influx is the trigger for contraction itself. The calcium entering through L-type channels during phase 2 is not, on its own, enough to drive a full contraction. Instead, it acts as a signal that causes a much larger release of calcium from internal stores inside the cell, a process called calcium-induced calcium release.8PubMed Central. Calcium and Excitation-Contraction Coupling in the Heart The sarcoplasmic reticulum, a calcium warehouse inside the muscle cell, detects the incoming calcium and opens its own release channels, flooding the cell with enough calcium to activate the contractile machinery.9PubMed Central. Contractions induced by a calcium-triggered release of calcium from the sarcoplasmic reticulum of single skinned cardiac cells This amplification step is why even modest changes in the action potential’s shape or duration can have outsized effects on how forcefully the heart squeezes.
Not Every Part of the Heart Fires the Same Way
The five-phase description above fits a typical ventricular muscle cell, but different regions of the heart produce action potentials with notably different shapes, durations, and mechanisms. These differences are not accidental; they are essential for the coordinated timing that keeps the heart pumping efficiently.
The sinoatrial (SA) node, the heart’s natural pacemaker, does not wait for an external signal. Its cells lack a stable resting potential. Instead, after each action potential ends, a special current called the “funny” current gradually depolarizes the membrane during phase 4, slowly pushing the voltage upward until it reaches the threshold for a new action potential.10PubMed. The role of the funny current in pacemaker activity The steepness of this slope determines heart rate: make the slope steeper and the heart beats faster; flatten it and the heart slows. SA node cells also rely more on calcium channels rather than sodium channels for their upstroke, which is why their action potentials rise more slowly and look smoother compared to the sharp spike in ventricular cells.
Atrial muscle cells have their own quirks. They express an ultra-rapid delayed rectifier potassium current that is largely absent from the ventricles.11Biophysical Journal. Role and Rate-Dependent Properties of Inactivation of the Ultrarapid Delayed-Rectifier K+ Current in Human Atrial Myocytes This extra repolarizing current shortens the atrial action potential compared to the ventricular one, which makes physiological sense: the atria only need to contract briefly to push blood into the ventricles, whereas the ventricles need a longer, more forceful contraction to send blood through the entire body. Because this current is atrial-specific, it has attracted interest as a drug target for atrial fibrillation, the idea being that blocking it would prolong atrial action potentials (potentially suppressing the arrhythmia) without affecting ventricular repolarization and risking dangerous side effects.
Purkinje fibers, the specialized conduction cells that deliver the electrical signal deep into the ventricular muscle, have the longest action potentials of any cardiac cell type, along with some of the fastest conduction speeds. They also express a mix of ion channels from both pacemaker and working muscle cells, which gives them a latent ability to fire on their own if the normal pacemaker fails.12PubMed Central. Purkinje fibers and arrhythmias
How the Signal Spreads Between Cells
A single cell’s action potential would be useless if it could not pass its electrical impulse to neighbors. Heart cells are physically connected by gap junctions, clusters of protein channels (predominantly connexin43 in the atria, ventricles, and His-Purkinje system) that allow ions and small molecules to flow directly from one cell’s interior to the next.13PubMed. Regulation of connexin43 gap junctional conductance by ventricular action potentials When one cell depolarizes, positive ions stream through gap junctions into the neighboring cell, nudging its membrane voltage toward threshold and triggering a new action potential there. This process cascades across the tissue at speeds of roughly half a meter per second in ordinary ventricular muscle and several meters per second in Purkinje fibers, ensuring the entire ventricle contracts nearly in unison.
Gap junction function is itself voltage-sensitive, and changes in gap junction expression or function (from scarring after a heart attack, for instance) can slow conduction and create the conditions for reentrant arrhythmias, where the electrical signal loops back on itself instead of dying out normally.
How the Nervous System Adjusts the Action Potential
Your heart rate and contractile force change constantly in response to exercise, stress, sleep, and digestion. These adjustments happen because the autonomic nervous system modifies the cardiac action potential in real time. The two branches of the autonomic nervous system, adrenergic (sympathetic) and cholinergic (parasympathetic), act through distinct receptors on heart cells and produce opposite effects.14PubMed. Signaling and function of cardiac autonomic nervous system receptors: Insights from the GPCR signalling universe
Sympathetic stimulation (the “fight or flight” branch) releases norepinephrine and epinephrine, which activate beta-adrenergic receptors. This increases L-type calcium current, boosting calcium entry during the plateau and strengthening contraction. It also enhances potassium currents that speed up repolarization, slightly shortening the action potential so the heart can beat faster without sacrificing filling time. In the SA node, sympathetic stimulation steepens the phase 4 pacemaker slope, directly increasing heart rate.
Parasympathetic stimulation (via the vagus nerve) releases acetylcholine, which activates muscarinic receptors. The effects are largely the reverse: reduced pacemaker slope, a slower heart rate, and weaker atrial contraction. In the SA and AV nodes, acetylcholine opens a specific potassium channel that hyperpolarizes the membrane, making it harder to reach threshold and slowing conduction through the AV node. This is the physiological basis for the “vagal maneuvers” sometimes used to terminate certain fast heart rhythms.
When the Action Potential Goes Wrong
Because the cardiac action potential depends on so many ion channels opening and closing in precise sequence, even small disruptions can have large consequences. Arrhythmias, abnormal heart rhythms, often trace back to specific action potential defects.
Long QT syndrome (LQTS) is one of the clearest examples. In this condition, the action potential’s repolarization phase is abnormally prolonged. On an electrocardiogram (ECG), this shows up as a lengthened QT interval. LQTS can be inherited, caused by mutations in genes encoding cardiac ion channels or their regulatory proteins.15Heart Rhythm O2. From genes to clinical management: A comprehensive review of long QT syndrome pathogenesis and treatment One well-studied form (LQT3) involves mutations in SCN5A, the gene for Nav1.5, that cause the sodium channel to stay open longer than it should, adding extra inward current during the plateau and delaying repolarization.16PubMed. Characterization of human cardiac Na+ channel mutations in the congenital long QT syndrome
The danger of a prolonged action potential is that it creates a window for abnormal depolarizations to occur before the cell has fully recovered. These early afterdepolarizations can trigger a chaotic ventricular rhythm called torsades de pointes, a form of polymorphic ventricular tachycardia that can degenerate into ventricular fibrillation and sudden death.17Heart Rhythm. Mechanisms of early afterdepolarizations and torsades de pointes
Ischemia (reduced blood flow, as during a heart attack) disrupts the action potential through a different mechanism. Oxygen-starved cells lose the energy needed to run their ion pumps, and potassium accumulates in the extracellular space. This elevated extracellular potassium partially depolarizes neighboring cells, slowing conduction and creating zones where the electrical wave can fragment or re-enter, setting the stage for ventricular fibrillation.18PubMed. Extracellular K+ and H+ shifts in early ischemia: mechanisms and relation to changes in impulse propagation
How Drugs Target the Action Potential
Most antiarrhythmic drugs work by modifying one or more of the ion currents that shape the action potential. The classic Vaughan Williams classification system groups them into four classes based on their primary target: sodium channels (Class I), beta-adrenergic receptors (Class II), potassium channels (Class III), and calcium channels (Class IV). A modernized version of this framework subcategorizes each class further to account for the many subtypes of channels and receptors now known, and adds newer molecular targets related to calcium handling and other signaling pathways.19PubMed. Modernized Classification of Cardiac Antiarrhythmic Drugs
Class III drugs, which prolong the action potential by blocking potassium channels, illustrate both the promise and the peril of targeting cardiac electrophysiology. By extending repolarization, they increase the refractory period and can suppress reentrant arrhythmias. But if they prolong the action potential too much, they risk causing the very problem they are meant to prevent: torsades de pointes. The hERG potassium channel, which carries the rapid delayed rectifier current, turns out to be unusually susceptible to blockade by a wide range of drugs, not just heart medications. Inhibition of hERG is a shared feature of many drugs associated with torsades de pointes, from certain antibiotics to antipsychotics to antihistamines.20PubMed. The hERG potassium channel and hERG screening for drug-induced torsades de pointes Because of this, testing for hERG channel blockade has become a standard step in drug development for any new compound, not just cardiac drugs.21PubMed. Predictive value of the hERG assay for anticipating the arrhythmogenic potential of new drugs
The Action Potential Changes as the Heart Develops
The cardiac action potential you see in an adult heart is not the same one present in an embryonic or neonatal heart. During fetal development, ventricular cells actually fire spontaneously, much like pacemaker cells, because they have not yet developed the strong inward rectifier potassium current that stabilizes the resting potential and suppresses automaticity in mature cells.22PubMed Central. Simulation of developmental changes in action potentials with ventricular cell models As the heart matures, repolarizing potassium currents increase and the action potential shortens, the internal calcium stores (sarcoplasmic reticulum) expand, and the cell’s internal tubule system develops to couple electrical excitation more tightly to contraction.23PubMed Central. Developmental Changes in the Excitation-Contraction Mechanisms of the Ventricular Myocardium and Their Sympathetic Regulation in Small Experimental Animals
These developmental shifts are one reason why neonatal and fetal hearts handle certain stresses and drugs differently from adult hearts. They also complicate efforts to use stem-cell-derived heart cells for research or therapy: lab-grown cardiomyocytes often remain in an immature state, producing action potentials that look more fetal than adult, which limits how reliably they predict adult drug responses.
Computational Models and Why Species Differences Matter
Because the cardiac action potential involves dozens of interacting ion channels, pumps, and exchangers, researchers rely heavily on computational models to understand how changes in one current ripple through the whole system. Modern approaches can link events at the molecular scale, such as how a single gene mutation alters one channel’s gating behavior, all the way up to tissue-level phenomena like arrhythmias and ECG abnormalities.24PubMed Central. A multiscale model linking ion-channel molecular dynamics and electrostatics to the cardiac action potential Newer modeling methods have expanded this even further, tracking processes that span from nanometers and femtoseconds at the channel level to centimeters and minutes at the arrhythmia level.25PubMed Central. Emerging methods to model cardiac ion channel and myocyte electrophysiology
These models are especially valuable because the animal species most commonly used in cardiac research do not always have the same ion channel profile as humans. Mice and rats, for example, have much shorter action potentials and rely on different potassium currents for repolarization compared to humans, making direct extrapolation of drug effects tricky.26PubMed Central. A comparative review on heart ion channels, action potentials and electrocardiogram in rodents and human: extrapolation of experimental insights to clinic Rabbits and guinea pigs produce action potentials that more closely resemble human ones, which is why they are often preferred for safety pharmacology studies. Still, no animal model fully replicates human cardiac electrophysiology, and some human channelopathies simply cannot be faithfully reproduced in other species.27PubMed Central. Differences in ion channel phenotype and function between humans and animal models This gap is one of the driving forces behind the development of human-iPSC-derived cardiomyocytes and increasingly sophisticated computational tools as complementary platforms for drug testing and disease modeling.
Reading the Action Potential on an ECG
The ECG you see in a doctor’s office is not a direct recording of any single cell’s action potential. It is a surface-level summation of millions of action potentials occurring across the heart at slightly different times. Still, the major features of the ECG trace map onto action potential events in a recognizable way. The P wave reflects atrial depolarization (phase 0 across atrial cells). The QRS complex corresponds to ventricular depolarization. The ST segment and T wave together represent the ventricular plateau and repolarization phases.
This relationship is why ECG changes can point to specific ion channel problems. A prolonged QT interval signals delayed ventricular repolarization, which could stem from a genetic channelopathy or a drug blocking potassium channels. ST-segment elevation during a heart attack reflects the injury current created when ischemic cells lose their normal resting potential and repolarization pattern while neighboring healthy cells maintain theirs. Understanding the action potential turns the ECG from a squiggly line into a readable story about what the heart’s electrical machinery is doing, and where it might be failing.