Cardiac Action Potentials: Phases 0-4 Made Easy, Diagram and Ions Explained
Cardiac Action Potentials: Phases 0-4 made easy with our diagram and memory trick! Pacemaker and non-pacemaker cells and their ions explained!
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Cardiac Action Potential
What is an Action Potential?
An action potential is a change in voltage across a cell membrane, specifically a rise in voltage followed by a fall.
What is the Function of an Action Potential?
Action potentials are used to send information throughout the body.
Action potentials are also necessary for some types of cells to function, as they trigger intracellular processes (i.e. contraction of muscle cells).
What Cells Use Action Potentials?
Cells that use action potentials are also called excitable cells.
Examples include:
Neurons
Muscle cells (skeletal, cardiac, and smooth)
Pacemaker cells (specialized cardiac muscle cells)
Endocrine cells
Among others
This lecture focuses on action potentials in the heart, called cardiac action potentials.
This includes the action potentials of pacemaker cells and non-pacemaker cells (contractile cells).
You will learn a simple memory trick to remember the different phases of cardiac action potentials and which ions are involved.
Understanding cardiac action potentials becomes clinically relevant when using antiarrhythmic drugs or managing conduction disorders.
Cardiac Action Potential Phases: Diagram of pacemaker vs non-pacemaker cell action potentials and their ions explained.
Types of Cardiac Cells
Let’s briefly review the 2 types of cardiac cells, as this will help us understand cardiac action potentials.
There are 2 main types of cardiac cells:
Pacemaker cells (conduction)
Non-pacemaker cells (contraction)
1. Pacemaker Cells (conduction)
What are Pacemaker Cells?
Pacemaker cells are specialized heart muscle cells (cardiac myocytes) capable of generating their own spontaneous action potentials.
This is called automaticity.
These action potentials lead to cardiac conduction and contraction.
Of note: Cardiac (heart) muscle cells are also called cardiac myocytes, cardiomyocytes, or myocardiocytes.
Where are Pacemaker Cells Located?
Pacemaker cells are primarily located in the:
Sinoatrial node (SA node)
Atrioventricular node (AV node)
Pacemaker cells are also present in other parts of the conduction system including:
Bundle of His
Purkinje fibers
The SA node has a faster inherent pacemaker rate than the other pacemaker cells (AV node, bundle of His, etc.)
As a result, the SA node is the primary pacemaker in a normal functioning heart.
In other words, the SA node generates the initial action potential in the heart.
If the SA node becomes suppressed, then the other pacemaker cells (AV node, etc.) are capable of generating spontaneous action potentials, but at a slower heart rate.
The SA node is located in the back of the right atrium near the superior vena cava entry.
The action potential generated by the SA node travels through the heart’s electrical conduction system.
Conduction System Pathway
The action potential generated by the SA node (right atrium) travels to the left atrium via Bachmann’s bundle, and to the AV node via internodal pathways.
This depolarizes the atria and leads to atrial contraction.
Conduction velocity is slowed through the AV node (base of the right atrium)
This allows for atrial contraction and the movement of blood from the atria to the ventricles to occur, before ventricular contraction occurs.
From the AV node, the impulse travels through the bundle of His to the right and left bundle branches.
The right bundle branch depolarizes the right ventricle, leading to contraction.
The left bundle branch depolarizes the left ventricle, leading to contraction.
The bundle branches terminate/connect to Purkinje fibers.
Purkinje fibers also help depolarize the ventricles to contract.
Cardiac Conduction System: Labeled diagram of the conduction system pathway. Action potentials generated by the SA node travel to the AV node, bundle of His, right and left bundle branches, and Purkinje fibers.
2. Non-Pacemaker Cells (contraction)
What are Non-Pacemaker Cells?
Non-pacemaker cells are heart muscle cells (cardiac myocytes) that physically contract the heart and pump blood.
Unlike the pacemaker cells, they do not generate their own action potentials.
Instead, their action potentials are generated when neighboring cardiac myocytes are depolarized or when pacemaker cells stimulate them.
Where are Non-Pacemaker Cells Located?
Non-pacemaker cells are primary located in the:
Atria (atrial myocytes)
Ventricles (ventricular myocytes)
Non-pacemaker cells are responsible for atrial and ventricular contraction.
Cardiac Action Potentials
There are 2 main types of cardiac action potentials:
Fast-response action potential
Non-pacemaker contractile cells (i.e. atrial and ventricular myocytes)
Slow-response action potential
Pacemaker cells (i.e. SA and AV nodes)
Pacemaker and non-pacemaker cells, discussed above, have slightly different cardiac action potentials.
Let’s discuss each type and provide a simple memory trick to remember them.
Non-Pacemaker Cells: Atrial & Ventricular Myocytes
Let’s first discuss the action potentials of non-pacemaker cells (contractile cells).
The action potentials of non-pacemaker cells (i.e. atrial and ventricular myocytes) are also called fast-response action potentials.
Reminder: The atrial and ventricular myocytes are non-pacemaker cells responsible for the physical contraction of the heart.
Memory Trick
All you need to memorize is the following phrase:
“Summit, Plummet, Continue, Plummet”.
Through this simple catchphrase, you just learned ALL of the action potential phases AND the ion channels involved with non-pacemaker cells.
Here’s how:
“Summit” = Sodium = Phase 0
When a non-pacemaker cell becomes stimulated, its resting membrane potential (-90 mV) becomes more positive.
An action potential is generated if the threshold (-70 mV) is met.
The action potential will first rise or “summit” as the voltage across the cell membrane becomes more positive.
This is referred to as depolarization and is phase 0 of the action potential.
What ion is involved in depolarizing the cell?
Here’s where the catchphrase is helpful.
Summit starts with the letter “S”, and this will help you remember when the action potential summits it is sodium involved.
In order for the cell to become more positive and depolarize, will sodium ions need to enter or exit the cell?
Sodium ions will need to enter the cell (because sodium is a positive ion).
And this is exactly what occurs during phase 0 of the non-pacemaker cell action potential.
Sodium ion channels open when the threshold cell membrane voltage is met, and an influx of sodium ions into the cell leads to depolarization.
Recap: Phase 0 = “Summit” phase of action potential = Sodium ion influx
Cardiac Action Potentials: Phase 0 of non-pacemaker cells involves an influx of sodium ions into the cell, leading to depolarization (summit = sodium).
“Plummet” = Potassium = Phase 1
As the cell becomes more positive from the influx of sodium ions, the sodium channels begin to close, thereby reducing the influx of sodium.
The action potential will then fall or “plummet” as the voltage across the cell membrane becomes slightly more negative.
This causes the cell to slightly repolarize and is phase 1 of the action potential.
What ion is involved in repolarizing the cell?
Again, here is where the catchphrase is helpful.
Plummet starts with the letter “P”, and this will help you remember when the action potential plummets it is potassium involved.
In order for the cell to become more negative and repolarize, will potassium ions enter or exit the cell?
Potassium ions will need to exit the cell (because potassium is a positive ion).
And this is exactly what occurs during phase 1 of the non-pacemaker cell action potential.
The sodium channels from phase 0 have closed, thereby reducing the influx of sodium ions, and the potassium channels are open, leading to an efflux of potassium ions and a slight repolarization of the cell.
Recap: Phase 1 = “Plummet” phase of action potential = Potassium ion efflux
Cardiac Action Potentials: Phase 1 of non-pacemaker cells involves an efflux of potassium ions, leading to a slight repolarization (plummet = potassium).
“Continue” = Calcium = Phase 2
After slight repolarization has occurred in phase 1, the action potential will then plateau or “continue” as the voltage across the cell membrane remains fairly constant.
This plateau in the voltage is phase 2 of the action potential.
If potassium is exiting the cell, then there must be another positive ion entering the cell to counteract the loss of the positive charge and keep the action potential level.
Which ion is responsible?
Again, use the catchphrase.
Continue starts with the letter “C”, and this will help you remember when the action potential continues it is calcium involved.
We know that when calcium enters muscle cells it will lead to contraction.
And this is exactly what occurs during phase 2 of the non-pacemaker cell action potential.
L-type calcium channels are open, and an influx of calcium ions into the cell leads to myocyte contraction.
Recap: Phase 2 = “Continue” phase of action potential = Calcium ion influx
Cardiac Action Potentials: Phase 2 of non-pacemaker cells involves an influx of calcium ions into the cell, leading to contraction (continue = calcium).
“Plummet” = Potassium = Phase 3
After myocyte contraction has occurred in phase 2, the action potential will then fall or “plummet” again as the voltage across the cell membrane becomes negative.
This is referred to as repolarization and is phase 3 of the non-pacemaker cell action potential.
We know from phase 1 that plummeting involves the efflux of potassium ions.
And this is exactly what occurs during phase 3 of the non-pacemaker cell action potential.
The calcium channels are now closed, thereby reducing the influx of calcium, and the potassium channels are open, leading to an efflux of potassium ions and repolarization of the cell.
Recap: Phase 3 = “Plummet” phase of action potential = Potassium ion efflux
Cardiac Action Potentials: Phase 3 of non-pacemaker cells involves an efflux of potassium ions, leading to repolarization (plummet = potassium).
Resting Phase = Phase 4
After the cell has repolarized, it is now back at its resting membrane potential.
This is phase 4 of the non-pacemaker cell action potential, in which the cell is at rest until the next stimulus occurs.
Cardiac Action Potentials: Phase 4 of non-pacemaker cells involves the cell being at rest until the next stimulus occurs (resting membrane potential).
Pacemaker Cells: SA and AV Node
Let’s now review the action potentials of pacemaker cells (conduction cells).
The action potentials of pacemaker cells (i.e. SA and AV nodes) are also called slow-response action potentials.
Reminder: The SA node and AV node are made up of pacemaker cells responsible for cardiac conduction.
Memory Trick
All you need to memorize is the following phrase:
“Climb and Plummet”.
Through this simple catchphrase, you just learned ALL of the action potential phases AND the ion channels involved with pacemaker cells.
Here’s how:
“Climb” = Calcium = Phase 0
Unlike non-pacemaker cells, the pacemaker cells have the ability to spontaneously generate their own action potential.
Pacemaker cells do not require an external stimulus, and their cell membrane at “rest” slowly becomes more positive on its own (discussed below in phase 4).
Once the threshold cell membrane voltage is met, then an action potential is generated.
The action potential will first rise or “climb” as the voltage across the cell membrane becomes more positive.
This is referred to as depolarization and is phase 0 of the action potential (similar to non-pacemaker cells).
What ion is involved in depolarizing pacemaker cells?
Use the catchphrase.
Climb starts with the letter “C”, and this will help you remember when the action potential climbs it is calcium involved.
In order for the cell to become more positive and depolarize, will calcium ions enter or exit the cell?
Calcium ions will need to enter the cell (because calcium is a positive ion).
And this is exactly what occurs during phase 0 of the pacemaker cell action potential.
Calcium channels open, and an influx of calcium ions into the cell leads to depolarization.
Recap: Phase 0 = “Climb” phase of action potential = Calcium ion influx
We can see how phase 0 of pacemaker cells differs from non-pacemaker cells.
Non-pacemaker cell depolarization is a result of sodium ions entering the cell (“summit”), whereas pacemaker cell depolarization is the result of calcium ions entering the cell (“climb”).
Cardiac Action Potentials: Phase 0 of pacemaker cells involves an influx of calcium ions into the cell, leading to depolarization (climb = calcium).
“Plummet” = Potassium = Phase 3
Pacemaker cells do not have a phase 1 or phase 2 in their action potentials because they do not need to contract.
Instead, they are conduction cells, and simply need to depolarize and repolarize over and over to generate and conduct electrical impulses.
As the cell becomes more positive from the influx of calcium ions in phase 0, the calcium channels begin to close, thereby reducing the influx of calcium.
The action potential will then fall or “plummet” as the voltage across the cell membrane becomes more negative.
This causes the cell to repolarize and is phase 3 of the action potential.
We know from non-pacemaker cells that a plummet/repolarization phase is due to the efflux of potassium ions out of the cell.
This is the case for the plummet phase of pacemaker cells as well.
Calcium channels are closed and potassium channels are open, leading to an efflux of potassium ions and repolarization.
Recap: Phase 3 = “Plummet” phase of action potential = Potassium ion efflux
Cardiac Action Potentials: Phase 3 of pacemaker cells involves an efflux of potassium ions, leading to repolarization (plummet = potassium).
“Resting” Phase = Phase 4
Unlike non-pacemaker cells, the pacemaker cells do not have a true “resting phase”.
During phase 4, the pacemaker cells continue to become more positive from a baseline influx of positive ions.
Once a threshold voltage (-40 mV) is met, an action potential is generated.
The slow influx of positive ions during phase 4 is why pacemaker cells are capable of generating their own action potentials (called automaticity).
Cardiac Action Potentials: Phase 4 of pacemaker cells involves a slow influx of positive ions, causing the cell to become more positive until the threshold is met (not a true “resting” phase)
Practical Applications
Understanding cardiac action potentials is important for several reasons:
Antiarrhythmics
Medications can target the different phases of pacemaker and non-pacemaker cell action potentials.
Antiarrhythmics are an example.
There are 4 different classes of antiarrhythmics including:
Sodium channel blockers (Class I)
Beta blockers (Class II)
Potassium channel blockers (Class III)
Calcium channel blockers (Class IV)
Each class of medication functions by blocking different phases of the pacemaker and non-pacemaker cell action potentials.
Pathology
Problems with the conduction system of the heart can lead to heart blocks or dysrhythmias.
Conduction disorders can affect the SA node (sick sinus syndrome), the AV node (1st, 2nd, and 3rd degree AV blocks), and the bundle branches (right or left bundle branch blocks).
There are also disorders of the ion channels that can lead to long QT syndrome and Brugada syndrome.
Regulation
The autonomic nervous system influences cardiac action potentials.
Increased sympathetic activity increases heart rate and cardiac contraction by increasing the slope of phase 4 in pacemaker cells and augmenting phase 2 in non-pacemaker cells, respectively.
Parasympathetic activity normalizes heart rate by slowing the rate of depolarization.
Summary
Non-Pacemaker Cells:
“Summit, Plummet, Continue, Plummet”
Phase 0 = Summit = Sodium in — depolarization
Phase 1 = Plummet = Potassium out — slight repolarization
Phase 2 = Continue = Calcium in — contraction
Phase 3 = Plummet = Potassium out — repolarization
Phase 4 = Resting phase
Pacemaker Cells:
“Climb and Plummet”
Phase 0 = Climb = Calcium in — depolarization
Phase 3 = Plummet = Potassium out — repolarizatoin
Phase 4 = “Resting” phase
Reminder: Pacemaker cells do not have a phase 1 or 2 because they do not contract.
Last updated 11/26/2025
https://www.nhlbi.nih.gov/health-topics/conduction-disorders
http://www.pathophys.org/physiology-of-cardiac-conduction-and-contractility/
https://www.cvphysiology.com/Arrhythmias/A010