What structure passes the action potential from the the atria to the ventricles?
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In the simplest of terms, the heart is a pump made up of muscle tissue. The heart's pumping action is controlled by an electrical conduction system that coordinates the contraction of the heart chambers. Show
 Click Image to Enlarge How does the heart beat?An electrical stimulus is generated in a special part of the heart muscle called the sinus node. It's also called the sinoatrial node (SA node). The sinus node is a small mass of special tissue in the right upper chamber of the heart (right atrium). In an adult, the sinus node sends out a regular electrical pulse 60 to 100 times per minute. This electrical pulse travels down through the conduction pathways and causes the heart's lower chambers (ventricles) to contract and pump out blood. The right and left atria are stimulated first and contract to push blood from the atria into the ventricles. The ventricles then contract to push blood out into the blood vessels of the body. The original electrical impulse travels from the sinus node across the cells of your heart's right and left atria. The signal travels to the AV node (atrioventricular node). This node is located between the atria and the ventricles. In the AV node, the impulses are slowed down for a very short period. This allows the atria to contract a fraction of a second before the ventricles. The blood from the atria empties into the ventricles before the ventricles contract. After passing through the AV node, the electrical current then continues down the conduction pathway, through a pathway called the bundle of His, and into the ventricles. The bundle of His divides into right and left pathways (bundle branches) to give electrical stimulation to the right and left ventricles. Normally at rest, the heart contracts about 60 to 100 times a minute depending on your age. In general, your heart rate slows as you age. What can go wrong with the heart's electrical system?Under some abnormal conditions, certain heart tissue is capable of starting a heartbeat, or becoming the "pacemaker," just like the sinus node. An abnormal heartbeat (arrhythmia) may occur when:
Symptoms of an arrhythmia can include a feeling that your heart is fluttering (heart palpitations), shortness of breath, dizziness, or fainting. Your doctor may do an ECG (electrocardiogram) to assess the rhythm of the heart. This painless test involves recording the electrical activity of your heart with several small stickers attached to your chest. If the electrical rhythm is abnormal, you may need medicine or a procedure. Action potentials (electrical impulses) in the heart originate in specialized cardiac muscle cells, called autorhythmic cells. These cells are self‐excitable, able to generate an action potential without external stimulation by nerve cells. The autorhythmic cells serve as a pacemaker to initiate the cardiac cycle (pumping cycle of the heart) and provide a conduction system to coordinate the contraction of muscle cells throughout the heart. The autorhythmic cells are concentrated in the following areas:
A membership gives you exclusive, unlimited access to a membership page of ALL the study guides, flashcards, and PDF lectures! Sign Up Sign Up Download the Study Guide!Ace the conduction system of the heart by getting this amazing study guide! Enjoy :) sale Quick View Conduction System of the Heart | Study Guide | 10 Pages Sale Price:$3.99 Original Price:$5.99 Add To Cart Conduction System of the HeartThe cardiac conduction system is the electrical pathway of the heart that leads to atrial and ventricular contraction. The conduction system consists of pacemaker cells that generate spontaneous action potentials, and then deliver those impulses throughout the heart. The cardiac conduction system comprises the following structures in order: SA node, internodal pathway and Bachmann’s bundle, AV node, bundle of His, bundle branches, and Purkinje fibers. This lecture will walk you through the conduction pathway step-by-step using a labeled diagram of the heart. Once you understand the conduction system of the heart, you will be able to apply it to conduction system diseases, disorders, and abnormalities (discussed in other EZmed posts). You will be able to apply the conduction system to the different parts of an electrocardiogram (EKG/ECG) waveform as well. As with every EZmed lecture, the material will be presented simply and concisely. We will outline the conduction system sequence using labeled ppt images, as well as provide a summary video above. Let’s get started! Anatomy of the HeartBefore we discuss the cardiac conduction system, let’s briefly review the gross anatomy of the heart as the diagram below will be used throughout this lecture. For a great step-by-step guide filled with memory tricks to remember the anatomy of the heart, check out the EZmed lecture below! Heart Anatomy: Labeled Diagram, Structures, Function, and Blood Flow Cardiac ChambersThe heart has 4 chambers: the right atrium, right ventricle, left atrium, and left ventricle. The atria are positioned at the superior/upper portion of the heart, and the ventricles are located at the inferior/lower portion of the heart. Great VesselsThe main pulmonary artery, also known as the pulmonary trunk, emerges from the right ventricle and delivers deoxygenated blood to the pulmonary circulation and lungs. The aorta emerges from the left ventricle and delivers oxygenated blood to the rest of the body. The superior vena cava and inferior vena cava are the main veins that deliver deoxygenated venous blood from the rest of the body back to the heart, specifically the right atrium. The pulmonary veins are the main veins that deliver oxygenated blood from the lungs back to the heart, specifically the left atrium. ValvesThere are 4 valves in the heart: the tricuspid valve, mitral valve, pulmonic valve, and aortic valve. The tricuspid and mitral valves are positioned between the atria and ventricles. Specifically, the tricuspid valve is located between the right atrium and right ventricle, and the mitral valve is positioned between the left atrium and left ventricle. The pulmonic and aortic valves are located between the ventricles and great vessels. Specifically the pulmonic valve is positioned between the right ventricle and pulmonary trunk, and the aortic valve is located between the left ventricle and aorta. View fullsize RA = Right Atrium; RV = Right Ventricle; LA = Left Atrium; LV = Left Ventricle; TV = Tricuspid Valve; MV = Mitral Valve; PV = Pulmonic Valve; AV = Aortic Valve; SVC = Superior Vena Cava; IVC = Inferior Vena Cava; PA = Pulmonary Artery (main) 2 Types of Cardiac Muscle CellsThere are 2 main types of cardiac myocytes (muscle cells) in the myocardium:
Pacemaker CellsThe heart has the innate ability to generate its own spontaneous action potentials without any external stimuli, a phenomenon known as automaticity. It does this using pacemaker cells, which are specialized cardiac myocytes (muscle cells) within the myocardium that have the ability to generate spontaneous action potentials. The pacemaker cells are located in structures that make up the electrical pathway of the heart, known as the conduction system, and they generate and transmit electrical impulses throughout the myocardium. As the action potential travels through the conduction system and myocardium, it will lead to atrial and ventricular depolarization and contraction. The rate at which the pacemaker cells fire is the heart rate. The pacemaker cells do not have a true “resting phase” in their action potential cycle. Once a pacemaker cell repolarizes, the voltage across the cell membrane slowly becomes more positive until the action potential threshold is met and rapid depolarization occurs again. For more information about pacemaker cell action potentials, make sure to check out the EZmed lecture that makes cardiac action potentials easy! The pacemaker cells are located within the SA node, AV node, bundle of His, right and left bundle branches, and Purkinje fibers. These structures make up the conduction system of the heart, which will be the focus of this post. Contractile CellsThe contractile cells are the second type of cardiac myocytes found within the myocardium. The contractile cells make up the bulk of the myocardium (99%), and they are the cardiac myocytes (muscle cells) responsible for contraction of the heart. They mainly rely on the above conduction system to become depolarized, which will lead to cardiac contraction and movement of blood forward. For more information about the contractile cell action potentials, make sure to check out the cardiac action potential EZmed blog! View fullsize The myocardium has 2 main cell types: Pacemaker Cells and Contractile Cells The pacemaker cells have the capability of generating spontaneous action potentials. They are located in the SA node, AV node, bundle of His, right and left bundle branches, and the Purkinje fibers. They make up the conduction system of the heart. The contractile cells are the muscle cells that lead to contraction of the heart once depolarized. SA NodeAs mentioned above, the heart has the ability to generate its own spontaneous action potentials, a phenomenon known as automaticity. In a normal functioning heart, the SA node is the primary pacemaker that produces spontaneous action potentials that will determine the heart rate. The SA node is composed of many pacemaker cells, and it is located at the back of the right atrium near the superior vena cava entry. The conduction system of the heart can be influenced by the sympathetic nervous system to speed up the heart rate by activating cardiac beta receptors. Alternatively, the parasympathetic nervous system can facilitate slowing the heart rate down. While the autonomic nervous system can influence the heart rate extrinsically, the SA node can produce spontaneous action potentials at a rate of 60-100 beats per minute intrinsically without any external stimuli. This is known as the normal sinus rhythm. Once an action potential is generated by the SA node, it will travel through the right atrium via the internodal pathways. There are 3 internodal tracts: Anterior, Middle, and Posterior The action potential will also travel from the right atrium to the left atrium via Bachmann’s bundle, a branch of the anterior internodal tract. As the action potential travels through the atria, the atria depolarize and contract to further push blood into the ventricles during diastole. Atrial depolarization is represented by the P wave on EKG. For more information about how the conduction system can be applied to the different parts of an EKG, make sure to check out the EZmed blog that makes EKGs easy! View fullsize In a normal functioning heart, the SA node is the pacemaker that sets the heart rate and is the starting point of the conduction system. The pacemaker cells within the SA node generate action potentials at 60-100 beats per minute. The action potential travels from the SA node through the right atrium via the internodal pathway, and to the left atrium via Bachmann’s bundle. As the action potential travels through the atria, the atria depolarize and contract. AV NodeAfter the action potential travels through the atria, it will converge onto another node called the AV node. The AV node is located at the base of the right atrium near the interventricular septum. It is the “gatekeeper” that sends the action potential from the atria to the ventricles. Similar to the SA node, the AV node consists of many pacemaker cells that have the ability to generate their own spontaneous action potentials as well. The key difference, however, is that the pacemaker cells within the AV node generate their action potentials at a slower rate than the SA node. The rate at which the AV node produces spontaneous action potentials is approximately 40-60 beats per minute. Since the SA node produces action potentials at much faster rate than the AV node, the SA node depolarizes the pacemaker cells within the AV node before they have time to spontaneously depolarize. For this reason, the SA node is the primary pacemaker. If the SA node were eliminated or stopped functioning properly, then it would be up to the AV node to spontaneously depolarize the heart. As a result, the heart rate would be approximately 40-60 beats per minute rather than the 60-100 beats per minute produced by the SA node. The other important function of the AV node is that it slows down the conduction velocity of the action potential. This is a critical function of the AV node because by slowing down the conduction velocity of the action potential, it gives time for the atria to contract before depolarizing and contracting the ventricles. If there were no delay in conduction through the AV node, then the atria and ventricles would contract at the same time making it difficult for blood to flow properly. We want the atria to contract first to push the blood into the ventricles, then the ventricles can contract to pump blood to the pulmonary and systemic circulation. Therefore, the AV node is the transition from the end of diastole to the start of systole in the cardiac cycle. The time between atrial depolarization (P wave) and ventricular depolarization (QRS complex) is represented by the PR segment on an EKG. The PR segment mainly reflects the slow impulse conduction through the AV node. View fullsize The AV node is the “gatekeeper” that sends the action potential from the atria to the ventricles. The pacemaker cells within the AV node generate action potentials at 40-60 beats per minute, and are therefore masked by the SA node (60-100 beats per minute). The AV node slows down the conduction velocity of the action potential to allow time for the atria to contract before depolarizing the ventricles. Bundle of HisAfter the action potential travels through the AV node it will enter the bundle of His, also known as the atrioventricular bundle. The bundle of His is located in the interventricular septum. It also comprises pacemaker cells, and they can generate their own action potentials spontaneously at a rate of 40-60 beats per minute. View fullsize The action potential exits the AV node and enters the bundle of His. The bundle of His has pacemaker cells that can generate action potentials at 40-60 beats per minute. Right and Left Bundle BranchesThe action potential then travels from the bundle of His to the right and left bundle branches, also known as the atrioventricular bundle branches. The right bundle branch mainly supplies the right ventricle, and the left bundle branch mainly supplies the left ventricle. The bundle branches consist of pacemaker cells that can generate spontaneous action potentials at a rate of 20-40 beats per minute. Again, this slow action potential rate is masked by the SA node and/or the AV node (if the SA node were not functioning properly.) View fullsize From the bundle of His, the action potential travels through the right and left bundle branches. The right bundle branch depolarizes the right ventricle, and the left bundle branch depolarizes the left ventricle. The pacemaker cells within the bundle branches generate action potentials at 20-40 beats per minute. Purkinje FibersLastly, the action potential travels from the right and left bundle branches to the Purkinje fibers. The Purkinje fibers conduct the impulse throughout the right and left ventricles. As the action potential travels through the bundle of His, the bundle branches, and the Purkinje fibers, the ventricular contractile myocytes depolarize and contract. The heart is now in systole. Ventricular depolarization is represented by the QRS complex on EKG. The pacemaker cells within the Purkinje fibers have the ability to generate spontaneous action potentials at a rate of 20-40 beats per minute. View fullsize The action potential travels from the bundle branches to the Purkinje fibers, which are the terminal branches throughout the inner right and left ventricular wall. As the action potential disperses through the ventricles, the ventricular myocytes depolarize and contract. The pacemaker cells within the Purkinje fibers produce action potentials at 20-40 beats per minute. Practical ApplicationAbnormalities within the conduction system can lead to diseases such as heart blocks, sick sinus syndrome, arrhythmias, etc which will be discussed in other EZmed posts. Depending on the conduction abnormality, antiarrhythmics may be required. Antiarrhythmics include sodium channel blockers, beta blockers, potassium channel blockers, and calcium channel blockers. ConclusionHopefully this provided you with a clear understanding of the conduction system of the heart. The SA node is the primary pacemaker, spontaneously depolarizing at a rate of 60-100 beats per minute. The action potential generated by the SA node then travels through the right atrium via the internodal pathway, and to the left atrium via Bachmann’s bundle. As the action potential travels through the atria, the atrial contractile myocytes depolarize and contract. The action potential converges onto the AV node, located at the base of the right atrium at the interventricular septum. The AV node is the gatekeeper that sends the action potential from the atria to the ventricles. The AV node also slows down the conduction velocity to allow time for the atria to contract before depolarizing the ventricles. The action potential then exits the AV node and enters the bundle of His, followed by the right and left bundle branches, and lastly through the Purkinje fibers. As the action potential travels through this portion of the conduction system, the ventricles depolarize and contract. Before You Go, Make Your Learning Experience Even Easier! If you enjoyed the content in this post, don’t forget to join the EZmed community for free so you don’t miss out on future posts that make medicine easy! A weekly notification is sent right to your inbox filled with new blog posts, new videos, and exam prep! Sign up is on the bottom of this page or in the navigation bar. Perform well in class, ace your exams, and keep up with your medical knowledge throughout your career using: Instagram: @ezmedlearning - High yield exam content YouTube Channel: EZmed - Simple animations and videos Pinterest: ezmedlearning - Easy illustrations and flashcards Feel free to use the contact button to reach out with any feedback or suggestions you may have for future topics! Thank you for using EZmed! How does action potential spread from atria to ventricles?The electrical signal travels from the sinoatrial node (SAN), which stimulates the atria to contract, to the atrioventricular node (AVN) which slows down conduction of the action potential, from the atria to the ventricles. This delay allows the ventricles to fully fill with blood before contraction.
Which group conducts action potential from atria to the ventricles?The atrioventricular (AV) bundle (bundle of His) receives the action potential from the AV node and transmits the impulse to the ventricles by way of the right and left bundle branches. Except for the AV bundle, which provides the only electrical connection, the atria are electrically insulated from the ventricles.
What structure is the only place where an action potential can cross to ventricles?Normally, the only pathway available for action potentials to enter the ventricles is through a specialized region of cells (atrioventricular node, or AV node) located in the inferior-posterior region of the interatrial septum.
What is the pathway of the action potential through the heart?Sinoatrial (SA) node normally generates the action potential, i.e. the electrical impulse that initiates contraction. The SA node excites the right atrium (RA), travels through Bachmann's bundle to excite left atrium (LA). The impulse travels through internodal pathways in RA to the atrioventricular (AV) node.
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