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Conducting System of the Heart

Pearson
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>> Welcome to another Anatomy and Physiology SmartArt Video, where we guide you through an important piece of art. After watching this video, you should be able to describe the conducting system of the heart and the electrical events shown on electrocardiogram. Let's begin by looking at this image showing the heart and its conducting system, a network of pacemaker cells and conducting cells. And electrocardiogram, or ECG, also known as an EKG, measures the electrical activity of the conducting system. Let's move to step one. Each heartbeat begins with an action potential generated by the sinoatrial or SA node. The SA node is a cluster of pacemaker cells embedded in the posterior wall of the right atrium. In step two, the action potential travels along the conducting cells of the intermodal pathways and spreads across both atria. We see this depolarization of the atria as the P wave on an ECG. In step three, the action potential continues to the pacemaker cells of the atrioventricular or AV node located at the junction between the atria and the ventricles, and can be seen as the P-R segment. At this time, atrial contraction actually occurs. Now let's move to step four. As atrial contraction completes, the impulse moves along the intraventricular septum within the conducting cells of the AV bundle, the right and left bundle branches to the Purkinje fibers, as well as the moderator band and the papillary muscles of the right ventricle. In step five, the Purkinje fibers distribute the impulse throughout the ventricular myocardium, causing the ventricles to contract. We can see this ventricular depolarization on ECG as the QRS complex. Ventricular contraction, however, begins shortly after the peak of the R wave. The atria are re-polarizing at this time, but the repolarization is masked by the QRS complex. To summarize, the heart's conducting system consists of the pacemaker cells of the SA and AV nodes and conducting cells of the internodal pathways, AV bundle, bundle branches, and Purkinje fibers. The electrical events of the conducting system can be seen on an electrocardiogram with the P wave representing atrial depolarization and the QRS complex corresponding to ventricular depolarization. Remember that the atria actually contract during the PR segment. The ventricles contract shortly after the R wave peak, and the QRS complex masks atrial repolarization. Notice in this ECG tracing, the T wave, which represents ventricular repolarization. So what? Why is it important to understand the heart's conducting system and the electrocardiogram? Well, this knowledge is crucial to detecting and diagnosing cardiac arrhythmias, abnormal patterns of the heart's electrical activity. Cardiac arrhythmias can be caused by drugs and hormones, a heart attack, or for unknown reasons and can range from temporary and relatively mild conditions such as premature contractions of the atria or ventricles, to life-threatening, as in ventricular fibrillation when the ventricles quiver and are unable to eject blood. Many other cardiac arrhythmias are possible and treatments range from medications to the surgical implantation of an artificial pacemaker to regulate heart rate.
>> Welcome to another Anatomy and Physiology SmartArt Video, where we guide you through an important piece of art. After watching this video, you should be able to describe the conducting system of the heart and the electrical events shown on electrocardiogram. Let's begin by looking at this image showing the heart and its conducting system, a network of pacemaker cells and conducting cells. And electrocardiogram, or ECG, also known as an EKG, measures the electrical activity of the conducting system. Let's move to step one. Each heartbeat begins with an action potential generated by the sinoatrial or SA node. The SA node is a cluster of pacemaker cells embedded in the posterior wall of the right atrium. In step two, the action potential travels along the conducting cells of the intermodal pathways and spreads across both atria. We see this depolarization of the atria as the P wave on an ECG. In step three, the action potential continues to the pacemaker cells of the atrioventricular or AV node located at the junction between the atria and the ventricles, and can be seen as the P-R segment. At this time, atrial contraction actually occurs. Now let's move to step four. As atrial contraction completes, the impulse moves along the intraventricular septum within the conducting cells of the AV bundle, the right and left bundle branches to the Purkinje fibers, as well as the moderator band and the papillary muscles of the right ventricle. In step five, the Purkinje fibers distribute the impulse throughout the ventricular myocardium, causing the ventricles to contract. We can see this ventricular depolarization on ECG as the QRS complex. Ventricular contraction, however, begins shortly after the peak of the R wave. The atria are re-polarizing at this time, but the repolarization is masked by the QRS complex. To summarize, the heart's conducting system consists of the pacemaker cells of the SA and AV nodes and conducting cells of the internodal pathways, AV bundle, bundle branches, and Purkinje fibers. The electrical events of the conducting system can be seen on an electrocardiogram with the P wave representing atrial depolarization and the QRS complex corresponding to ventricular depolarization. Remember that the atria actually contract during the PR segment. The ventricles contract shortly after the R wave peak, and the QRS complex masks atrial repolarization. Notice in this ECG tracing, the T wave, which represents ventricular repolarization. So what? Why is it important to understand the heart's conducting system and the electrocardiogram? Well, this knowledge is crucial to detecting and diagnosing cardiac arrhythmias, abnormal patterns of the heart's electrical activity. Cardiac arrhythmias can be caused by drugs and hormones, a heart attack, or for unknown reasons and can range from temporary and relatively mild conditions such as premature contractions of the atria or ventricles, to life-threatening, as in ventricular fibrillation when the ventricles quiver and are unable to eject blood. Many other cardiac arrhythmias are possible and treatments range from medications to the surgical implantation of an artificial pacemaker to regulate heart rate.