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Learning Objectives

Learning Objectives

By the end of the chapter the student will be able to:

  • Draw a typical action potential in a ventricular muscle and a pacemaker cell.

  • Describe the ionic basis of the phases of the action potential.

  • Explain the significance of the long duration of the cardiac action potential and the resultant long refractory period.

  • Describe the normal sequence of cardiac activation and conduction and predict the consequence of its abnormalities.

  • Discuss the significance of “overdrive suppression” in a natural pacemaker and the abnormalities leading to an “ectopic pacemaker.”

  • Explain the effects of the sympathetic and parasympathetic nervous system on the heart rate, cardiac conduction, and contractility.

  • Describe the main components of a normal ECG recording, their significance, and abnormalities.

Introduction

The contraction and relaxation of the cardiac muscle follows a specific synchronized pattern between the atria and the ventricles. This rhythmic contraction and relaxation is preceded by electrical activity called the action potential that is represented in the depolarization and repolarization of the cardiac muscle, respectively. The special origin and sequence of the initiation and propagation of the action potential is essential for maintaining normal heart function. The action potential is created by ions fluxes across the plasma membrane of the cardiac muscle cells via specific channels, transporters, and other proteins. Normally, the action potential originates in the sinoatrial (SA) node known as “the pacemaker of the heart,” which propagates a specific sequence in the atria first and then in the ventricle through specialized conducting tissues as shown in Fig. 4.1. Although the SA node activity sets up the heart rate, the autonomic nervous system modulates this heart rate as well as the electrical conduction and contraction of the heart. Electrical activities of the heart can be precisely measured and monitored via electrocardiography (ECG). Any disturbances of the heart electricity result in various types of arrhythmias as discussed in Chapter 9, which can lead to serious consequences including death. This chapter will briefly explain the major components of the electrical activity of the heart and how it is regulated and measured.

Figure 4.1

Sectional view of the heart. The basic anatomy of the heart is indicated, including chambers, major blood vessels, and conducting system (yellow), including the sinoatrial node, atrioventricular node, and Purkinje fibers. (Reproduced, with permission, from Barrett KE, Barman SM, Boitano S, Brooks HL. Ganong’s Review of Medical Physiology. 24th ed. New York: McGraw-Hill; 2012.)

Origin and Pathway of the Action Potential of the Heart

There are 3 types of cardiac muscles according to their excitability: (1) the pacemaker cells, (2) the conducting tissues (bundle of His, Purkinje fibers), and (3) the ventricular and atrial muscle fibers. Each of these types has its unique action potential as will be discussed. In general, the action potential originates ...

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