thaler chapter 1

The Cells of the Heart

13

The Cells of the Heart

From the standpoint of the electrocardiographer, the heart consists of three types of cells: • Pacemaker cells —under normal circumstances, the electrical power source of the heart

• Electrical conducting cells —the hard wiring of the heart • Myocardial cells —the contractile machinery of the heart

Pacemaker

Conduction pathway

Myocardial cells

Pacemaker Cells Pacemaker cells are small cells approximately 5 to 10 μm long, about the same as the width of a single strand of a spider’s web. These cells are able to depolarize spontaneously over and over again. The rate of depolarization is determined by the innate electrical characteristics of the cell and by external neurohormonal input. Each spontaneous depolarization serves as the source of a wave of depolarization that initiates one complete cycle of cardiac contraction and relaxation.