CHAPTER 12 



Excitation of the heart 



ALLEN M. SCHER 



Department of Physiology and Biophysics, University of Washington 

 School of Medicine, Seattle, Washington 



CHAPTER CONTENTS 



FUNCTIONAL ANATOMY 



Functional Anatomy 



Specialized Tissue 



Myocai'dial Cells 



Pacemaker Cells 



The Atria : Atrial Conduction System 



Atrioventricular Conduction System 



A-V Conduction System in Lower Forms 



Purkinje Fibers of the Ventricle 



Accessory Pathway for A-V Conduction 



Other Anatomical Details 

 Excitation of the Heart 



The Cardiac Pacemaker, Excitation of Atrium 



Atrial Repolarization 



Atrioventricular Conduction 



Potentials from the A-V Region in the Frog Heart 



Summary 



VVolff-Parkinson-White Syndrome: Alternate A-V Conduc- 

 tion Pathways 



Dual A-V Clonduction System 



Conduction in the Common Bundle and Its Branches 



Ventricular Activation 



Excitation of the Ventricular Surface 



Excitation of the Ventricular Walls 



Activation of the Interventricular Septum 



Details of Ventricular Excitation in Two and Three Di- 

 mensions 



Map of Ventricular Activation 



Conduction During Ventricular Extrasystoles 



Ventricular Activation and the QRS Complex 



Ventricular Repolarization 



Ventricular Activation in the Ungulate Heart 



Three-Dimensional Activation in the Goat 



Derivation of the Ventricular Electrocardiogram in the Un- 

 gulate 

 Abnormal Excitation 



Bundle Branch Block 



Flutter and Fibrillation 



Myocardial Injury; Ischemia and Infarction 



THE WAY in which the electrical impulse spreads 

 through a particular heart is determined by the gross 

 geometry of that heart, and by the position within it 

 of the specialized conduction tissue that both generates 

 and "distributes" the impulse. It is an anatomical 

 rather than a physiological problem to discuss in 

 detail the geometry of a great many hearts and we 

 will presume a rather standard cardiac conformation 

 (figs. I, q). The distribution of the specialized tissue 

 in a variety of invertebrate and vertebrate hearts has 

 been described in a number of excellent papers (i, 2, 

 27, 28, 35-37, 63-65, 78, 81, 83, 86-88, 93, 96, 97, 127, 

 129. 135-137. '39. 143. '44)- 



Specialized Tissue 



The specialized tissue comprises two or three cell 

 types with a fairly constant distribution through the 

 hearts of various warm-blooded species. There are 

 similar cells in lower forms. It includes the pacemaker 

 cells of the sinus node, which initiate the beat of the 

 mammalian heart, and similar cells that lie in the 

 sinus venosus in lower forms. These pacemaker cells 

 anatomically resemble the A-V nodal cells (see below), 

 which are generally considered the "next" group of 

 specialized cells and which are first in the train of 

 cells transmitting the impulse from atrium to ventricle. 

 The A-V nodal cells are connected to the cells of the 

 common bundle and the right and left conducting 

 bundles. The right and left conducting bundles also 

 give rise to a large number of cells lining the endo- 

 cardium and distributing the impulse within the 



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