In the denervated state the mammalian heart, both in vivo and in vitro, is excited at very regular intervals, the coefficient of variance of the interbeat intervals not exceeding 2%. The pacemaker that is the source of this regular ex citation is localised normally within the sinus node (" sino-atrial node " node of Keith and Flack), a most intriguing small piece of tissue in the caval corner of the right atrium. A small portion of this node containing a group of probably only a few thousands of cells fires spontaneously, that means without any exter nal influence to trigger their activity. The so called pacemaker cells do this by letting their membrane potential fall to the level where an action potential will start which subsequently activates surrounding cells to fire an action po tential. The first question which is tackled in this book is which processes underly this spontaneous diastolic depolarization. This is discussed in section I, concerning the fundamental properties of pacemaker cells with special refer ence to ionic membrane currents. Although views still quite differ about the exact nature of the membrane processes that cause the automatic pacemaker dis charge there is agreement that diastolic depolarization is brought about by the interaction of a number of ionic current systems, including both inward and out ward going currents.
Table of ContentsSection One: Fundamentals of Pacemaker Activity.- Pacemaker Mechanisms.- On the Ultrastructural Identification of Pacemaker Cell Types within the Sinus Node.- Pacemaker Mechanisms of Rabbit Sinoatrial Node Cells.- The Relative Contributions of Various Time-dependent Membrane Currents to Pacemaker Activity in the Sino-Atrial Node.- The Current IK2 in Purkinje Fibres Reinterpreted and Identified with the Current If in the SA node.- Implications of the Re-interpretation of IK2 for the Modelling of Electrical Activity of Pacemaker Tissues in the Heart.- Pacemaker Mechanisms in Myocardial Cells during Development of Embryonic Chick Hearts.- Section Two: Interaction of Pacemaker Cells.- Cellular Communication: Introductory Remarks.- On the Control of Cell Communication in Heart.- Electrical Cell Coupling in Rabbit Sino Atrial Node and Atrium: Experimental and Theoretical Evaluation.- Gap Junction Structure under Coupled and Uncoupled Conditions.- Formation and Growth of Myocardial Gap Junctions: In Vivo and In Vitro Studies.- Intercellular Coupling between Embryonic Heart Cell Aggregates.- Intercellular Coupling between Cardiac Cells and its Disturbances.- Ectopic Activity in the Early Phase of Regional Myocardial Ischemia.- Section Three: Establishment of Cardiac Rhythm.- In Vitro Models of Entrainment of Cardiac Cells.- Synchronization of Cardiac Pacemaker Cells by Electrical Coupling: a Study with Embryonic Heart Cell Aggregates and Pacemaker Cell Models.- Factors Influencing Regularity and Synchronization of Beating of Tissue Cultured Heart Cells.- Synchronization of Hodgkin-Huxley Type and Related Oscillators.- Dynamic Properties of Electrically Interacting Excitable Cells.- Fibrillation as a Consequence of Pacemaker Phase-resetting.- Section Four: Neural Control of Rate and Rhythm.- On the Intrinsic Cardiac Rhythm.- Mode of Action of Acetylcholine on the Rabbit SA Node Cell.- Interaction of Adrenaline and Acetylcholine on Sinus Node Function.- Conduction within the Sinus Node and its Modification by Autonomic Drugs.- The Effects of Brief Vagal and Sympathetic Stimulation on Rate and Rhythm Changes in the Sinus Node.- The Phase-dependent Effects of Repetitive Bursts of Vagal Activity on Heart Rate.- Entrainment of the SA Nodal Pacemaker by Brief Vagal Bursts in Relation to AV Conduction.- Properties of the Fast Sodium Channel and of the Muscarinic Receptor during Development of Embryonic Heart Cells In Ovo and In Vitro.- Initiation of Transmitter Secretion by Adrenergic Neurons and its Relation to Morphological and Functional Innervation of the Embryonic Chick Heart.- References.