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BOOK CHAPTER CARDIAC ELECTROPHYSIOLOGY AND THE ELECTROCARDIOGRAM      W. Jonathan Lederer Medical Physiology, CHAPTER 21, 504­528 Different cardiac cells serve different and very specialized functions, but all are electrically active. The heart's electrical signal normally originates in a group of cells high in the right atrium that depolarize spontaneously; it then spreads throughout the heart from cell to cell ( Fig. 21­1 (f1) ). As this action potential propagates through the heart—somet
  BOOK CHAPTER CARDIAC ELECTROPHYSIOLOGY AND THEELECTROCARDIOGRAM W. Jonathan Lederer  Medical Physiology, CHAPTER 21, 504-528 Different cardiac cells serve different and very specialized functions, but all are electrically active.The heart's electrical signal normally srcinates in a group of cells high in the right atrium thatdepolarize spontaneously; it then spreads throughout the heart from cell to cell ( Fig. 21-1 (f1)  ). Asthis action potential propagates through the heart—sometimes carried by cells that formspecialized conducting pathways and sometimes by the very cells that generate the force of contraction—it assumes different appearances within the different cardiac cells ( Fig. 21-2 (f2)  ). By the speed of the upstroke, we can characterize action potentials as either slow   (SA and AV nodes)or fast  (atrial myocytes, Purkinje fibers, and ventricular myocytes). Figure 21-1Conduction pathways through the heart. A section through the long axis of the heart is shown.  Figure 21-2Cardiac action potentials. The distinctive shapes of action potentials at five sites along the spread of excitationare shown. Because the excitation of cardiac myocytes triggers contraction—a process called excitation-contraction coupling  (see Chapter 9 )—the propagation of action potentials must be carefully timedto synchronize ventricular contraction and thereby optimize the ejection of blood. This chapterfocuses on the membrane currents responsible for the generation and transmission of actionpotentials in heart tissue. We also examine how to record the heart's electrical flow by placementof electrodes on the surface of the body, creating one of the simplest and yet one of the most usefuldiagnostic tools available to the clinician—the electrocardiogram. ELECTROPHYSIOLOGY OF CARDIAC CELLS The cardiac action potential starts in specialized muscle cells of the sinoatrialnode and then propagates in an orderly fashion throughout the heart The cardiac action potential srcinates in a group of cells called the sinoatrial (SA) node ( Fig.21-1 (f1)  ), located in the right atrium. These cells depolarize spontaneously and fire off actionpotentials at a regular, intrinsic rate that is usually between 60 and 100 times per minute for anindividual at rest. Both parasympathetic and sympathetic neural input can modulate this intrinsic pacemaker activity, or automaticity (see Chapter 16 ).  Because cardiac cells are electrically coupled through gap junctions ( Fig. 21-3A (f3)  ), the actionpotential propagates from cell to cell in the same way that an action potential in nerve conductsalong a single, long axon. A spontaneous action potential srcinating in the SA node will conductfrom cell to cell throughout the right atrial muscle and spread to the left atrium. The existence of discrete conducting pathways in the atria is still disputed. About one tenth of a second after itssrcination, the signal arrives at the atrioventricular (AV) node ( Fig. 21-1 (f1)  ). The impulsedoes not spread directly from the atria to the ventricles because of the presence of a fibrous atrioventricular ring . Instead, the only available pathway is for the impulse to travel from the AV node to the His-Purkinje fiber system , a network of specialized conducting cells thatcarries the signal to the muscle of both ventricles.  Figure 21-3Conduction in the heart. A,  An action potential conducting from left to right causes intracellular current to flowfrom fully depolarized cells on the left, through gap junctions, and into cell A. Depolarization of cell A causescurrent to flow from cell A to cell B ( I  AB ). Part of I  AB discharges the capacitance of cell B (depolarizing cellB), and part flows downstream to cell C. B, Subthreshold depolarization of cell A decays with distance. C, Thespeed of conduction increases with greater depolarization of cell A ( blue versus red curves ) or with a morenegative threshold. The cardiac action potential conducts from cell to cell through gap junctions The electrical influence of one cardiac cell on another depends on the voltage difference betweenthe cells and on the resistance of the gap junction connection between them. A gap junction (see Chapter 8 ) is an electrical synapse ( Fig. 21-3A (f3)  ) that permits electrical current to flow  between neighboring cells. According to Ohm's law, the current flowing between cell A and theadjacent cell B (  I  AB ) is proportional to the voltage difference between the two cells (Δ V  AB ) butinversely proportional to the electrical resistance between them (  R  AB ): When  R  AB is very small (i.e., when the cells are tightly coupled), the gap junctions are minimal barriers to the flow of depolarizing current.Imagine that several interconnected cells are initially all at their normal resting potentials ( Fig. 21-3B (f3)  ). An action potential propagating from the left of cell A now injects depolarizing currentinto cell A. As a result, the cell depolarizes to V  A , which is now somewhat positive compared with V B . Thus, a small depolarizing current (i.e., positive charges) will also move from cell A to cell Band depolarize cell B. In turn, current flowing from cell B will then depolarize cell C. By thisprocess, the cells closest to the current source undergo the greatest depolarization.Imagine that the injected current, coming from the active region of the heart to the left,depolarizes cell A just to its threshold ( Fig. 21-3C, red curve (f3)  ) but that cell A has not yet fired anaction potential. At this instant, the current passing from cell A to cell B cannot bring cell B to itsthreshold. Of course, cell A will eventually fire an action potential and, in the process, depolarizeenough to inject enough current into cell B to raise cell B to its threshold. Thus, the actionpotential propagates down the chain of cells, but relatively slowly. On the other hand, if the activeregion to the left injects more current into cell A ( Fig. 21-3C, blue curve (f3)  )—producing a largerdepolarization in cell A—the current passing from cell A to cell B will be greater and sufficient todepolarize cell B beyond its voltage threshold for a regenerative action potential. However, at thisinstant, the current passing from cell B to cell C is still not sufficient to trigger an action potentialin cell C. That will have to wait until the active region moves closer to cell C, but the wait is not aslong as in the first example (red curve). Thus, the action potential propagates more rapidly in thissecond example (blue curve). ==     I     AB    −     V       A     V       B     R       AB     ΔV       AB     R       AB    
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