PSIO 202, Lecture 6 - PSIO 202 PSIO Human Anatomy and...

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Unformatted text preview: PSIO 202 PSIO Human Anatomy and Physiology Lecture 6 The Heart; Conduction System and EKG Reading: Tortora, pages 730-738 Objectives Objectives and Reading Assignment Describe each of the components of the cardiac conduction system Describe the sequence of excitation of the cardiac conduction system Discuss the features of the cardiac muscle action potential, including the ionic basis of the potential changes Compare and contrast the cardiac muscle and SA node potentials Explain how the cardiac muscle action potential is generated Identify the components of a normal EKG, and discuss the physiologic relevance of each component Main Components of the Cardiac Muscle Conduction System SA node, or pacemaker AV node AV bundle, or bundle of His right and left bundle branches conducting myofibers (Purkinge fibers) Conduction System of Heart The Pacemaker the pacemaker, or SA node, is a mass of cells in the right atrial wall pacemaker cells spontaneously discharge action potentials at a rate of 100-120 / minute or more in the intact animal, autonomic nerves modify the rate of discharge, so that the resting “heart rate” is about 70 b/min Histology of Cardiac Muscle cardiac muscle consists of branched, striated fibers with one or two centrally located nuclei actin and myosin are packaged in myofibrils, as they are in skeletal muscle particularly important for the current discussion is the branching pattern, which forms a network that can facilitate the transmission of electrical impulses in all directions. Histology of Cardiac Muscle, Con’t. “intercalated discs” are irregular, transverse thickenings of plasma membrane that lie between the muscle cells they contain desmosomes and gap junctions desmosomes hold adjacent cells together gap junctions are small channels, which allow electrical impulses to pass quickly from one cell to the next they allow action potentials to spread rapidly throughout heart muscle Sequence of Excitation of Heart Muscle the atria and ventricles must contract in a coordinated fashion the sequence of cardiac muscle excitation is such that the first event is depolarization of the SA node the impulses then travel down and across both atria, causing atrial muscle fiber contraction Sequence of excitation of heart muscle, Con’t. at the AV border there is band of poorlyconducting tissue (small fibers, few gap junctions) the impulse must pass through the AV node and through this tissue mass in order to reach the ventricles the presence of the poorly-conducting tissue slows the impulse by about 0.1 sec this gives the atria time to fully empty before the ventricles begin to contract. Sequence of excitation of heart muscle, Con’t. The right and left AV bundles are connected by the short Bundle of His This bundle transmits the action potential into the muscle of both ventricles the sequence of excitation is such that the lower portions of the ventricles contract first, pushing the blood upwards Cardiac Muscle Action Potential an action potential is the result of depolarization of a muscle or nerve cell the action potential propagates along nerves and into the muscle fibers, causing muscle contraction the cardiac muscle action potential has three distinct phases; rapid depolarization; the plateau; and repolarization Cardiac Muscle Action Potential The Refractory Period refractory means “unresponsive”, or stubborn in physiology, it refers to the period of time when the muscle or nerve cell is unresponsive to stimulation the absolute refractory period refers to the time when the cell will not respond regardless of the strength of stimulus the relative refractory period refers to the time when the cell will respond only if the stimulus is “suprathreshold” The absolute refractory period in skeletal and heart muscle lasts about as long as the action potential (~ 250 ms in heart muscle) Action Potential and Ion Permeabilities Heart Physiology: Intrinsic Conduction System Autorhythmic cells: Initiate action potentials Have unstable resting potentials called pacemaker potentials Use calcium influx (rather than sodium) for rising phase of the action potential Heart Physiology: Intrinsic Conduction System EKG EKG Action potentials of all active cells can be detected and recorded P wave atrial depolarization P to Q interval conduction time from atrial to ventricular excitation QRS complex ventricular depolarization T wave ventricular repolarization ...
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This note was uploaded on 01/30/2010 for the course PSIO 202 taught by Professor Staff during the Spring '08 term at University of Arizona- Tucson.

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