Cardiovascular system- L2 Faisal I. Mohammed, MD, PhD University of Jordan

Autorhythmic Fibers Specialized cardiac muscle fibers Self-excitable Repeatedly generate action potentials that trigger heart contractions 2 important functions Act as pacemaker Form conduction system University of Jordan

Conduction system Begins in sinoatrial (SA) node in right atrial wall Propagates through atria via gap junctions Atria contact Reaches atrioventricular (AV) node in interatrial septum Enters atrioventricular (AV) bundle (Bundle of His) Only site where action potentials can conduct from atria to ventricles due to fibrous skeleton Enters right and left bundle branches which extends through interventricular septum toward apex Finally, large diameter Purkinje fibers conduct action potential to remainder of ventricular myocardium Ventricles contract University of Jordan

Anterior view of frontal section SINOATRIAL (SA) NODE Frontal plane Left atrium Left ventricle Anterior view of frontal section SINOATRIAL (SA) NODE ATRIOVENTRICULAR (AV) NODE ATRIOVENTRICULAR (AV) BUNDLE (BUNDLE OF HIS) RIGHT AND LEFT BUNDLE BRANCHES 1 2 3 4 Right atrium Right ventricle Frontal plane SINOATRIAL (SA) NODE ATRIOVENTRICULAR (AV) NODE Left atrium Left ventricle Anterior view of frontal section ATRIOVENTRICULAR (AV) BUNDLE (BUNDLE OF HIS) RIGHT AND LEFT BUNDLE BRANCHES PURKINJE FIBERS 1 2 3 4 5 Right atrium Right ventricle Frontal plane Left atrium Left ventricle Anterior view of frontal section SINOATRIAL (SA) NODE ATRIOVENTRICULAR (AV) NODE ATRIOVENTRICULAR (AV) BUNDLE (BUNDLE OF HIS) 1 2 3 Right atrium Right ventricle Frontal plane Left atrium Left ventricle Anterior view of frontal section SINOATRIAL (SA) NODE ATRIOVENTRICULAR (AV) NODE 1 2 Right atrium Right ventricle Frontal plane Right atrium Right ventricle Left atrium Left ventricle Anterior view of frontal section Frontal plane Left atrium Left ventricle Anterior view of frontal section SINOATRIAL (SA) NODE 1 Right atrium Right ventricle University of Jordan

Intrinsic Cardiac Conduction System Approximately 1% of cardiac muscle cells are autorhythmic rather than contractile 75/min 40-60/min 30/min

Autorhythmicity During embryonic development, about 1% of all of the muscle cells of the heart form a network or pathway called the cardiac conduction system. This specialized group of myocytes is unusual in that they have the ability to spontaneously depolarize. For the conduction system, the heart acts like a “top” and a “bottom” 6

Autorhythmicity The rhythmical electrical activity they produce is called autorhythmicity. Because heart muscle is autorhythmic, it does not rely on the central nervous system to sustain a lifelong heartbeat. When transplanted hearts are re- warmed following cardiopulmonary bypass, they once again begin to beat without the need to connect outside nerves or use life-long pacemaker devices. 7

Autorhythmicity Autorhythmic cells spontaneously depolarize at a given rate, some groups faster, some groups slower. Once a group of autorhythmic cells reaches threshold and starts an action potential (AP), all of the cells in that area of the heart also depolarize. Membrane of two cells clearly seen. The spread of ions through gap junctions of the Intercalated discs (I) allows the AP to pass from cell to cell 8

Cardiac Conduction The self-excitable myocytes that "act like nerves" have the 2 important roles of forming the conduction system of the heart and acting as pacemakers within that system. Because it has the fastest rate of depolarization, the normal pacemaker of the heart is the sinoatrial (SA) node, located in the right atrial wall just below where the superior vena cava enters the chamber. 9

Cardiac Conduction Spontaneous Depolarization of autorhythmic fibers in the SA node firing about once every 0.8 seconds, or 75 action potentials per minute 10

Fast Response Action Potential of Contractile Cardiac Muscle Cell

Pacemaker and Action Potentials of the Heart

Slow Response Action Potential (Pacemaker Potential)

Cardiac Conduction The action potential generated from the SA node reaches the next pacemaker by propagating throughout the wall of the atria to the AV node in the interatrial septum. At the AV node, the signal is slowed, allowing the atrium a chance to mechanically move blood into the ventricles. 14

Cardiac Conduction From the AV node, the signal passes through the AV bundle to the left and right bundle branches in the interventricular septum towards the apex of the heart. Finally, the Purkinje fibers rapidly conduct the action potential throughout the ventricles (0.2 seconds after atrial contraction). 15

Frontal plane Left atrium Left ventricle Anterior view of frontal section SINOATRIAL (SA) NODE ATRIOVENTRICULAR (AV) NODE ATRIOVENTRICULAR (AV) BUNDLE (BUNDLE OF HIS) RIGHT AND LEFT BUNDLE BRANCHES 1 2 3 4 Right atrium Right ventricle Frontal plane SINOATRIAL (SA) NODE ATRIOVENTRICULAR (AV) NODE Left atrium Left ventricle Anterior view of frontal section ATRIOVENTRICULAR (AV) BUNDLE (BUNDLE OF HIS) RIGHT AND LEFT BUNDLE BRANCHES PURKINJE FIBERS 1 2 3 4 5 Right atrium Right ventricle Frontal plane Left atrium Left ventricle Anterior view of frontal section SINOATRIAL (SA) NODE ATRIOVENTRICULAR (AV) NODE ATRIOVENTRICULAR (AV) BUNDLE (BUNDLE OF HIS) 1 2 3 Right atrium Right ventricle Frontal plane Left atrium Left ventricle Anterior view of frontal section SINOATRIAL (SA) NODE ATRIOVENTRICULAR (AV) NODE 1 2 Right atrium Right ventricle Frontal plane Left atrium Left ventricle Anterior view of frontal section SINOATRIAL (SA) NODE 1 Right atrium Right ventricle Frontal plane Right atrium Right ventricle Left atrium Left ventricle Anterior view of frontal section

Conduction System SA node acts as natural pacemaker Faster than other autorhythmic fibers Initiates 100 times per second Nerve impulses from autonomic nervous system (ANS) and hormones modify timing and strength of each heartbeat Do not establish fundamental rhythm University of Jordan

Action Potentials and Contraction Action potential initiated by SA node spreads out to excite “working” fibers called contractile fibers Depolarization Plateau Repolarization University of Jordan

Autonomic regulation Originates in cardiovascular center of medulla oblongata Increases or decreases frequency of nerve impulses in both sympathetic and parasympathetic branches of ANS Noreprinephrine has 2 separate effects In SA and AV node speeds rate of spontaneous depolarization In contractile fibers enhances Ca2+ entry increasing contractility Parasympathetic nerves release acetylcholine which decreases heart rate by slowing rate of spontaneous depolarization University of Jordan

Thank You 20