Human Anatomy Physiology II PSIO202 The Heart Conduction ECGs Pacemaker Cardiac APs Learning Objectives Exam Emphases 1 Describe each of the components of the cardiac conduction system 2 Describe the sequence of excitation of the cardiac conduction system 3 Discuss the features of the cardiac muscle action potential including the ionic basis of the potential changes 4 Compare and contrast the cardiac muscle and SA node potentials 5 Explain how the cardiac muscle action 6 potential is generated Identify the components of a normal ECG and discuss the physiological relevance of each component Cardiac Muscle Structure cardiac muscle consists of branched striated fibers usually one centrally located nuclei actin and myosin are packaged in myofibrils in the same arrangement as they are in skeletal muscle in sarcomeres the branching pattern of cardiac muscle forms a network that can facilitate the multi directional transmission of electrical impulses in all directions Connected by intercalated disks Cardiac Muscle Structure Myofibrils Actin Myosin Chains Cardiac Muscle Structure cont d Cardiac Muscle Structure cont d The Importance of Gap Junctions and Desmosomes Gap junctions are small channels which allow electrical impulses to pass quickly from one cell to the next Gap junctions are located in intercalated discs which lie between adjacent muscle fibers Intercalated discs also contain desmosomes which hold adjacent cells together Gap junctions allow the myocardium to behave as a single unit i e a functional syncytium Desmosomes Arrangement of components in a cardiac muscle fiber Sarcolemma Mitochondrion Transverse tubule Sarcoplasmic reticulum Calcium Pool Thin filament Thick filament Zdisc Iband Zdisc Iband Hzone Aband Sarcomere Z disc to Z disc Sarcomere Length A Band Thin Thick Filament Overlap I Band No Overlap Cause stripes i e striations Conduction System of the Heart Conduction System of the Heart 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 bpm In an intact animal autonomic nerves modify the rate of discharge so that resting HR is 70 bpm Conduction System of the Heart the atria and ventricles must contract in a coordinated fashion sequence of cardiac muscle excitation FIRST depolarization of SA node next impulses travel down and across both atria causing atrial muscle fiber contraction Conduction System of the Heart Bundle of His at the AV border there is poorly slowly conducting tissue the presence of this poorly conducting tissue slows the impulse by 0 1 sec this gives the atria time to fully empty before the ventricles begin to contract Conduction System of the Heart Bundle of His right left AV bundles are connected by the short Bundle of His this bundle transmits APs into bundle branches Purkinje fibers and the muscle of both ventricles the sequence of excitation is such that the lower portions of the ventricles contract first pushing the blood upwards Action Potentials of Contractile Fibers a k a Cardiac Myocytes Unresponsive to outside stimuli Action Potentials of Contractile Fibers a k a Cardiac Myocytes Na INTO CELL Influx Depolarize Ca INTO CELL Influx Prolongs Contracts K OUT OF CELL Efflux Repolarize Ion Permeabilities during Action Potentials in Contractile Fibers a k a Cardiac Myocytes Remember 1 Depolarize Na Influx 2 Prolong Contract Ca2 Influx 3 Repolarize K Efflux Sodium Calcium Potassium Refractory Period of a Cardiac Muscle Fiber Refractory unresponsive or stubborn In physiology refractory refers to the period of time when a muscle or nerve cell is unresponsive to stimulation Absolute refractory period the time when a cell will not respond regardless of the strength of stimulus Relative refractory period the time when a cell will respond only if the stimulus is supra threshold The absolute refractory period in skeletal and heart muscle lasts roughly the same time as the action potential 250 ms in heart muscle https opentextbc ca anatomyandphysiology chapter 19 2 cardiac muscle and electrical activity Action Potentials of Autorhythmic Fibers a k a Pacemaker Cells Pacemaker cells are autorhythmic They auto initiate action potentials They have unstable leaky resting membrane potentials called pacemaker potentials They use CALCIUM INFLUX rather than sodium for the major depolarization phase of the action potential Ion Permeabilities during Action Potentials in Autorhythmic Fibers a k a Pacemaker Cells Slow Na Leak Into the Cell Comparison SA Node Action Potential vs Nerve SA Node 800 ms Nerve 4ms Slow Na Leak Into the Cell Action Potentials Across the Heart During Contraction Cycle Pacemaker Cells no contraction Contractile Tissue An Electrocardiogram ECG or EKG An ECG records heart s electrical currents An ECG is a composite record of action potentials of all active cells at points in time during a heartbeat P wave atrial depolarization P to Q interval conduction time from atrial to ventricular excitation QRS complex atrial repolarization ventricular depolarization T wave ventricular repolarization Cardiac events recorded by the ECG Key Wave of depolarization Wave of repolarization P P R Q R Q S Atria begin depolarizing Atrial depolarization complete Ventricular depolarization begins at apex and progresses superiorly as atria repolarize Ventricular depolarization complete P T P T Ventricular repolarization begins at apex and progresses superiorly Ventricular repolarization complete heart is ready for the next cycle P P R Q S R Q S Figure 19 16 1 Atrial depolarization start 2 Atrial depolarization end Delay Bundle of His AV Bundle 3 Ventricular depolarization start AND atrial repolarization 4 Ventricular depolarization end 5 Ventricular repolarization start 6 Ventricular repolarization end 3 5 1 2 4 6 Normal Sinus Rhythm Uncoordinated Atrial Contraction multiple P waves Atrial Fibrillation Feeling unwell but upright Get the AED defibrillator Ventricular Fibrillation i e Heart Attack complete uncoordinated chaos