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12-Mar-17 Cardiovascular System Prof Dr Waqas Hameed HOD, Physiology Pak Int’l Med Colg A Businessman with heart attack • A 55 years old businessman got a Myocardial infarction. Timely treatment was given and he was saved. However it was noted by the physician that his heart rate has dropped to just 40 beats per minute. • He was all right when he was lying in bed but on the second day when he tried to go to washroom he collapsed and felt dizziness. The cardiologist advised to put ‘an artificial pace maker’ to control his heart rate. Immediately! • • What is the problem with this patient? How will artificial pacemaker help him? Electrical Activity of the Heart 1 12-Mar-17 Electrical Activity of Heart • Heart beats rhythmically - result of action potentials it generates itself (autorhythmicity) • Two specialized types of cardiac muscle cells – Contractile cells 99% of cardiac muscle cells Do mechanical work of pumping Normally do not initiate own action potentials – Autorhythmic cells Do not contract Specialized for initiating & conducting action potentials responsible for contraction of working cells Membrane Ion Channels in Cardiac Muscle Fast Na+ channels Slow Na+ - Ca++ channels K+ channels 2 12-Mar-17 Action Potential in Contractile Muscle Autorhythmicity Action Potential is generated by specialized heart cells itself This generation of action potential is rhythmic It spreads to the whole heart by specialized cardiac muscle It leads to Rhythmic Contraction of Heart (Heart Beat) Sinoatrial Node / Sinus Node small, flattened, ellipsoid strip of specialized cardiac muscle Size: 3 mm wide, 15 mm long & 1 mm thick Location: superior posterolateral wall of right atrium immediately below & slightly lateral to the opening of superior vena cava 3 12-Mar-17 SA Node to AV Node Anterior Middle (tract of Wenckebach) Posterior (tract of Thorel) Anterior Interatrial Band (Bachmann’s bundle) connects right atria with left Intrinsic Cardiac Conduction System Approximately 1% of cardiac muscle cells are autorhythmic rather than contractile 70-80/min 40-60/min 20-40/min 4 12-Mar-17 Special Property of Autorhythmic Cells (Drift & Fire) • (Please Recall that the Nerve & Skeletal Muscle cells have a constant Resting Membrane Potential) ‘Fire’ Cardiac Muscle cells do not have Resting Potential Instead they have PACEMAKER POTENTIAL DRIFT to threshold ‘f ’ channels: ‘ f ’ from funny T-Type Ca++ Channels: “Transient” Ca++ Channel L-Type Ca++ Channel: “Longer Lasting” Ca++ Channel P: Permeability S-A Node (sinoatrial node) Their membrane potential slowly depolarizes between the two action potentials (drifts to threshold), when the threshold for action potential generation is reached, the membrane generates an impulse(fires) An autorythmic cell membrane’s slow drift to threshold is called pacemaker potential Atrioventricular Node Internodal pathways posterior wall of right atrium A-V Node (atrioventricular node) immediately behind the tricuspid valve A-V bundle Left & Right bundle branches of Purkinje fibers 5 12-Mar-17 Conducting System of Heart 6 12-Mar-17 7 12-Mar-17 Targets of Cardiac Excitation Atrial Excitation & Contraction should be complete before the onset of ventricular contraction Excitation of cardiac muscle fibers should be coordinated to ensure that each heart chamber contracts as a unit (syncytium) to pump efficiently The pair of atria & the pair of ventricles should be functionally coordinated so that both members of the pair contract simultaneously Autorhythmic Cells: Initiation of Signals 8 12-Mar-17 Pacemaker & Action Potentials of Heart Conduction between the Atria & the Ventricles • Action potential cannot be transferred from atria to ventricles at any other place except for AV Node • Slow conduction across AV Node – ventricular filling • Atria completely depolarize, contract & empty into ventricles • delay of impulse at AV Node - AV Nodal delay Control of Excitation & Conduction in Heart Role of the Purkinje System: – Efficient fast conducting system - ensures very fast spread of impulse in all portions of ventricles A-V Nodal delay = 0.13 sec A-V Node = 0.09 sec A-V bundle = 0.03 sec – Effective & Synchronous pumping by ventricles is achieved only by this system 9 12-Mar-17 10 12-Mar-17 Abnormal Pacemakers “Ectopic” Pacemaker Pacemaker elsewhere than SA Node abnormal sequence of contraction debility of heart pumping Site: most frequently at the A-V node penetrating portion of the A-V bundle Stokes-Adams syndrome A-V block occurs • atria continue to beat at the normal rate of rhythm of the sinus node • new pacemaker usually develops in the Purkinje system of the ventricles • new rate b/w 15 to 40 beats/ min • Purkinje fibers had been “overdriven” by the rapid sinus impulses • Gap of 5 to 20 seconds; ventricles fail to pump blood • Person faints after first 4 to 5 seconds Action potential in cardiac contractile cell Travels down T tubules Entry of small amount of Ca2+ from ECF Addition of Ca+2 initiates contraction Release of large amount of Ca2+ from sarcoplasmic reticulum Cytosolic Ca2+ Troponin-tropomyosin complex in thin filaments pulled aside Removal of Ca+2 causes relaxation Cross-bridge cycling between thick and thin filaments Thin filaments slide inward between thick filaments Contraction 11 12-Mar-17 Control of Excitation & Conduction in the Heart • Cardiac Nerves Heart rate – Sympathetic Stimulation • Distributed to all parts of the heart • Sympathetic stimulation releases “norepinephrine” that increases permeability of cardiac cells for Na+ & Ca++ ions Sympathetic activity (& epinephrine) Parasympathetic activity – ↑’s rate of sinus nodal discharge – ↑’s the rate of conduction at all levels – ↑’s force of contraction Control of Excitation and Conduction in the Heart (contd.) Pacemaker Function • Cardiac Nerves – Parasympathetic Stimulation • Parasympathetic supply (Vagus Nerve) mainly distributed to SA & AV nodes • Parasympathetic stimulation releases ‘acetylcholine’ - causes increased leakage of K+ causing hyperpolarization of cardiac cells leading to: - Decreased Heart rate - Slowed impulse transmission in ventricular muscle fibers leading to decreased rate of heart pumping 12 12-Mar-17 Sympathetic and Parasympathetic Sympathetic – speeds heart rate by Ca++ & I-f channel flow Parasympathetic – slows rate by K+ efflux & Ca++ influx 13