Presentation on theme: "Electrophysiology (Conduction System of Heart)"— Presentation transcript:
1Electrophysiology (Conduction System of Heart) بسم الله الرحمن الرحيمElectrophysiology (Conduction System of Heart)Dr. Mohammed Sharique Ahmed QuadriAssistant Prof. physiologyAl maarefa college
2Objectives Identify the components of conducting system of heart . Know the sequence of conduction of impulse in the heartRecognize the concept associated with pacemakerAppreciate the role of ANS in controlling rate of generation and conduction of impulseRecognize the difference between A.P of SA Node and ventricular muscle fiber
3Electrical Activity of Heart Heart beats rhythmically as result of action potentials it generates by itself (Autorhythmicity)Two specialized types of cardiac muscle cellsContractile cells( atrial & ventricular muscles99% of cardiac muscle cellsDo mechanical work of pumpingNormally do not initiate own action potentialsAuto rhythmic cells ( conductive tissue )Do not contractSpecialized for initiating ( excitation)and conducting action potentials responsible for contraction of working cells
4What is AutoRhythmicity? Cardiac autorhythmic cells do not have resting potential instead they show PACE MAKER POTENTIALMembrane potential slowly depolarizes between action potential until threshold is reached.This spontaneous depolarization to threshold is known as PACE MAKER POTENTIAL
8Conducting Tissues of the Heart (autorhythmic cells) APs spread through myocardial cells through gap junctions.Impulses cannot spread to ventricles directly because of fibrous tissue.Conduction pathway:SA node.AV node.Bundle of His.Purkinje fibers.
10Conducting Tissues of the Heart Sinoatrial Node (SA node)Specialized region in right atrial wall near opening of superior vena cavaPacemaker of the heartINTERNODAL FIBERSInternodal Fibers – Anterior, Middle and Posterior [Bachman, Wenchkeback, Thorel].Atrioventricular Node (AV node)Small bundle of specialized cardiac cells located at base of right atrium near inter atrial septum.
11Conducting Tissues of the Heart Bundle of His (atrioventricular bundle)It is a tract of specialized cardiac cells that originate at AV Node and passes through the fibrous ring and enters interventricular septumDivides to form right and left bundle branches which travel down septum, curve around tip of ventricular chambers, travel back toward atria along outer wallsNOTE – Lt Bundle Branch has 2 fascicles Left Anterior Fascicle and Left Posterior Fascicle.Purkinje fibersSmall, terminal fibers that extend from bundle of His and spread throughout ventricular myocardium
13CONDUCTIVE TISSUE Why SA-Node is a Pace-maker? Because its discharge rate is high 70-80/min.This action potential/min drive rest of the heart, therefore, it is known as pace-maker of the heart.
14CONDUCTIVE TISSUEOther auto - rhythmic tissue are firing at slow rate.They can work as pace-maker, if SA-Node is not functioning e.g. if AV Node takes over as pace-maker, heart rate will be about 50/min.Any pace-maker other than SA-Node is called ‘Ectopic Pace-maker’. ( associated with organic heart disease or lack of sleep, anxiety, excess caffeine, nicotine)
16APPLIED – HEART BLOCKS There are three types of heart blocks: FIRST DEGREE HEART BLOCK – Every impulse is conducted but very slowly, therefore, there is increase in conduction time [we can see on ECG].SECOND DEGREE HEART BLOCK – Some impulses are conducted and other are not conducted.
17APPLIED – HEART BLOCKSTHIRD DEGREE HEART BLOCK – Complete heart block, no conduction occurs from SA Node to the ventricle through AV node, therefore, atrial rate is separate [75/min] from the ventricular rate which follows the Purkinje fibers and is about 30/min.IMPORTANTIf ventricular rate is very slow e.g. complete heart block, we need artificial pace-maker [implanted device which generates impulse].
20Epinephrine & Norepinephrine Effect 0f Sympathetic and parasympathetic Stimulation on Prepotential (Pace Maker Potential)Epinephrine & Norepinephrine(Adrenaline and Noradrenaline) causes Prepotential to occur faster therefore increase the heart rateAcetylcholine causes Prepotential to occur at slow rate therefore decrease the heart rate
21Effect 0f Sympathetic Stimulation on Prepotential Why Sympathetic Stimulation causesPrepotential to occur faster?Because Sympathetic Stimulation causes- more Na+ influx [entry]- more Ca2+ influx [entry]- decreased K+ efflux [going outside]Therefore, membrane potential changes quickly from -60mV to -40mV [increases the slope of Prepotential] and when it reaches the threshold level, AP starts.
22Effect 0f parasympathetic Stimulation on Prepotential Why parasympathetic causes Prepotential tooccur after long time?Because Parasympathetic Stimulation causes- decreased Na+ influx [entry]- decreased Ca2+ influx [entry]- increased K+ efflux [going outside]Therefore, membrane potential changes slowly from -60mV to -40mV [decreases the slope of Prepotential] and when it reaches the threshold level, AP starts.
23Control of heart rate:Heart rate is determined by balance between Inhibition of SA node by vagus(parasympathetic) & stimulation by sympatheticUnder resting condition parasympathetic discharge dominates
24POINT TO PONDERIn Transplanted Heart, where there is no sympathetic and parasympathetic nerve supply, what will be the rate of SA Node discharge [Heart Rate] ?
25SPREAD OF CARDIAC EXCITATION Cardiac impulse originates at SA node and spread to the atria [via gap junction] – Atrial Syncytium, therefore, both atria depolarize same time.Impulse [AP] goes to AV-Node by Internodal pathway.AV-Node is the only point of electrical contact between atria and ventricle [as atria and ventricle are separated by fibrous ring which is non-conductive].
26SPREAD OF CARDIAC EXCITATION AV – NodeAt AV-Node, there is delay of 0.1 sec [100 milli- sec].This delay is important to allow complete ventricular fillingbecause it allows the atria to contract and empty their blood into the ventricle, before impulse reaches the ventricle and causes ventricular depolarization and contraction
27SPREAD OF CARDIAC EXCITATION Ventricular ExcitationAfter AV delay of 0.1sec, impulse [AP] travels quickly via Right Bundle Branch and Left Bundle Branch [branches of Bundle of His] to Purkinje Fibers to the ventricles.Both ventricle depolarize, than contract at same time.Conduction in Purkinje Fiber is fastest2-4 meter/sec, therefore, both ventricle depolarize quickly and at the same time.
31SPREAD OF CARDIAC EXCITATION Why Conduction is slow at AV-Node?Because there are less gap junctions.Diameter of the fiber is small.
32Myocardial Action Potential ( Excitability ) Once myocardial cells are stimulated by action potential originating in SA node, it produces its own action potentialVentricular Muscle membrane has resting membrane potential of -90mV.Action Potential of ventricular muscle fiber has four phases 0, 1, 2, 3 ,4.
33Ventricular action potential Rapid depolarization (Phase 0) – due to Na+ influxRapid Repolarization (Phase 1) - Due to closure of Na+ channelsSlow depolarization (Phase 2) - this is called Plateau phase and is maintained for 200 – 300 ms – due to Ca++ influxRepolarization (Phase 3) – due to K+ effluxResting Membrane Potential (Phase 4)
34Relationship of an Action Potential and the Refractory Period to the Duration of the Contractile Response in Cardiac Muscle
35Electrical Activity of Heart Because long refractory period occurs in conjunction with prolonged plateau phase, summation and tetanus of cardiac muscle is impossibleEnsures alternate periods of contraction and relaxation which are essential for pumping blood
36References Human physiology by Lauralee Sherwood, seventh edition Text book physiology by Guyton &Hall,11th editionText book of physiology by Linda .s contanzo,third edition