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Electrophysiology (Conduction System of Heart)

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1 Electrophysiology (Conduction System of Heart)
بسم الله الرحمن الرحيم Electrophysiology (Conduction System of Heart) Dr. Mohammed Sharique Ahmed Quadri Assistant Prof. physiology Al maarefa college

2 Objectives Identify the components of conducting system of heart .
Know the sequence of conduction of impulse in the heart Recognize the concept associated with pacemaker Appreciate the role of ANS in controlling rate of generation and conduction of impulse Recognize the difference between A.P of SA Node and ventricular muscle fiber

3 Electrical Activity of Heart
Heart beats rhythmically as result of action potentials it generates by itself (Autorhythmicity) Two specialized types of cardiac muscle cells Contractile cells( atrial & ventricular muscles 99% of cardiac muscle cells Do mechanical work of pumping Normally do not initiate own action potentials Auto rhythmic cells ( conductive tissue ) Do not contract Specialized for initiating ( excitation)and conducting action potentials responsible for contraction of working cells

4 What is AutoRhythmicity?
Cardiac autorhythmic cells do not have resting potential instead they show PACE MAKER POTENTIAL Membrane potential slowly depolarizes between action potential until threshold is reached. This spontaneous depolarization to threshold is known as PACE MAKER POTENTIAL


6 Cause of Prepotential Na+ going inside Ca++ going inside
↓ K+ going outside After Prepotential we get Depolarization and Repolarization Cause of Depolarization - Ca++ going inside Cause of Repolarization - K+ going outside

7 S A NODE POTENTIAL PHASE 4 = Prepotential PHASE 0 = Depolarization

8 Conducting 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.

9 Specialized Conduction System of Heart

10 Conducting Tissues of the Heart
Sinoatrial Node (SA node) Specialized region in right atrial wall near opening of superior vena cava Pacemaker of the heart INTERNODAL FIBERS Internodal 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.

11 Conducting 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 septum Divides to form right and left bundle branches which travel down septum, curve around tip of ventricular chambers, travel back toward atria along outer walls NOTE – Lt Bundle Branch has 2 fascicles Left Anterior Fascicle and Left Posterior Fascicle. Purkinje fibers Small, terminal fibers that extend from bundle of His and spread throughout ventricular myocardium


13 CONDUCTIVE 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.

14 CONDUCTIVE TISSUE Other 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)


16 APPLIED – 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.

17 APPLIED – HEART BLOCKS THIRD 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. IMPORTANT If ventricular rate is very slow e.g. complete heart block, we need artificial pace-maker [implanted device which generates impulse].

18 Control of Excitability by ANS


20 Epinephrine & 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 rate Acetylcholine causes Prepotential to occur at slow rate therefore decrease the heart rate

21 Effect 0f Sympathetic Stimulation on Prepotential
Why Sympathetic Stimulation causes Prepotential 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.

22 Effect 0f parasympathetic Stimulation on Prepotential
Why parasympathetic causes Prepotential to occur 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.

23 Control of heart rate: Heart rate is determined by balance between Inhibition of SA node by vagus(parasympathetic) & stimulation by sympathetic Under resting condition parasympathetic discharge dominates

24 POINT TO PONDER In Transplanted Heart, where there is no sympathetic and parasympathetic nerve supply, what will be the rate of SA Node discharge [Heart Rate] ?

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].

AV – Node At AV-Node, there is delay of 0.1 sec [100 milli- sec]. This delay is important to allow complete ventricular filling because it allows the atria to contract and empty their blood into the ventricle, before impulse reaches the ventricle and causes ventricular depolarization and contraction

Ventricular Excitation After 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 fastest 2-4 meter/sec, therefore, both ventricle depolarize quickly and at the same time.

28 Spread of Cardiac Excitation

29 Normal Impulse Conduction
Sinoatrial node AV node Bundle of His Bundle Branches Purkinje fibers

30 Conduction speed in cardiac tissue
Slowest Conduction at AV – Node Fastest Conduction – Purkinje Fibers

Why Conduction is slow at AV-Node? Because there are less gap junctions. Diameter of the fiber is small.

32 Myocardial Action Potential ( Excitability )
Once myocardial cells are stimulated by action potential originating in SA node, it produces its own action potential Ventricular Muscle membrane has resting membrane potential of -90mV. Action Potential of ventricular muscle fiber has four phases 0, 1, 2, 3 ,4.

33 Ventricular action potential
Rapid depolarization (Phase 0) – due to Na+ influx Rapid Repolarization (Phase 1) - Due to closure of Na+ channels Slow depolarization (Phase 2) - this is called Plateau phase and is maintained for 200 – 300 ms – due to Ca++ influx Repolarization (Phase 3) – due to K+ efflux Resting Membrane Potential (Phase 4)

34 Relationship of an Action Potential and the Refractory Period to the Duration of the Contractile Response in Cardiac Muscle

35 Electrical Activity of Heart
Because long refractory period occurs in conjunction with prolonged plateau phase, summation and tetanus of cardiac muscle is impossible Ensures alternate periods of contraction and relaxation which are essential for pumping blood

36 References Human physiology by Lauralee Sherwood, seventh edition
Text book physiology by Guyton &Hall,11th edition Text book of physiology by Linda .s contanzo,third edition

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