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

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. 2

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 3

Conducting Tissues of the Heart 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. 4

Conducting Tissues of the Heart Locations of noncontractile cells capable of autorhymicity – Sinoatrial Node (SA node) Specialized region in right atrial wall near opening of superior vena cava It is 15mm long, 3mm wide, 1mm thick. Pacemaker of the heart It is supplied by right vagus. 5

Conducting Tissues 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 at posterior part of inter atrial septum. It is supplied by left vagus. 6

Specialized Conduction System of Heart 7

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 8

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SPREAD 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]. 11

SPREAD OF CARDIAC EXCITATION  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 12

SPREAD OF CARDIAC EXCITATION  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. 13

Spread of Cardiac Excitation 14

Normal Impulse Conduction Sinoatrial node AV node Bundle of His Bundle Branches Purkinje fibers 15

Conduction speed in cardiac tissue  Slowest Conduction at AV – Node  Fastest Conduction – Purkinje Fibers 16

SPREAD OF CARDIAC EXCITATION  Why Conduction is slow at AV-Node? Because there are less gap junctions. Diameter of the fiber is small. 17

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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. 20

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’. 21

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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. 23

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]. 24

ECTOPIC PACE-MAKER Any pace-maker other than SA Node is called ECTOPIC Pace-maker. [Ectopic means out of place]. It may be in atria, ventricle. If Ectopic Pace-maker is faster than SA node, it will take over and heart rate will be high. Ectopic pace-maker can occur in heart disease or some factors can precipitate e.g. anxiety, lack of sleep, excess caffeine, alcohol. 25

Control of Excitability by ANS 26

Autonomic effects on Heart Excitability Sympathetic from the cardiac plexus supplies all parts of the heart (atria, ventricle and all parts of the conduction system) Parasympathetic from Vagus nerves supply mainly the atria, SA and AV nodes, very little supply to ventricles Sympathetic: increase the permeability of the cardiac cells to Na + and Ca 2+ i.e Positive Chronotropic and positive Inotropic action Parasympathetic: Increase the permeability of the cardiac cells to K+ and decrease its permeability to Na + and Ca

Area affectedEffect of parasympathetic stimulation Effect of sympathetic stimulation SA node Decrease rate of depolarization, heart rate Increase rate of depolarization, increase heart rate AV node Decrease excitability, increase AV nodal delay Increase excitability,decrease AV nodal delay Ventricular conduction pathway No effect Increase excitability, hasten the conduction through bundle of hiss & purkinje fibers Atrial muscleDecrease contractility,Increase contractility Ventricular muscle No effectIncrease contractility Adrenal medullaNo effect Promotes secretion of epinephrine,that augments the sympathetic nervuos system actions on heart VeinsNo effect Increaser venous return which increases the strength of cardiac contraction through Frank-starling mechanism

Autonomic control of heart rate -+-+ Heart rate Increase Parasympathetic activity Increase sympathetic activity

– Heart rate is determined by balance between Inhibition of SA node by vagus(parasympathetic) & stimulation by sympathetic – Under resting condition parasympathetic discharge dominates – Although heart rate is primarily regulated by autonomic innervation the other factor affect it as well,the most imp is EPINEPHRINE,a hormone secreted by adrenal medulla and that act on heart & increases heart rate Control of heart rate:

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] ? 31

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