Conduction system of the heartDr. Niranjan Murthy HL Asst Prof of Physiology SSMC, Tumkur

Two types of muscle fiberscontractile and conducting Contractile fibers in atria and ventricles- form two functional syncytia due to presence of gap junctions Conducting system includes SA Node, internodal tracts, AV Node, Bundle of His, Bundle branches and purkinje fibers Conducting system has i) less cross-striations ii) less glycogen

The Conduction System Conduction system Specialized electrical (pacemaker) cells in the heart arranged in a system of pathways

Normally, the pacemaker site with the fastest firing rate controls the heart

Sinoatrial (SA) Node Initiates electrical impulses at a rate of 60 to 100 beats/min Normally the primary pacemaker of the heart

Small, flattened, ellipsoid strip of specialized muscle Size- 3 x 15 x 1mm Situation- superior lateral wall of right atrium below and lateral to opening of superior venacava Pacemaker of heart P cells- primitive cells- pale- rhythm generators

Atria Fibers of SA node connect directly with fibers of atria Impulse leaves SA node and is spread from cell to cell across the atrial muscle

Internodal Pathways Conduction through the AV node begins before atrial depolarization is completed Impulse is spread to AV node via internodal pathways Pathways merge gradually with cells of AV node

Connect SA Node and AV Node Faster rate of conduction than Atrial muscles AnteriorBachmans bundle MiddleWenkebachs bundle Posterior-

AV Junction Area of specialized conduction tissue Provides electrical links between atrium and ventricle

AV Node Located in the posterior septal wall of the right atrium Supplied by right coronary artery in most individuals

As the impulse from the atria enters the AV node, there is a delay in conduction of the impulse to the ventricles Allows time for atria to empty contents into ventricles

AV Node Divided into three functional regions according to their action potentials and responses to electrical and chemical stimulation Atrionodal (AN) or upper junctional region Nodal (N) region

AV Node The primary delay in the passage of the electrical impulse from the atria to the ventricles occurs in the AN and N areas of the

Only conducting pathway between atria and ventricles normally Has thinner fibers with more negative RMP & fewer gap junctions causing conduction delay Velocity of conduction0.05m/sec It acts as pacemaker when SA Node is damaged

Bundle of His Also called the common bundle or the AV bundle Normally the only electrical connection between the atria and the ventricles Connects AV node with bundle branches Has pacemaker cells capable of discharging at an intrinsic rate of 40 to 60 beats/min

It begins from AV Node, passes downwards in the intraventricular septum for 5-15mm Divides into right and left bundle branches Left branch divides into anterior and posterior fasciculus

Right & Left Bundle Branches Right bundle branch Innervates the right ventricle

Left bundle branch Spreads the electrical impulse to the interventricular septum and left ventricle Divides into three divisions (fascicles) Anterior fascicle Posterior fascicle Septal fascicle

Purkinje Fibers Elaborate web of fibers that penetrate about 1/3 of the way into the ventricular muscle mass Become continuous with cardiac muscle fibers

Receive impulse from bundle branches and relay it to ventricular myocardium Fastest conducting 1-2 mm thick; largest conducting fiber Intrinsic pacemaker ability of 20 to 40 beats/min

ORIGIN AND SPREAD OF IMPULSESSA Node Anterior bundle of bachman Middle bundle of wenkebach AV Node Bundle of His Right & left bundle branches Purkinje fibers Posterior bundle Of thorel

0.09 0.00 0.19 0.03 0.16 0 0.19.17 0.18 0.21

0.22

0.21 0.18

0.20

CONDUCTION RATESTISSUE Atrial muscle Internodal tract AV Node Purkinje fibers Ventricle muscle m/sec 0.3 1.0 0.05 1.5-4 1.0

AV Nodal delay Delay in transmission of impulses to ventricles by 0.13sec-( 0.09 at AVN & 0.04 at AV bundle) Causes of delayi) smaller size of fibers ii) smaller number of gap junctions iii) more negative RMP Significancea) atria contracts 0.1sec earlier than ventricle b) limits the number impulses transmitted to ventricles-