Conduction system of the heart

Dr. Niranjan Murthy H LAsst Prof of PhysiologySSMC, Tumkur

• Two types of muscle fibers- contractile 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• iii) do not contract

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

• Anterior- Bachman’s bundle

• Middle- Wenkebach’s bundle

• Posterior- Thorell’s bundle

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

– Nodal-His (NH)

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 AV node

• Only conducting pathway between atria and ventricles normally

• Has thinner fibers with more negative RMP & fewer gap junctions causing conduction delay

• Velocity of conduction- 0.05m/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

• Both divide repeatedly & lie subendocardially

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 IMPULSES

SA Node

Anterior bundle of bachman

Middle bundle of wenkebach

Posterior bundle Of thorel

AV Node

Bundle of His

Right & left bundle branches

Purkinje fibers

0.00

0.03

0.09

0.16

0.17

0.18

0.19

0.20

0.21

0.22

0.21

0.18

0.19

CONDUCTION RATESTISSUE m/sec

Atrial muscle 0.3

Internodal tract 1.0

AV Node 0.05

Purkinje fibers 1.5-4

Ventricle muscle 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 delay- i) smaller size of fibers ii) smaller number of gap junctions iii) more negative RMPSignificance- a) atria contracts 0.1sec earlier than ventricle b) limits the number impulses transmitted to

ventricles- <230/min

STOKES ADAMS SYNDROME

• Seen during acute, complete AV block• Ventricles stop beating • Person faints due reduced blood supply to

brain• Ventricle recovers after few seconds & starts

generating own impulses• Rx- artificial pacemaker

FACTORS AFFECTING CONDUCTIVITY

• 1) Nervous stimulation• 2) Hormones• 3) Drugs• 4) Ions• 5) temperature