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