Upload
nirilib
View
460
Download
3
Tags:
Embed Size (px)
DESCRIPTION
it gives the pysiological anatomy of conduction system of heart and helps in understanding basis of ECG in part
Citation preview
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