Electrophysiology
(Conduction System of Heart)
Dr. Mohammed Sharique Ahmed Quadri
Assistant Prof. physiology
Al maarefa college
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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.
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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
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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.
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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.
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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.
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Specialized Conduction System of
Heart
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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
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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 nonconductive].
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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
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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.
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Spread of Cardiac Excitation
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Normal Impulse Conduction
Sinoatrial node
AV node
Bundle of His
Bundle Branches
Purkinje fibers
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Conduction speed in cardiac tissue
 Slowest Conduction at
AV – Node
 Fastest Conduction –
Purkinje Fibers
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SPREAD OF CARDIAC EXCITATION
Why Conduction is slow at AV-Node?
• Because there are less gap junctions.
• Diameter of the fiber is small.
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CONDUCTIVE TISSUE
Why SA-Node is a Pace-maker?
• Because its discharge rate is high 70-80/min.
• This 70-80 action potential/min drive rest of
the heart, therefore, it is known as pacemaker of the heart.
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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’.
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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.
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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].
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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.
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Control of Excitability by ANS
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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 Ca2+ 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 Ca2+.
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Area affected
Effect 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 Increase excitability ,decrease AV
nodal delay
nodal delay
Ventricular
conduction
pathway
No effect
Increase excitability, hasten the
conduction through bundle of hiss
& purkinje fibers
Atrial muscle
Decrease contractility,
Increase contractility
Ventricular
muscle
No effect
Increase contractility
Adrenal medulla No effect
Promotes secretion of epinephrine
,that augments the sympathetic
nervuos system actions on heart
Veins
Increaser venous return which
increases the strength of cardiac
contraction through Frank-starling
mechanism
No effect
Autonomic control of heart rate
Heart rate
Increase
Parasympathetic
activity
+
Increase
sympathetic activity
Control of heart rate:
– 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
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] ?
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References
• Human physiology by Lauralee Sherwood,
seventh edition
• Text book physiology by Guyton &Hall,11th
edition
• Text book of physiology by Linda .s
contanzo,third edition