Electrophysiology
(Conduction System of Heart)
Dr. Mohammed Sharique Ahmed Quadri
Assistant Prof. physiology
Al maarefa college
1
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
• Recognize the difference between A.P of SA
Node and ventricular muscle fiber
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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
3
Conducting Tissues of the Heart
(autorhythmic cells)
• 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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Specialized Conduction System of
Heart
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Conducting Tissues of the Heart
– Sinoatrial Node (SA node)
• Specialized region in right atrial wall near
opening of superior vena cava
• Pacemaker 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 near inter atrial septum.
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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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AUTORHYTHMICITY
What is AutoRhythmicity ?
• Cardiac autorhythmic cells do not have stable
resting membrane potential instead they
show PACE MAKER POTENTIAL
• Membrane potential slowly depolarizes
between action potential until threshold is
reached.
• This spontaneous depolarization to threshold
is known as PACE MAKER POTENTIAL
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AUTORHYTHMICITY( PACE MAKER POTENTIAL)
Cause of Prepotential
•
•
•
•
Na+ going inside
↓ K+ going outside
Ca++ going inside
After Pre-potential we get Depolarization and
Repolarization
Cause of Depolarization - Ca++ going inside
Cause of Repolarization - K+ going outside
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S A NODE POTENTIAL
PHASE 4 =
Prepotential
PHASE 0 =
Depolarization
PHASE 3 =
Repolarization
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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’. ( associated with organic
heart disease or lack of sleep, anxiety, excess
caffeine, nicotine)
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APPLIED – HEART BLOCKS
Sino-atrial block: The sinus node fires but
the stimulus does not excite the atria
because it is blocked at the junction
between the two.
AV junctional block: The blocks at the level
of the AV junction are classified according
to the severity of the block.
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APPLIED – 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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APPLIED – HEART BLOCKS
Bundle Branch block: Conduction at one of
the branches become blocked. The wave
of excitation spread from the intact branch
to depolarize the whole ventricle, which
take longer time than if both branches are
intact.
A)Right bundle branch block.
B)Left bundle branch block.
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Control of Excitability by ANS
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SA NODAL POTENTIAL & CHANGES
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Effect 0f Sympathetic and parasympathetic Stimulation
on Prepotential
(Pace Maker Potential)
• Epinephrine & Norepinephrine
(Adrenaline and Noradrenaline) causes
Prepotential to occur faster therefore increase
the heart rate
• Acetylcholine causes Prepotential to occur at
slow rate therefore decrease the heart rate
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Effect 0f Sympathetic Stimulation on
Prepotential
Why Sympathetic Stimulation causes
Prepotential to occur faster?
• Because Sympathetic Stimulation causes
- more Na+ influx [entry]
- more Ca2+ influx [entry]
- decreased K+ efflux [going outside]
• Therefore, membrane potential changes quickly
from -60mV to -40mV [increases the slope of
Prepotential] and when it reaches the threshold
level, AP starts.
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Effect 0f parasympathetic Stimulation
on Prepotential
Why parasympathetic causes Prepotential to
occur after long time?
• Because Parasympathetic Stimulation causes
- decreased Na+ influx [entry]
- decreased Ca2+ influx [entry]
- increased K+ efflux [going outside]
• Therefore, membrane potential changes slowly
from -60mV to -40mV [decreases the slope of
Prepotential] and when it reaches the threshold
level, AP starts.
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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
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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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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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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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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Myocardial Action Potential
( Excitability )
• Once myocardial cells are stimulated by action
potential originating in SA node, it produces its own
action potential
•Ventricular
Muscle
membrane has
resting
membrane
potential of 90mV.
•Action Potential
of ventricular
muscle fiber has
four phases 0, 1,
2, 3 ,4.
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Ventricular action potential
• Rapid depolarization (Phase
0) – due to Na+ influx
• Early brief Repolarization
(Phase 1) - Due to closure
of Na+ channels & opening
of transient K+ channels.
• Slow depolarization (Phase
2) - this is called Plateau
phase and is maintained for
200 – 300 ms – due to Ca++
influx
• Rapid Repolarization (Phase
3) – due to K+ efflux
• Resting Membrane
Potential (Phase 4)
Relationship of an Action
Potential and the Refractory
Period to the Duration of the
Contractile Response in
Cardiac Muscle
Electrical Activity of Heart
• Because long refractory period occurs in
conjunction with prolonged plateau phase,
– This Ensures alternate periods of contraction and
relaxation which are essential for pumping blood
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