Download AV node - ISpatula

Cardiovascular system- L2
Faisal I. Mohammed, MD, PhD
Yanal A. Shafagoj, MD, PhD
University of Jordan
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Autorhythmic Fibers
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Specialized cardiac muscle fibers
Self-excitable
Repeatedly generate action potentials that trigger
heart contractions
2 important functions
1. Act as pacemaker
2. Form conduction system
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Conduction system
1. Begins in sinoatrial (SA) node in right atrial wall
 Propagates through atria via gap junctions
 Atria contact
2. Reaches atrioventricular (AV) node in interatrial septum
3. Enters atrioventricular (AV) bundle (Bundle of His)
 AV node is the only site where action potentials can
conduct from atria to ventricles due to fibrous skeleton
4. Enters right and left bundle branches which extends
through interventricular septum toward apex
5. Finally, large diameter Purkinje fibers conduct action
potential to remainder of ventricular myocardium
 Ventricles contract
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Frontal plane
Left atrium
Right
Right atrium
atrium
1 SINOATRIAL (SA) NODE
2 ATRIOVENTRICULAR
(AV) NODE
3 ATRIOVENTRICULAR (AV)
BUNDLE (BUNDLE OF HIS)
Left ventricle
4 RIGHT AND LEFT
BUNDLE BRANCHES
Right
Right ventricle
ventricle
5 PURKINJE FIBERS
University ofAnterior
Jordan
view of frontal section
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Intrinsic Cardiac Conduction System
Approximately 1% of cardiac muscle cells are
autorhythmic rather than contractile
75/min
40-60/min
30/min
Transmission of electrical impulse
(fraction of second)
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Autorhythmicity
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During embryonic development, about 1% of all of the
muscle cells of the heart form a network or pathway
called the cardiac conduction system. This
specialized group of myocytes
is unusual in that
they have the ability
to spontaneously
depolarize.
Autorhythmicity
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The rhythmical electrical activity they produce is called
autorhythmicity. Because heart muscle is autorhythmic, it does
not rely on the central nervous
system to sustain a lifelong heartbeat.
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When transplanted hearts are rewarmed following cardiopulmonary
bypass, they once again begin to beat
without the need to connect outside
nerves or use life-long pacemaker devices.
Autorhythmicity
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Autorhythmic cells spontaneously depolarize at a given
rate, some groups faster, some groups slower. Once a
group of autorhythmic cells reaches threshold and starts
an action potential (AP), all of the cells in that area of
the heart also depolarize.
Membrane of two cells clearly seen. The spread of ions through
gap junctions of the Intercalated discs (I) allows the AP to pass
Cardiac Conduction
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The self-excitable myocytes that "act like nerves" have
the 2 important roles of forming the conduction
system of the heart and acting as pacemakers within
that system.
Because it has the fastest rate of depolarization, the
normal pacemaker of the heart is the
sinoatrial (SA) node, located in the
right atrial wall just below
where the superior vena
cava enters the chamber.
Cardiac Conduction
Spontaneous Depolarization of autorhythmic fibers in the
SA node firing about once every 0.8 seconds, or 75 action
potentials per minute (75 bpm)
Pacemaker and Action Potentials of the
Heart….SA nodal cells
Slow Response Action Potential (Pacemaker
Potential)
Fast Response Action Potential of
Contractile Cardiac Muscle Cell
Cardiac Conduction
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The action potential generated from the SA node
reaches the next pacemaker by propagating throughout
the wall of the atria to the AV node in the interatrial
septum.
At the AV node, the signal is
slowed, allowing the atrium
a chance to mechanically
move blood into
the ventricles (AV Delay).
Cardiac Conduction
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From the AV node, the signal passes through the AV
bundle to the left and right bundle branches in the
interventricular septum towards the
apex of the heart.
Finally, the Purkinje
fibers rapidly
conduct the action
potential throughout
the ventricles (0.2
seconds after atrial contraction).
Frontal plane
Left atrium
Right
Right atrium
atrium
1 SINOATRIAL (SA) NODE
2 ATRIOVENTRICULAR
(AV) NODE
3 ATRIOVENTRICULAR (AV)
BUNDLE (BUNDLE OF HIS)
Left ventricle
4 RIGHT AND LEFT
BUNDLE BRANCHES
Right
Right ventricle
ventricle
5 PURKINJE FIBERS
Anterior view of frontal section
Conduction System
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SA node acts as natural pacemaker
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Faster than other autorhythmic fibers
Initiates 100 beats per min (bpm)
Nerve impulses from autonomic nervous system
(ANS) and hormones modify timing and strength
of each heartbeat makes it 72 bpm
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ANS do not establish fundamental rhythm
Parasympathetic system normally predominates
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Action Potentials and Contraction
Action potential initiated by SA node spreads out to
excite “working” fibers called contractile fibers
1. Depolarization
2. Plateau
3. Repolarization
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Autonomic regulation
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Originates in cardiovascular center of medulla oblongata
Increases or decreases frequency of nerve impulses in both
sympathetic and parasympathetic branches of ANS
Noreprinephrine has 2 separate effects
 In SA and AV node speeds rate of spontaneous
depolarization..it increases Na+ leakage
 In contractile fibers enhances Ca2+ entry increasing
contractility…positive inotropic effect.
Parasympathetic nerves release acetylcholine which
decreases heart rate by slowing rate of spontaneous
depolarization
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Thank You
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