Download Cardiovascular Lecture-2

Cardiac Muscle
• Made up of many interconnected individual
cardiac cells that are highly coordinated
• The cell network allows the efficient spread of
an AP from cell to cell throughout the heart
• Composed of two coordinated functional
units: atrial and ventricular
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Anatomy of the Heart
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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Cardiac Conduction Pathway
Sinoatrial node ("pacemaker")
Atrial cardiac muscle
AV node
AV bundle
Right and left bundle branches
Purkinje fibers
Ventricular cardiac muscle
Conducting System of Heart
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0.16 0.19
0.19
0.18
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• The pulse cannot travel directly from atrium to ventricle
• Contraction of heart follows direction of electrical pulses
• Moves upwards and following
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Sinoatrial (SA) Nodal Cells
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“Pacemaker cells” - self electrical excitation
Located on right atrial wall
Smaller than contractile cells or Purkinje cells
75 AP/min at rest (acetylcholine release)
Lack fast Na channels
Spontaneous depolarization is a
pacemaker potential 1 Sinoatrial node
(Pacemaker)
triggering an AP
2 Atrioventricular node
when it reaches
3 Atrioventricular Bundle
(Bundle of His)
threshold
4 Left & Right Bundle
branches
5 Bundle Branches
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Purkinje Cells
• Larger than ordinary cardiac fibers and
bundle fibers
• Conduct action potentials up to four times
faster than a ventricular myocyte (4m/sec)
• Subendocardial location
• Linked to cardiac fibers and bundle fibers by
gap junctions and desmosomes
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 ventricle
5 PURKINJE FIBERS
Right
Right ventricle
ventricle
Anterior 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
 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
 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.
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.
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 Autorhythmic cells spontaneously depolarize at a
given rate (some groups faster and some slower)
 Once a group of cells reach threshold it starts an
action potential, all cells in that area depolarize
 The spread of ions through gap junctions of the
Intercalated discs (I) allows the action potential to
pass from cell to cell
Cardiac Conduction
 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.
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Cardiac Conduction
Spontaneous depolarization of autorhythmic fibers in
the SA node firing about once every 0.8 seconds, or
75 action potentials per minute
Cardiac Conduction Pathway
Sinoatrial node ("pacemaker")
~70 b/min
Atrial cardiac muscle
AV node
~50 b/min
AV bundle
~50 b/min
Right and left bundle branches
Purkinje fibers
~30 b/min
Ventricular cardiac muscle
• Fastest pacemakers controls rate of excitation
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Fast Response Action Potential of
Contractile Cardiac Muscle Cell
Puffer Fish
(tetrodotoxin)
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Pacemaker and Action Potentials
of the Heart
Self-induced
action potential
Cardiac Conduction
 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.
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PHYSICAL EXERCISE
Cannot happen !
(no diastolic phase means no
refilling of the heart with blood)
Cardiac Conduction
 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).
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Conduction System
 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

ANS do not establish fundamental rhythm
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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
• In contractile fibers enhances Ca2+ entry
increasing contractility
Parasympathetic nerves release acetylcholine which
decreases heart rate by slowing rate of
spontaneous depolarization
Extrinsic factors increasing or decreasing
conduction velocity within the heart
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