CARDIAC MUSCLE
Dr. Abdelrahman Mustafa
Department of Basic Medical Sciences
Division of Physiology
Faculty of Medicine
Almaarefa Colleges
Learning Objectives
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By the end of this lecture you should be able to
Identify the types of cardiac muscle cells
Define the cardiac muscle Autorhythmicity
Describe the pace maker potential
Define the cardiac muscle excitability
Describe the Ventricular potential
Define the cardiac muscle contractility
Describe the mechanism of cardiac muscle contraction
Define cardiac muscle conductivity
Identify the conductive system of the heart
Describe the starling law of the heart
CARDIAC MUSCLE
3Layers:
• Epicardium,
• Endocardium
• Myocardium ,
4 Chambers
• Rt atrium
• Rt Ventricle
• Lt atrium
• Lt ventricle
4 large vessels
• Aorta
• Pulmonary artery
• Vena Cava
• Pulmonary veins
4 Valves
• Tricuspid
• Mitral
• Aortic
• Pulmonary
Types of Cardiac Muscle Cells
• Contractile Cells
– 99% of Cardiac Muscle
– Do mechanical work of contraction
– Do not generate action Potential in
normal conditions
(Can generate Action Potential in
abnormal conditions)
• Autorhythmic cells:
– Initiate Action Potential
– Conduct Action Potential
• Do not contract
• Interconnected by intercalated discs and
form functional syncytia
• Within intercalated discs – two kinds of
membrane junctions
– Desmosomes
– Gap junctions
Cardiac Muscles Action=Action of Pump
5
What are the Properties that allowed the Cardiac Muscles to Act as
PUMP?
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AUTORHYTHMICITY
• Definition Is the ability of cardiac muscle
to contract in self regular constant manner
• IS IT MYOGENIC OR
NEUROGENIC IN ORIGIN ?
• MYOGENIC This is evident by :• 1)The frogs apex beating
• After deinnrevetion in Ringr Lactate
soulation
• 2)The embryo heart started to beat
BEFORE CNS DEVELOPMENT
AUTORHYTHMICITY
• Cardiac autorhythmic
structure that initiate
AutoRhythmicity Called
PACE MAKER
spontaneous
depolarization to
threshold is known as
PACE MAKER
POTENTIAL
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AUTORHYTHMICITY( PACE MAKER
POTENTIAL)
PHASES OF PACEMAKER POTENTIAL
Prepotential
Causes :
Na+ going inside(Infflux )
Ca++ going inside(Infflux )
↓ K+ going outside(Efflux)
After Prepotential we get
Depolarization and Repolarization
Depolarization Cause: Ca++ going
inside(Infflux )
Repolarization Cause : K+ going
outside(Efflux)
EXCITABILITY
Definition is the ability to respond to
stimuli by generation action
potential.
The resting membrane potential of
the contractile fibers is stable at
about– 90 mV.
• Once myocardial cells are
stimulated by action potential
originating in PACEMAKER , it
produces its own action potential
Action Potential of ventricular muscle
fiber has four phases 0, 1, 2, 3 ,4.
Excitability (Myocardial Action
0 = Rapid Depolarization
Potential )
+
(inward Na current)
1
1= Early partial repolarization
2 = Plateau
(inward Ca++ current)
2
0
0
3
PHASE
3 = Repolarization
(outward K+ current)
4 = Resting Potential
4
-90
TIME
EXCITABILITY
• The heart passes through two phases of
excitability :1)The absolute refractory period
2)The relative refractory period
• Because long refractory period occurs in
conjunction with prolonged plateau phase,
– Ensures alternate periods of contraction and
relaxation which are essential for pumping blood
CONTRACTILITY
• Definition The ability of cardiac
muscle to convert chemical
energy to mechanical work
• Ca2+ entry through L-type
channels in T tubules triggers
larger release of Ca2+ from
sarcoplasmic reticulum
– Ca2+ induced Ca2+ release leads
to cross-bridge cycling and
contraction
– 90% of Ca2+ needed for
contraction comes from
sarcoplasmic reticulum
Excitation-Contraction Coupling in Cardiac Contractile Cells
CONDUCTIVITY
• Defienion Is the ability of cardiac muscle fiber to
conduct cardiac impulses that are initiated in SA
node, which is the pacemaker of the heart
1. Sinoatrial (SA) node
The
conductive
system
2. Atrioventricular (AV)
node
3. Atrioventricular
bundle (bundle of
His)
4. Bundle branches
5. Purkinje fibers
Cardiac Muscle Mechanics
Starling’s law of the heart :• Law states that : with in
physiological limit , the force
of myocardial contraction is
directly proportional to the
initial length of the cardiac
muscle fibers
10 MINs for study
Q1
• The cells that responsible to convert the
chemical energy to mechanical work called
• A)Contractile Cells
• B)Autorhythmic cells:
• C)Conductive cells
• D)Desmosomes
Q2
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Pacemaker depolarization caused by :
A)Na+ Infflux
B)Ca++ Infflux
C)K+ Efflux
D)Ca++ Efflux
Q3
• the ability to respond to stimuli by generation
action potential known as :
• A)cardiac muscle Auto rhythmicity
• B)cardiac muscle excitability
• C)cardiac muscle contractility
• D)cardiac muscle conductivity
Q4
• Starling’s law of the heart Describe the
relationship between :
• A) Contraction and cardiac muscle Length
• B) Contraction and cardiac muscle
conductivity
• C) Contraction and cardiac muscle excitability
• D) Contraction and cardiac muscle Auto
rhythmicity
References
• Human physiology by Lauralee Sherwood, 7th
edition
• Text book physiology by Guyton &Hall,12th
edition
• Text book of physiology by Linda .s
contanzo,third edition
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