Download CARDIAC PROPERTIES LECTURE – 4

CARDIAC PROPERTIES
ACTION POTENTIALS
I. RHYTHMICITY
II. EXCITABILITY
LECTURE – 2
DR. ZAHOOR ALI SHAIKH
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I. RHYTHMICITY
What is Rhythmicity?
• Action Potential in SA-Node is called Auto
Rhythmicity or self excitability.
• It is also called Pace-maker potential.
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Pacemaker Potential
( Rhythemicity )
• SA node is pacemaker of
normal heart
• SA node has a spontaneous
slow depolarization called as
pacemaker potential or pre
potential
• Membrane potential begins at 60mv and slowly depolarizes to
-40mv, which is threshold for
producing Action Potential
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Cause of Prepotential
•
•
•
•
Na+ going inside
Ca++ going inside
↓ K+ going outside
After Prepotential we get Depolarization and
Repolarization
Cause of Depolarization - Ca++ going inside
Cause of Repolarization - K+ going outside
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Phase 0
Phase 3
Phase 4
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S A NODE POTENTIAL
PHASE 4 =
Prepotential
PHASE 0 =
Depolarization
PHASE 3 =
Repolarization
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WHY SA-NODE IS PACE-MAKER OF
THE HEART?
• SA-Node is Pace-maker because
- It has pace-maker or Prepotential
- It has fastest rate of auto Rhythmicity, 70-80
Action Potential/min, which drives the rest of
the heart at this rate.
NOTE
• SA-Node is specialized cardiac muscle cell,
situated in the atrial wall, near the opening of
superior vena-cava.
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Effect 0f Sympathetic and parasympathetic Stimulation on
Prepotential (Pace Maker Potential)
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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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II.EXCITIBILITY
Action Potential in
Ventricular Muscle
Fiber
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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
Action Potential From Ventricular Muscle Fiber
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EXCITABILITY
•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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Action Potential in Myocardial
cell
• Resting membrane potential is
about -90mv
• Rapid depolarization (Phase 0) –
due to Na+ influx
• Rapid Repolarization (Phase 1) Due to closure of Na+ channels
• Slow depolarization (Phase 2) this is called Plateau phase and is
maintained for 200 – 300 ms – due
to Ca++ influx
• Repolarization (Phase 3) – due to
K+ efflux
• Resting Membrane Potential
(Phase 4)
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ACTION POTENTIAL IN
VENTRICULAR MUSCLE
FIBER & REFRACTORY
PERIODS
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• AP in Ventricular
Muscle Fiber and
Refractory Periods
• AP followed by
contraction
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REFRACTORY PERIOD
Absolute Refractory Period [ARF]
• ARF is that period, where second action
potential can not be triggered by the
stimulus.
• ARF last for about 200-250ms and
includes depolarization, plateau phase,
and half of Repolarization phase.
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REFRACTORY PERIODS
Relative Refractory Period
Relative Refractory Period occurs after ARP.
During Relative Refractory Period, second action potential can be triggered by
very strong stimulus.
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NOTE
• Ventricular AP is followed by ventricular
contraction. Ventricular contraction occurs
during ARP, therefore, ventricular muscle
can not be Tetanized [contraction can not
be summated].
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Action
Potential &
Contractile
Response
In Cardiac
Muscle
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Difference between
Myocardial AP & Pacemaker
Potential
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Difference between Myocardial AP &
Pacemaker Potential
Myocardial AP
• Resting Memb. Potential is
about -90mv
• It is stable
• Needs stimulus
• No Prepotential
• Phases 0,1,2,3 & 4
• Rapid depolarization due to
Na+ influx
• Has plateau phase
Pacemaker Potential
• Memb. Potential is about
-60mv
• It is unstable
• Automatic
• Prepotential
• Phase 0,3 ,4.(no phase 1 & 2).
• slow depolarization due to
ca++ influx
• Has no plateau phase
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ACTION POTENTIAL OF DIFFERENT CARDIAC TISSUES
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THANK YOU
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