CARDIAC PROPERTIESACTION POTENTIALS

I. RHYTHMICITYII. 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

RepolarizationCause of Depolarization - Ca++ going insideCause of Repolarization - K+ going outside

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Phase 4

Phase 0 Phase 3

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S A NODE POTENTIAL

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PHASE 4 = Prepotential

PHASE 0 =Depolarization

PHASE 3 =Repolarization

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 causesPrepotential 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 tooccur 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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Action Potential in Ventricular Muscle

Fiber13

II.EXCITIBILITY

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 14

EXCITABILITY

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•Ventricular Muscle membrane has resting membrane potential of -90mV.•Action Potential of ventricular muscle fiber has four phases 0, 1, 2, 3 ,4.

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 PERIODAbsolute 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 PeriodRelative 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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