cardiac properties action potentials i. rhythmicity ii. excitability
DESCRIPTION
CARDIAC PROPERTIES ACTION POTENTIALS I. RHYTHMICITY II. EXCITABILITY . LECTURE – 2 DR. ZAHOOR ALI SHAIKH. I. RHYTHMICITY. What is Rhythmicity? Action Potential in SA-Node is called Auto Rhythmicity or self excitability. It is also called Pace-maker potential. - PowerPoint PPT PresentationTRANSCRIPT
CARDIAC PROPERTIESACTION POTENTIALS
I. RHYTHMICITYII. EXCITABILITY
LECTURE – 2 DR. ZAHOOR ALI SHAIKH
1
I. RHYTHMICITY
What is Rhythmicity?• Action Potential in SA-Node is called Auto Rhythmicity or self excitability.• It is also called Pace-maker potential.
2
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
3
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
4
Phase 4
Phase 0 Phase 3
5
S A NODE POTENTIAL
6
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.
7
Effect 0f Sympathetic and parasympathetic Stimulation on Prepotential (Pace Maker Potential)
8
SA NODAL POTENTIAL & CHANGES
9
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
10
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.
11
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.
12
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
15
•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)
16
ACTION POTENTIAL IN VENTRICULAR MUSCLE FIBER & REFRACTORY
PERIODS
17
• AP in Ventricular Muscle Fiber and Refractory Periods
• AP followed by contraction
18
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.
19
REFRACTORY PERIODS
Relative Refractory PeriodRelative Refractory Period occurs after ARP.
During Relative Refractory Period, second action potential can be triggered by very strong stimulus.
20
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].
21
Action Potential & Contractile Response In Cardiac
Muscle
22
Difference between Myocardial AP & Pacemaker
Potential
23
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
24
ACTION POTENTIAL OF DIFFERENT CARDIAC TISSUES
25
THANK YOU
26