Chapter 4 BLOOD CIRCULATION. Key points in this unit: 1.the action potential of the ventricular...
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Transcript of Chapter 4 BLOOD CIRCULATION. Key points in this unit: 1.the action potential of the ventricular...
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Chapter 4
BLOOD CIRCULATION
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Key points in this unit:
1.the action potential of the ventricular muscle and the action potential of the pacemaker cells.
2.electrophysiological properties of cardiac muscle.
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1.Physiology of the heart
Function of heart Pumping Endocrine atrial natriuretic pep
tide(ANP, 心房钠尿肽 ) brain natriuretic pep
tide(BNP, 脑钠尿肽 )
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General process of excitation and contraction of cardiac muscle
• Initiation of action potential in sinoatrial node • Conduction of action potential along specialized
conductive system• Excitation-contraction cou
pling• Muscle contraction
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+30
-55
0
-70
阈电位
静息电位
时间( ms )
细胞内电位(m
V
)
除极化
复极化
后电位
0 2 4 6 8 10 12
神经或骨骼肌 AP 心室肌 AP
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1.1 Action potentials in cardiac muscle
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Two major types of cardiac muscle: Working cardiac muscle cell: atrial and ventricular muscle cells
Specialized excitatory and conductive muscle: sinoatrial (SA) node, Purkinje fiber
non-autorhythmic cardiac cell
autorhythmic cardiac cell
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1.1.1 AP of ventricular muscle
Phase 0: rapid depolarization-90mV - +30mV
Phase1: early phase of rapid repolarization+30mV - +10mV
Phase2: the plateau or slow phase of repolarization+10 - 0mV
Phase3: late phase of rapid repolarization0mV - 90mV
Phase4: resting phase-90mV
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Mechanism
Phase 0: Na+ influx
Phase 1: K+ outflux
Phase 2: K+ outflux, Ca2+ and Na+ influx Phase 3: K+ outflux
Phase 4: Na+ - K+ pump Ca2+ pump Na+ - Ca2 exchanger
Na+
K+,Ca2+,Na+K+
K+
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Phase2 needs 100-150ms.
the main cause of long duration of cardiac action potential.
the main characteristic which differs from the action potential in nervous fiber or skeletal muscle.
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working cells phase4 autorhythmic cells phase4stable spontaneous depolarization
resting potential
maximal repolarization potential
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1.1.2 AP of Purkinje cell quite similar to that of working cells except
spontaneous depolarization during phase 4.
Phase 4:
If (inward current, caused b
y Na+ influx) increases
outward current ( caused by K+ outflux) decreases gradually.
If
内向电流
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1.1.3 AP of the P cell of SA node
Phase 0: Ca2+ influx
Phase 3: K+ outflux
Phase 4: fast spontaneous depolarization
4
0 3 IK decreasesIf increasesICa-T
ICa
IK
IK, If, ICa
Slow response cell
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outflow of K+ < inflow of Na+
outflow of K+ inflow of Na+
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2. Electrophysiological Characteristics of cardiac muscle
2.1 Excitability
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2.1.1 Factors affecting excitability
o Level of RP ↓ :→ excitability ↓
o Level of TP:↑→ excitability ↓
o States of sodium channel: resting state: excitability is normal
activated state: Na+ diffuse in
inactivated state: excitability is zero
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2.1.2 Excitability changes:
o Effective refractory period (ERP ):Any strong stimulus fails to elicit an AP.
Cause: The sodium channels are at inactivated state and so the excitability is zero.
ERP is nearly 100-250ms.
Significance:
the heart can not be
tetanized.
the heart can
eject and fill continually.
ARP:phase0---phase3 -55mV
ERP
LRP:phase0---phase3 -60mVERP
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o Relative refractory period phase3 -60-- -80mV
Effective stimulus is suprathreshold stimulus.
Cause: the most sodium channels are at resting
state but their opening ability are not good as
normal, so the excitability is lower than norm
al.
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o Supranormal period
phase3 -80-- -90mV
subthreshold stimulus can elicit an AP.
Cause: the sodium channels are at resting state and the
distance between the resting potential and the thresh
old potential is nearer, so the excitability is better tha
n normal.
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2.1.3 Extrasystole and compensatory pause
①extrasystole ( 期前收缩) premature systole
②compensatory pause(代偿间歇)
After ERP, a stimulus can evoke a extrasystole.
compensatory pause:
a S-A node AP meets the ERP of extrasystole.
①
②
ERP
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2. 2 Autorhythmicity the ability that the cardiac muscle can gene
rate an action potential by itself.
Index: frequency of AP
SA node
AV node
left,right bundle branch
terminal Purkinje cells
(beats/min)
50100
40
25
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Natural pacemaker: the pacemaker which can control the activity of the heart under normal condition.(SA node)
Latent pacemaker: the pacemaker which doesn’t show its autorhythmicity under normal condition. (other rhythmic regions of the heart)
Ectopic pacemaker: from latent pacemaker
① autorhythmicity of latent pacemaker increases
② autorhythmicity of SA node decreases
③ severe conduction block
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Why can SA node be natural pacemaker?
① Preoccupation( 抢先抑制 ): SA node autorhythm
icity is much higher than latent pacemakers an
d elicit excitations before they finish phase 4.
② Overdrive suppression( 超速抑制 ):The higher ra
te pacemaker suppresses the autorhythmicity
of the lower rate pacemakers
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Factors affecting Autorhythmicity o Velocity of spontaneous depolarization: ↑(b to a)→autorhythmicity o Threshold potential: (TP2 to TP1)→ autorhythmicity ↑
o Maximum repolarization potential
(d to a)→ autorhythmicity ↑
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2.3 Conductivity
2.3.1Specialized conducting system
SA node
preferential pathway
( 优势通路 )
AV node
AV bundle
right and left bundle branch
terminal Purkinje fibers
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0.1s
synchronization contraction
传导最慢 纤维最小 , 缝隙连接少 , 慢反应细胞
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2.3.2 Factors affecting conduction velocity
Rate and amplitude of phase0 depolarization: local current↑→ conduction↑ rate and number of Na+or Ca2+ influx level of resting potential
concentration and potential gradient of Na+, Ca2+ Structure of cardiac muscle: diameter↑→ local current↑→conduction↑ number of intercalated disc (gap junction) Excitability of the adjacent region: ↑→conduction ↑
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2.3.3 Spread of cardiac impulse
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Electrocardiogram (ECG)
With an electrocardiogragh, electrical activity of the heart can be recorded from the surface of the body.
When a large number of cells are simultaneously depolarizing or repolarizing, large voltages are observed on ECG.
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• Normal ECG
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P wave : depolarization of the atria.
PQ(PR) interval: from the onset of atrial excitation to the onset of QQRS complex: depolarization of the ventricles.S-T segment : from the end o
f QRS complex to the onset of T wave (plateau)
T wave : repolarization of the ventricles. Q-T interval : from the onset of QRS complex to the end of the T
wave. (AP duration)
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+30
-55
0
-70
a b c d
阈电位
静息电位
时间( ms )
细胞内电位(m
V
)
除极化
复极化
反极化
e
f
2 4 6 8 10 12 14
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• Cardiac muscle as a syncytium