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...
Chapter 4
BLOOD CIRCULATION
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.
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 )
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
+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
1.1 Action potentials in cardiac muscle
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
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
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+
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.
working cells phase4 autorhythmic cells phase4stable spontaneous depolarization
resting potential
maximal repolarization potential
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
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
outflow of K+ < inflow of Na+
outflow of K+ inflow of Na+
2. Electrophysiological Characteristics of cardiac muscle
2.1 Excitability
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
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
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.
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.
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
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
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
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
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 ↑
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
0.1s
synchronization contraction
传导最慢 纤维最小 , 缝隙连接少 , 慢反应细胞
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 ↑
2.3.3 Spread of cardiac impulse
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.
• Normal ECG
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)
+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
• Cardiac muscle as a syncytium