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