Section 2 Electrophysiology of the Heart

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Section 2 Electrophysiology of the Heart
Two kinds of cardiac cells 1, The working
cells. Special property contractility 2, Special
conduction system, including the sinoatrial node,
atrioventricular node, atrioventricular bundle
(bundle of His), and Purkinje system. Special
property automaticity
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• Transmembrane Potentials of Myocardial Cells
• 1. Transmembrane Potentials in Working
  Myocardial Cells

• General description
• Resting potential -90mv (-85-95mv)
• Action Potential
• Phase 0 rapid depolarization, 1-2ms
• Phase 1 early rapid repoarization, 10 ms
• Phase 2 plateau, slow repolarization, the
  potential is around 0 mv. 100 150ms
• Phase 3, late rapid repolarization. 100 150 ms
• Phase 4 resting potentials

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2
0
3
4
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(2) Ionic bases of transmembrane
potentials Resting potential the equilibration
potential of potassium Action potential Phase 0,
Na influx. The threshold potential, -70 mv Phase
1, K outward flow Phase 2, Ca2 inward flow and
K outward flow Phase 3, K outward flow Phase 4,
Na - K pump and Na - Ca2 exchange (primary
and secondary active transport)
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• 2. Transmembrane Potential of Rhythimic Cells
• Purkinje cell

The phases 0 3 are almost the same with that of
ventricle cells. At phase 4, the membrane
potential does not maintain at a level, but
depolarizes automatically the automaticity
Mechanism If, a kind of Na channel that is
activated by the hyperpolarization. It is not the
same with the fast Na channel at phase 0. It
could be blocked by Cs but is not affected by TTX
.
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(2) The SA node cell

• Transmembrane potential of the sinoatrial node
  cell
• Maximal repolarization (diastole) potential,
  70mv
• Low amplitude and long duration of phase 0. It
  is not so sharp as ventricle cell and Purkinje
  cell.
• No phase 1 and 2
• Comparatively fast spontaneous depolarization at
  phase 4

A, Cardiac ventricular cell B, Sinoatrial node
cell
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Mechanism of the membrane potential
Phase 0, I Ca-L, slow channel and slow response
cell Phase 3, Ik, the time dependent channel, was
slowly inactivated near the maximal
repolarization potential (-60 mv)
Phase 4 Ik, ICa-T and If ICa-T, activated at 50
mv during the repolarization and contribute to
the spontaneous deplorization during the Phase 4
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• Fast and slow response, rhythmic and non-rhythmic
  cardiac cells
• Fast response, non rhythmic cells working cells
• Fast response, rhythmic cells cells in special
  conduction system of A-V bundle and Purkinje
  network.
• Slow response, non-rhythmic cells cells in nodal
  area
• Slow response rhythmic cells S-Anode, atrionodal
  area (AN), nodal His (NH)cells

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• II Electrical Properties of Cardiac Cells
• Excitability, Conductivity and Automaticity
• Excitability of Cardiac Muscle
• Factors determining the excitability
• Resting potential or maximum diastole potential
  (rhythmic cell). Low concentration of K outside
  the cell --- resting potential lower
  excitability lower
• Threshold potential. High concentration of Ca2
  outside the cell threshold potential less
  negative excitability lower
• States of Na channel.
• Resting state close (could open at the
  threshold potential) activation (open)
  inactivation (close, but could not open at any
  potential), this state will transfer to resting
  state after a period of repolarization.

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• (2) Changes in excitability during an action
  potential
• Effective refractory period, including
• A, Absolute refractory period, from the beginning
  of phase 0 to 60mv of repolarization, no
  response to stimulus
• State of Na channel, inactivation
• B, Local potential period, form the 60mv to
  55mg of phase 3, very strong stimulus can elicit
  local response but not action potential
• State of Na channel, most of them are
  inactivation
• Common properties of A and B, from the beginning
  of phase 0 to 55mv of phase 3, no action
  potential can be elicited, no matter how strong
  the stimulus is effective refractory period
• 2) Relative refractory period, from 60 mv to 80
  mv of phase 3, a stronger stimulus can elicit
  action potential, although the duration,
  amplitude and slope of the upstroke is shorter
  and smaller
• State of Na channel, part of them return to
  the resting state

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• 3) Supernatural period
• From 80 mv to 90 mv of phase 3, the
  excitability is higher than normal
• Na state. Most of the Na channel have returned
  to the resting condition.
• The potential is higher than the resting
  potential
• (3) Relationship between the excitability and
  contraction of myocardium
• No tetanus in cardiac muscle, systole and
  diastole occur alternately. It is very important
  for pumping blood to arteries.
• Premature excitation, premature contraction and
  compensatory pause
• Extra-stimulus premature excitation
  premature contraction compensatory pause

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• 2. Automaticity (Autorhythmicity)
• Concept Some tissues or cells have the ability
  to produce spontaneous rhythmic excitation
  without external stimulus.
• Different intrinsic rhythm of rhythmic cells
• Purkinje fiber, 15 40 /min
• Atrioventricular node 40 60 /min
• Sinoatrial node 90 100 /min
• Concept normal pacemarker, latent
  pacemarker, ectopic pacemarker
• (2) The mechanism that SA node controls the
  hearts rhythm (acts as pacemaker) rather than the
  AV node and Purkinje fiber
• The capture effect
• Overdrive suppression
• (3) Factors determining automaticity

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• Depolarization rate of phase 4
• 2)Threshold potential
• 3)The maximal repolarization potential

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• 3. Conductivity
• Pathways and characteristics of conduction in
  heart
• Pathways S-A node -- A-V node --- Bundle of
  His --- R.L. bundle branches --- Purkinje network
  ventricular muscles
• Conductive speed of different cardiac
  muscles
• Atrial myocardium, 0.4m/s nodal area of A-V
  junction, 0.02 m/s Purkinje network, 4m/s
  ventricle myocardium, 1m/s
• Characteristics
• 1) Delay in transmission at the A-V node (150
  200 ms) sequence of the atrial and ventricular
  contraction physiological importance
• 2) Rapid transmission of impulses in the
  Purkinje system synchronize contraction of
  entire ventricles physiological importance
• (2) Factors determining conductivity

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• Anatomical factors
• A. Gap junction between working cells and
  functional atrial and ventricular syncytium

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• B. Diameter of the cardiac cell conductive
  resistance conductivity
• 2) Physiological factors
• Slope of depolarization and amplitude of phase 0
• Fast and slow response cells
• Factors that affect the depolarization rate
  of phase 0
• B. Excitability of the adjacent unexcited membrane

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• III. Neural and humoral control of the cardiac
  function
• Vagus nerve and acetylcholine (Ach)
• Vagus nerve release Ach from
  postganglionic fiber M receptor on cardiac
  cells - K channel permeability increase but Ca
  2 channel permeability decrease
• 1) K channel permeability increase resting
  potential (maximal diastole potential) more
  negative excitability decrease
• 2) On SA node cells, K channel permeability
  increase the depolarization velocity at phase 4
  decrease maximal diastole potential more
  negative automaticity decrease heart rate
  decrease --- Negative chronotropic action
• 3) Ca2 channel permeability decrease
  myocardial contractility decrease negative
  inotropic action
• 4) Ca2 channel permeability decrease
  depolarization rate of slow response cells
  decrease conductivity of these cell decrease
  negative dromotropic action

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2. Effects of Sympathetic Nerve and catecholamine
on the Properties of Cardiac Muscle Sympathetic
nerve release norepinephrine from the
postganglionic endings epinephrine and
norepinephrine released from the adrenal glands
binding with ß1 receptor on cardiac cells
increase the Ca2 channel permeability
Increase the spontaneous depolarization rate at
phase 4 automaticity of SA node cell rise
heart rate increase Positive chronotropic
action Increase the depolarization rate (slope)
and amplitude at phase 0 increase the
conductivity of slow response cells Positive
dromotropic action Increase the Ca2
concentration in plasma during excitation
myocardial contractility increase -- positive
inotropic action
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Effect of autonomic nerve activity on the heart
Region affected Sympathetic Nerve
Parasympathetic Nerve
Increased rate of diastole depolarization
increased cardiac rate
Decreased rate of diastole depolarization
Decreased cardiac rate
SA node
Increase conduction rate
Decreased conduction rate
AV node
Decreased strength of contraction
Increase strength of contraction
Atrial muscle
Ventricular muscle
Increased strength of contraction
No significant effect
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IV The Normal Electrocardiogram
Concept The record of potential fluctuations of
myocardial fibers at the surface of the
body Waves of Normal ECG 1, The P wave, spread of
the depolarization wave through the atria. 2, The
QRS wave result from the spread of the
depolarization wave through the ventricles 3, The
T wave represents repolarization of ventricular
muscle
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4, P-R internal, from the beginning of the P wave
to the beginning of QRS wave, represents the
beginning of contraction of atrium and the
beginning of contraction of ventricle. 0.12
0.20 ms. Atrial ventricular delay 5, Q-T
internal The duration between the beginning of
QRS wave and the end of the T wave, or the
duration between the beginning of contraction of
the ventricle and almost the end of the
contraction or the duration between the
beginning of depolarization and the end of
repolarization
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6, P-R segment The duration between the end of P
wave and the beginning of QRS wave 7, S-T
segment The duration between the end of QRS and
the beginning of T wave. All ventricles are in
complete depolarization
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