1. 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

2. 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 1 2 3 4

3. (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)

4. 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 .

5. (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

6. 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

7. 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

8. 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.

9. (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

10. 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

14. 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

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

16. 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

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

18. 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

19. 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

20. 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 β1 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

22. 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

23. 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

24. 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 – 0.20 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

25. 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