Action Potential

1. An introduction to Cardiology
Nikhil Vaishnav, Clinical coordinator
Department of Paramedical & Health Science

2. Cardiac Action Potential
CHAPTER-9

3. Introduction
Image source : Google
 Cardiac action potential is a brief changes in voltage(membrane potential )
across the cell membrane of the heart cells .
 This is caused by movement of charged ions between the inside and
outside of the cell through protein called ion channels.
Action potential in heart initiated by group of specialized cells called SA
node.
SA node produce roughly 60-100 action potential every minute .
 Action potential activity within the heart can be recorded to produce an
ECG

5. Terminology
 Resting membrane potential: It is the potential difference between inside
and outside of the cell under resting condition. Outside of the cells there is
sodium and chloride and inside potassium ion.
Depolarization: When impulse reaches the heart muscles , RMP is
abolished , the interior of heart muscles becomes positive and outside
becomes negative
Repolarization: Within a short time, muscle obtains RMP again; inside is
negative charged, outside positive charged.

6. Phases of Cardiac action potential
Phase 0 Depolarization
 Phase 1 Early repolarization
 Phase 2 Plateau phase
 Phase 3 Rapid Repolarization
 Phase 4 Resting phase

7. Phases of cardiac action potential

8. Phases of cardiac action potential

9. Phase 4 Resting phase
 It occurs when cell is at rest condition (diastole).
 The resting potential in a cardiomyocytes is -90 mv due to a constant
outward leak of potassium .
 Na+ , Ca+2 channels are closed at resting membrane potential.

11. Phase 0 Depolarization
 This phase consist of a rapid positive change in voltage across the cell
membrane .
 It occurs due to a net flow of positive charge into the cell .
 In ventricular cells it is produced by the activation of Na+ channels ,
there is increased membrane flow of sodium.
Na+ channels are activated when an action potential arrives from a
neighbouring cell through gap junctions .

12. When Na+ channel opens there is larger influx of sodium into the cells.
Voltage increased to +50 mv.
 In SA node membrane voltage is increased due to activation of L type
calcium channels.

15. Phase 1 Early Repolarization
 This phase begins with the rapid inactivation of Na+ channel .
There is reduction of Na+ movement into the cell .
 At the same time , K+ channels opens and closed rapidly .
 It makes the membrane potential slightly more negative

18. Phase 2 Plateau phase
 This phase is responsible for prolonging the cardiac action potential
Ca+2 channels open to keep the cells polarized .
Increased Ca+2 concentration increases the activity of Na+/Ca+2 exchanger
Increased Na+2 entry causes the Na+ /K+ pump . It results in membrane
potential remaining relatively constant.
This phase is responsible for large duration of action potential .

20. Phase 3 Rapid repolarization
 L type Ca+2 channels closed while slow delayed K+ channels opens.
Persistent outflow of potassium brings membrane potential brings towards
resting potential of -90 mv to prepare the cell for a new cycle of
depolarization.

22. Effective Refractory period(ERP)
 The effective refractory period is the amount of time in which the cell can
not respond to a new conducted stimuli .
 During this period heart stays in rhythm and prevents Arrthymia .
 It is composed of phase 0, 1 , 2 and 3 .
 The cell will not respond to a stimulus that comes from a neighboring cell.

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