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Cardiac Action Potentials

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Cardiac Action Potentials

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    1. Cardiac Action Potentials

    2. Reading: Klabunde, Cardiovascular Physiology Concepts Chapter 2 (Electrical Activity of the Heart) pages 18-26

    4. The Cardiac Action Potential Types Are Either Fast Or Slow Response Fast-response action potentials Atrial myocardial fibers Ventricular myocardial fibers Purkinje fibers Slow-response action potentials Sinoatrial node Atrioventricular node

    5. Differences between fast and slow cardiac action potentials:

    7. Fast Response Action Potential

    8. Phases of the Fast Response Action Potential Phase 0 = Depolarization Phase 1 = Partial Repolarization Phase 2 = Plateau Phase 3 = Repolarization Phase 4 = Resting Membrane Potential

    9. Phase 0 The characteristics of the upstroke of the action potential depend almost entirely on inward movement of Na+

    10. Phase 0

    11. Phase 1 Inactivation of Na+ channels ends Transient outward K+ current

    13. Phase 2 (Plateau) What produces the plateau? Slow inward Ca++ currents (L-type calcium channels) Counterbalanced by: Outward K+ currents

    14. Action potential and ionic fluxes

    15. Phase 2 (Plateau) Ventricular contraction persists throughout the action potential, so the long plateau produces a long action potential to ensure forceful contraction of substantial duration

    16. Calcium-Induced Calcium Release When the myocyte is depolarized calcium enters the cell via L-type calcium channels. The amount of calcium that enters the cell is small, but this triggers the release of a large amount of calcium into the cytosol from the sarcoplasmic reticulum which results in binding of myosin to actin and contraction of the myocyte.

    17. Phase 3 Outward K+ current is mainly responsible for repolarization Na+ channel recovery begins during Relative Refractory Period

    20. Phase 4 Restoration of ionic concentrations Na+,K+-ATPase Na+-Ca++ Exchanger ATP-driven Ca++ Pump

    21. Restoration of Ionic Gradients

    22. Na/Ca Exchanger

    23. Resting Membrane Potential in Cardiac Cells Depends mainly on the conductance of K+ Determined mainly by the ratio of intracellular to extracellular concentration of K+ Measured value is slightly less negative than predicted because of small but finite conductance of Na+ Na+,K+-ATPase

    24. Slow Response Action Potentials

    25. Slow Response Action Potential Phase 0 Phase 2 Very brief Phase 3 Not separated clearly from phase 2 Phase 4 Note: Phase 1 is absent

    26. Slow Response Action Potential: Phase 0 Depolarization is mainly by Ca++ influx

    28. Refractory Periods: Effective (ERP) and Relative (RRP)

    29. Automaticity (Pacemaker Cells) Diastolic Depolarization Inward Na+ (not via typical Na+ channels) Ca++ influx K+ efflux (opposes effects of other ions)

    30. Autonomic neurotransmitters

    31. Autonomic neurotransmitters

    33. Conduction System

    34. Sinus Rhythm The SA (Sinus) Node is the heart�s dominant pacemaker. The ability of a focal area of the heart to generate pacemaking stimuli is known as Automaticity. The depolarization wave flows from the SA Node in all directions.

    35. Overdrive Suppression

    36. Overdrive Suppression

    37. Overdrive Suppression Automaticity of pacemaker cells becomes depressed after a period of excitation at a high frequency Due to activity of Na+, K+-ATPase. At higher heart rates more Na+ is extruded than K+ enters the cell > tends to hyperpolarize the cells Slow diastolic depolarization requires more time to reach threshold

    38. THE END

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