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Electrophysiology of the Heart: Action Potentials and Cardiac Response

This chapter explains the different phases of the action potential in the heart, including depolarization and repolarization. It also discusses the properties of cardiac muscle cells and the conductive system of the heart. Furthermore, it explores the cardiac response to changes in the autonomic nervous system.

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Electrophysiology of the Heart: Action Potentials and Cardiac Response

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  1. CHAPTER 12 Electrophysiology of the Heart

  2. Action Potentials • Polarized or resting state • Resting membrane potential (RMP)

  3. Polarized State • Fig. 12-1. The polarized state. For each Na+ ion that diffuses into the cell, about 75 K+ ions diffuse out of the cell. The result is a deficiency of positive cations inside the cell; this is a cell with a negative charge.

  4. THE FIVE PHASES OF THE ACTION POTENTIAL

  5. Depolarization: Trigger for Myocardial Contraction • Phase 0: Rapid depolarization • Early phase

  6. Action Potentials • Fig. 12-2. The action potential and the Na+, K+, and Ca++ changes during phases 0, 1, 2, 3, and 4.

  7. Repolarization: • Process of heart cells returning to their resting state • Phase 1: Initial repolarization • Phase 2: Plateau state • Phase 3: Final rapid repolarization • Phase 4: Resting or polarized state

  8. Properties of the Cardiac Muscle Composed of Two Cardiac Cells • Contractile muscle fibers • Autorhythmic cells

  9. Four Specific Properties • Automaticity • Excitability • Conductivity • Contractility

  10. Four Specific Properties • Automaticity • Unique ability of the cells in the SA node (pacemaker cells) to generate an action potential without being stimulated.

  11. Action Potential of Pacemaker and Nonpacemaker Myocardial Cells • Fig. 12-3. Schematic representation comparing action potential of pacemaker and nonpacemaker (working) myocardial cells.

  12. Four Specific Properties • Excitability (irritability) • Ability of a cell to reach its threshold potential and respond to a stimulus or irritation

  13. Four Specific Properties • Conductivity • Ability of the heart cells to transmit electrical current from cell to cell throughout the entire conductive system.

  14. Four Specific Properties • Contractility • Ability of cardiac muscle fibers to shorten and contract in response to an electrical stimulus

  15. Refractory Periods • Absolute refractory period • Relative refractory period • Nonrefractory period

  16. Absolute Refractory Period • Time in which the cells cannot respond to a stimulus • Phases 0, 1, 2, and about half of 3 represent the absolute refractory period

  17. Relative Refractory Period • Time in which repolarization is almost complete • Strong stimulus may cause depolarization of some of the cells • Second half of phase 3 represents the relative refractory period

  18. Nonrefractory Period • Occurs when all the cells are in their resting or polarized state • Phase 4 represents the nonrefractory period

  19. Conductive System of the Heart • Fig. 12-4. Conductive system of the heart.

  20. Cardiac Response to Autonomic Nervous System Changes Table 12-1

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