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Electrical Properties of the Heart

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  1. Electrical Properties of the Heart

  2. Review of Heart Muscle • Cardiocytes, myocardium • Branched cells • Intercalated discs- (desmosomes) and electrical junctions (gap junctions). • Has actin and myosin filaments • Has low resistance (1/400 the resistance of cell membrane) • Atrial syncytium: Network of connecting cardiac cells • Ventricular syncytium:Network of connecting cardiac cells • Fibrous insulator exists (Bundle of His)between atrium and ventricle (what would this do to any electrical activity trying to go through?) Figure 9-1; Guyton & Hall

  3. If the electrical signals from the atria were conducted directly into the ventricles across the AV septum, the ventricles would start to contract at the top (base). Then the blood would be squeezed downward and trapped at the bottom of the ventricle. • The apex to base contraction squeezes blood toward the arterial opening at the base of the heart. • The AV node also delays the transmission of action potentials slightly, allowing the atria to complete their contraction before the ventricles begin their contraction. This AV nodal delay is accomplished by the naturally slow conduction through the AV node cells. (Why are they slow conductors? Small diameter cells, fewer channels)

  4. Fibers within the heart • Specialized Fibers • are the fibers that can spontaneously initiate an AP all by themselves! • The AP will spread to all other fibers via gap junctions • AKA “leading cells” • But they are also muscle, so they do contract, albeit feebly! • They are not nerves!!!! • Contractile Fibers • These maintain their RMP forever, unless brought to threshold by some other cell • They cannot generate an AP by themselves • AKA “following cells” • But they do have gap junctions, so once they’re triggered, they will help spread the AP to neighbors.

  5. Pathway of Heartbeat • Begins in the sinoatrial (S-A) node • Internodal pathway to atrioventricular (A-V) node • Impulse delayed in A-V node (allows atria to contract before ventricles) • A-V bundle takes impulse into ventricles • Left and right bundles of Purkinje fibers take impulses to all parts of ventricles KEY Red = specialized cells; all else = contractile cells

  6. Only conducting path between atria and ventricles Divides into left and right bundles Time delay of 0.04sec Fast conduction; many gap junctions at intercalated disks Purkinje System A-V Bundles(Bundle of His)

  7. How can these Specialized fibers spontaneously “fire?” • Can’t hold stable resting membrane potential • Potentials drift (gradual depolarization) • During this time, they have a gradually increasing perm to Na+ and less leaky to K+ (more “+” inside causes cell to depolarize, remember?) Na+

  8. Specialized fibers Notice slow rise from rest to threshold. This is called the “prepotential” or “pacemaker potential” Only specialized fibers of the heart can do this. This is what gives the heart it’s rhythm.

  9. Specialized fibers of conductive system • These rhythms can ALSO be modified by the ANS • Neurotransmitters can cause faster or slower rise to threshold by altering ion permeability. • Acetylcholine (ACh) slows the heart rate (parasympathetic division of ANS) • Norepinephrine (NE) speeds up the heart rate (sympathetic division of ANS) K+ efflux

  10. Sympathetic and Parasympathetic • Sympathetic – speeds heart rate by Ca++ & Na+ channel influx and  K+ permeability/efflux (increases sodium and calcium permeability) • Parasympathetic – slows rate by  K+ efflux & Ca++ influx (decreases sodium and calcium permeability) • Which neurotransmitter will cause your heart to pound rapidly? • Norepinephrine

  11. Spread of Depolarization

  12. Direction of Depol Resting Cell + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +

  13. Direction of Depol + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + Stim microelectrode

  14. Direction of Depol + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + Depolarizing Current! Stim microelectrode

  15. Direction of Depol + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + Depolarizing Current! Stim microelectrode

  16. Direction of Depol + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + + + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + Depolarizing Current! Stim microelectrode

  17. Direction of Depol - - - + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + Stim microelectrode

  18. Direction of Depol - - - - - - - - + + + + + + + + + + + + + + + + + + + + + + + + + + ++ + + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + ++ + + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -- - -- + + + + + + + + + + + + + + + + + + + + + + + + + + Stim microelectrode

  19. Direction of Depol - - - - - - - -- - - - -- - - + + + + + + + + + + + + + + + + + + + + + ++ + ++ + + + + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + ++ + ++ + + + + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -- - -- - - - - - -- - + + + + + + + + + + + + + + + + + + + + Stim microelectrode

  20. Direction of Depol - - - - - - - -- - - - -- - -- - - - - + + + + + + + + + + + + + + + + + ++ + ++ + + + ++ + + + - - - - - - - - - - - - - - - - - - - - - - - + ++ + ++ + + + ++ + + + - - - - - - - - - - - - - - - - - - - - - - - - - - -- - -- - - - - - -- -- - - - - + + + + + + + + + + + + + + + + + + Stim microelectrode

  21. Direction of Depol - - - - - - - -- - - - -- - -- - - - -- - - - - + + + + + + + + + + + + + ++ + ++ + + + ++ + + ++ + + + + - - - - - - - - - - - - - - - - + ++ + ++ + + + ++ + + ++ + + + + - - - - - - - - - - - - - - - - - - - -- - -- - - - - - -- -- - - - -- - - - - - + + + + + + + + + + + + + Stim microelectrode

  22. Direction of Depol - - - - - - - -- - - - -- - -- - - - -- - - - -- - - - - - - - + + + + + + + ++ + ++ + + + ++ + + ++ + + + ++ + + + + - - - - - - - - - + ++ + ++ + + + ++ + + ++ + + + ++ + + + + - - - - - - - - - - - - -- - -- - - - - - -- -- - - - -- - - - - -- - - - - - - - + + + + + + + Stim microelectrode

  23. Direction of Depol - - - - - - - -- - - - -- - -- - - - -- - - - -- - - - - - - -- - - - - - - + + + ++ + ++ + + + ++ + + ++ + + + ++ + + + ++ + + + - - - + ++ + ++ + + + ++ + + ++ + + + ++ + + + ++ + + + + - - - - - -- - -- - - - - - -- -- - - - -- - - - - -- - - - - - - -- - - - - - - + + Stim microelectrode

  24. Direction of Depol - - - - - - - -- - - - -- - -- - - - -- - - - -- - - - - - - -- - - - - - -- - - + ++ + ++ + + + ++ + + ++ + + + ++ + + + ++ + + ++ + + + ++ + ++ + + + ++ + + ++ + + + ++ + + + ++ + + + ++ + - - -- - -- - - - - - -- -- - - - -- - - - - -- - - - - - - -- - - - - - - - - - - Stim microelectrode

  25. Direction of Depol - - - - - - - -- - - - -- - -- - - - -- - - - -- - - - - - - -- - - - - - -- - - + ++ + ++ + + + ++ + + ++ + + + ++ + + + ++ + + ++ + + + ++ + ++ + + + ++ + + ++ + + + ++ + + + ++ + + + ++ + - - -- - -- - - - - - -- -- - - - -- - - - - -- - - - - - - -- - - - - - - - - - - Stim microelectrode

  26. Direction of Depol - - - - - - - -- - - - -- - -- - - - -- - - - -- - - - - - - -- - - - - - -- - - + ++ + ++ + + + ++ + + ++ + + + ++ + + + ++ + + ++ + + + ++ + ++ + + + ++ + + ++ + + + ++ + + + ++ + + + ++ + - - -- - -- - - - - - -- -- - - - -- - - - - -- - - - - - - -- - - - - - - - - - - Depol = spread of surface NEG charge Stim microelectrode

  27. Direction of Repolarization Begin Repolarization - - - - - - - -- - - - -- - -- - - - -- - - - -- - - - - - - -- - - - - - -- - - + ++ + ++ + + + ++ + + ++ + + + ++ + + + ++ + + ++ + + + ++ + ++ + + + ++ + + ++ + + + ++ + + + ++ + + + ++ + - - -- - -- - - - - - -- -- - - - -- - - - - -- - - - - - - -- - - - - - - - - - - - - - - Stim microelectrode

  28. Direction of Repolarization - - - - - - - -- - - - -- - -- - - - -- - - - -- - - - - - - -- - - - - - -- - - + ++ + ++ + + + ++ + + ++ + + + ++ + + + ++ + + ++ + + - - - + ++ + ++ + + + ++ + + ++ + + + ++ + + + ++ + + + ++ + - - -- - -- - - - - - -- -- - - - -- - - - - -- - - - - - - -- - - - - - - - - - - Stim microelectrode

  29. Direction of Repolarization + + + + + +- - - - -- - -- - - - -- - - - -- - - - - - - -- - - - - - -- - - - - - - - -+ + + + ++ + + ++ + + + ++ + + + ++ + + ++ + + - - - - - - ++ + + + ++ + + ++ + + + ++ + + + ++ + + + ++ + + + + + + - - - - - -- -- - - - -- - - - - -- - - - - - - -- - - - - - - - - - Stim microelectrode

  30. Direction of Repolarization + + + + + ++ + + + + + - -- - - - -- - - - -- - - - - - - -- - - - - - - - - - - - - -- - - - - - ++ + + ++ + + + ++ + + + ++ + + ++ + + - - - - - -- - - - - ++ + + ++ + + + ++ + + + ++ + + + ++ + + + + + ++ + + + + -- -- - - - -- - - - - -- - - - - - - -- - - - - - - - Stim microelectrode

  31. Direction of Repolarization + + + + + ++ + + + + ++ + + + + +- - - - -- - - - - - - -- - - - - - - - - - - - -- - - - - -- - - - - - - - + + + ++ + + + ++ + + ++ + + - - - - - -- - - - -- - - - - - - - - - + + ++ + + + ++ + + + ++ + + + + + ++ + + + ++ + + + + + + - - - -- - - - - - - -- - - - - - - - Stim microelectrode

  32. Direction of Repolarization + + + + + ++ + + + + ++ + + + + ++ + + + + + + + - - - - - - - - - - - - - - -- - - - - -- - - - - - - -- - - - - - - - - - --- + + ++ + + - - - - - - -- - - - - -- - - - - - - -- - - - - - - - - - --- + + ++ + + + + + + + ++ + + + + ++ + + + + ++ + + + + + + + - - - - - - - - Repolarization= spread of positive surface charge Stim microelectrode

  33. Direction of Repolarization + + + + + ++ + + + + ++ + + + + ++ + + + + + + ++ + + + + - - - - - - -- - - - - -- - - - - - - -- - - - - - - - - - ---- - - - - - - - - - - - - - - -- - - - - -- - - - - - - -- - - - - - - - - - ---- - - - - - - - + + + + + ++ + + + + ++ + + + + ++ + + + + + + ++ + + ++ + Stim microelectrode

  34. Direction of Repolarization + + + + + ++ + + + + ++ + + + + ++ + + + + + + ++ + + ++ + - - - - - - -- - - - - -- - - - - - - -- - - - - - - - - - ---- - - - - - - - - - - - - - - -- - - - - -- - - - - - - -- - - - - - - - - - ---- - - - - - - - + + + + + ++ + + + + ++ + + + + ++ + + + + + + ++ + + ++ + Repolarization= spread of POS surface charge Stim microelectrode

  35. Depolarization/Repolarization Cycle in the Atria

  36. Depolarization Begins =Resting cell =Depol cell

  37. =Resting cell =Depol cell

  38. =Resting cell =Depol cell

  39. =Resting cell =Depol cell

  40. Depolarization Complete =Resting cell =Depol cell

  41. Repolarization Begins =Resting cell =Depol cell

  42. =Resting cell =Depol cell

  43. =Resting cell =Depol cell

  44. Repolarization Complete =Resting cell =Depol cell

  45. Depolarization/Repolarization Cycle in the Ventricles

  46. Depolarization Begins =Resting cell =Depol cell

  47. =Resting cell =Depol cell

  48. =Resting cell =Depol cell

  49. =Resting cell =Depol cell

  50. Depolarization Complete =Resting cell =Depol cell