Therefore k ions are free to move and when they reach

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Unformatted text preview: c muscle permeability changes for: –  Na+ •  ⇑ at onset of depolarization, ⇓ during repolarization Slow vs Fast cardiac cell •  Relates to the channels that open during depolarization –  Typical cardiac muscle have both fast Na+ channels and slow Ca++/Na+ channels that open during depolarization –  Specialized excitatory cells like the SA node only slow Ca++/Na+ channels are operational during depolarization increasing depolarization time •  Tetradotoxin blocks fast Na+ channels selectively changing a fast response into a slow response –  Ca++ •  ⇑ at onset of depolarization, ⇓ during repolarization –  K+ •  ⇓ at onset of depolarization, ⇑ during repolarization Greatest permeability for K is during the resting potential 6 Any concentration gradient can be offset by an electric gradient Passive ion movement across cell •  Considerations –  Concentration gradient •  high to low Resting membrane potential (Er) •  During the Er in cardiac muscle, fast Na+ and slow Ca++/Na+ are closed, K+ channels are open. •  Therefore K+ ions are free to move, and when they reach their Nerst equilibrium potential, a stable Er is maintained –  Electrical gradient •  opposite charge attract, like charge repel –  Membrane permeability •  dependant on ion channels (open or closed) •  If ion channels are open, an ion will seek its Nerst equilibrium potential –  concentration gradient favoring ion movement in one direction is offset by electrical gradient Nernst Eq. = no net movement of ions Na+/K+ ATPase (pump) •  The Na+/K+ pump which is energy dependent operates to pump Na+ out & K+ into the cardiac cell at a ratio of 3:2 –  therefore as pumping occurs, there is net loss of one + charge from the interior each cycle, helping the interior of the cell remain negative –  the protein pump utilizes energy from ATP –  Digitalis binds to & inhibits this pump Ca++ exchange protein •  In the cardiac cell membrane is a protein that exchanges Ca++ from the interior in return for Na+ that is allowed to enter the cell. •  The function of this exchange protein is tied to the Na+/K+ pump –  if the Na+/K+ pump is inhibited, function of this exchange protein is reduced & more Ca++ is allowed to accumulate in the cardiac cell ⇑ contractile strength. More Ca inside the cell the greater the contraction Drug Stimuli that occur during the ERP are ineffective Refractory Period •  Absolute (effective = ERP) Longer duration –  unable to re-stimulate cardiac cell –  occurs during the plateau Wave of depolarization begins here SA node •  Relative (RRP) resting period •  Normal pacemaker of the heart •  Self excitatory nature –  less negative Er –  leaky membrane to Na+/CA++ –  only slow Ca++/Na+ channels operational –  spontaneously depolarizes at fastest rate •  overdrive suppression –  requires a supra-normal stimulus –  occurs during repolarization Beginning of •  In a Slow response cardiac muscle cell the relative refractory period is prolonged and the refractory period is about 25% longer –  in AV node & bundle this serves to protect the ventricles from supra-ventricular arrhythmias –  contracts feebly SA node doesnt have a stable resting potential due to its leaky membrane SA node cells are muscle cells but thuey contract weakly. Atrial arrhythmia During the RRP the mus le is difficult to Excite but a very strong excitatory signal will excite it Cardiac cycle begins at the sinus node in the w...
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