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14 Cv Excitation

14 Cv Excitation - CARDIAC EXCITATION page 1 AC Brown A6j-a...

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CARDIAC EXCITATION AC Brown page 1 A6j-a MYOCARDIAL CELL PROPERTIES A. Action and Resting Potentials 1. Ventricular myocardium a. resting potential: about -80 or -90 mv 1) similar to other excitable tissue 2) not normally self-excitatory b. action potential phases (see numbers on figure) 0) initial rapid depolarization 1) small initial repolarization 2) plateau (long) 3) repolarization 4) resting potential c. action potential duration 1) about 0.3 seconds at rest (ventricular) 2) shortens with increasing heart rate d. ionic basis: channels 1) fast Na + channels a) regenerative, Hodgkin cycle b) open briefly upon depolarization, then inactivated (A and I gates) c) remain inactivated until after repolarization 2) slow Ca 2+ - Na + channels a) admit both Ca 2+ (mainly) and Na + b) open upon depolarization, delayed 3) resting K + channels a) open when membrane resting, close upon depolarization 4) transient K + channels a) open briefly upon depolarization 5) slow (delayed) K + channels a) open later upon depolarization e. sequence 1) resting state: P K high, P Na & P Ca low 2) membrane depolarizes to threshold due to conducted excitation 3) fast Na + channels open (phase 0 ), leading to rapid depolarization 4) fast Na + channels close and transient K + channels open briefly, leading to small repolarization (phase 1 ) 5) slow Ca 2+ - Na +
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