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Lec 8

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University of Toronto St. George
Mc Kay+ French

46 Cardiac Potential From action potential to aortic flow January 25 2008 6BCardiac PotentialUGeneral Characteristics of Cardiac PotentialResting membrane potential and action potentials vary according to cell type o Sinus nodal fibres55 to 60 mV o Cardiac muscle85 to 95mV o Purkinje fibres90 to 100 mV Cardiac autorhythmic cells dont have a true resting potentialhave pacemaker potential o Potential never stays constantmembrane potential drifts between action potentialsDirectly related to how many times the heart beatsAction potential travels throughout the heart and triggers rhythmic beating contraction and relaxationinflux into membranehave o After an action potential leaky membrane allows Naslow depolarization to bring potential to threshold 40 mV to have another AP o No nervous stimulation required nonpacemaker potentialAction potential of the ventricular muscle contractile fibre 7BUPacemaker potentialthrough I channelcell membrane becomes positive and reaches 1 At the SA node the membrane is leaky for Nafthreshold of 40 mv 22Na channel openinflux of Ca and Na and causes AP 2 Ca23 CaNa channel become inactivated after a few 400 ms and closechannels open 4 The delayed K5 K efflux causes hyperpolarizationrepolarizationcell returns to resting potentialchannel closes and inward leaking of Na will begin again and repeat 6 After a few ms KAction potential in SA node fibres is slow develop than in ventricular fibres and recovers gradually electrical current from Na leakage at SA nodespreads quickly through myocardial fibres due to lattice network and gap junctions1
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