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BIOC63H3 (38)

lecture 2

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Biological Sciences
Ivana Stehlik

Cardiovascular system The HeartElectrophysiology of Cardiac Muscle and the Origin of the Heart Beat An action potential AP is the transient depolarization of a cell as a result of activity of ion channelsThe cardiac AP is considerably longer than those occurring in nerve or skeletal muscle 300 ms vs13 msThis is due to the presence of a plateau phase in cardiac muscle which lasts for 200300 msecVentricular muscle action potential Figure 1a and Figure 2 Initiation of the action potential At rest the membrane is most permeable to K and the resting potential is primarily dependent on the K concentration gradientAn AP is initiated when the membrane is depolarized to a threshold potential 55 mVThe initial depolarization results from transmission from an adjacent cell through intercalated disks Figure 4At the threshold potential the inward current caused by entry of Na through voltagegated Na channels Ia becomes large enough Nto overcome the outward current through K channels and thus causes further depolarizationThis in turn activates more Na channelsThe depolarization thus becomes selfgenerating and so results in a very rapid upstroke phase 0At this point the membrane is more permeable to Na than K because of the open Na channels The Na concentration gradient therefore becoming the major determinant of membrane potential and the cell moves towards the equilibrium potentialfor Na 65 mVIt does not reach this potential both because it is limited by the existing K permeability and because of rapid inactivation shutting of the Na channelsThe Na channels cannot be reactivated until the potential becomes more negative than55 mVTherefore another AP cannot be initiated until the cell repolarizes to at lesat this potential absolute refractory periodAt slightly more negative potentials some Na channels reactivate allowing an AP to be initiated by a sufficiently large stimulus relative refractory periodAll Na channels are reactivated by the time the cell is completely repolarizedThe refractory period and the length of the AP compared to the twitch means that unlike skeleton muscle cardiac muscle cannot be tetanized Figure 1cThe plateau phaseAt the end of the upstroke membrane Na permeability returns to its resisting valve and in skeleton muscle this results in rapid repolarizationIn cardiac muscle however the membrane potential decays slowly over250 ms before a more rapid repolarization phaseThis period of slow decay is the plateau phase phase 2 and is primarily due to Ca2 current entering the cell via voltagesensitive Ltype Ca2 channels which activate relatively slowly when the membrane potential becomes positive than35mV TheResultant Ca2 current slow inward current or I coupled with a reduced K outward SIcurrent is sufficient to slow repolarization until the potential falls to 20 mVThe length of the plateau is related to slow inactivation of Ca2 channelsCa2 entry during the plateau is vital for contraction blockers of Ltypes of Ca2 channels example dihydropyridines reduce force
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