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Lecture 11

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PHGY 210
Michael Guevara

Sarah Margareta IbrahimWednesdayJanuary30th 2013Lecture 11Cardiovascular Physiology Part 5 out 91 Action PotentialsLast time we talked about the ECG Ventricular Action Potential2We have a typical ventricular action potentialwe have the resting potential For roughlyof the cardiac cycle the cell is resting At some point it res and you have the upstrokeof the AP and then theres a long phase of about 300 milliseconds which is the phaseplateau phase and then you have the repolarization of the action potential Now if we had on the same scale a picture of the AP from a nerve neuron or skeletal muscle cell how would it be different The plateau would be a lot shorter The AP would 1 or 2 milliseconds long In cardiac cells the action potential is very long If we recorded from atrial muscle instead of ventricular muscle you would see the same thing except that the action potential would be much shorter 150 milliseconds in duration The ventricular muscles are the working myocardium the ones that contract and create force3 Ionic Basic Underlying the Ventricular APSimilar to what weve gone over in the past Voltage is sitting at about 90mV in atrial muscle or ventricular muscle At some point the cell next to it will be excited and there will be an injection of current intracellularly and extracellularly and that will depolarize the cell So this rst part of the upstroke of the action potential is created by the local circuit current Depolarizes membrane at about a voltage of 60mV the sodium channels of the cell membrane will say hey The voltage is depolarizing I need to open up So the sodium channels will open up The concentration of sodium outside the cell is much higher 140 mmol than outside 510 mmol So that sodium will ood into the cell Is the inside of the cell at this time a negative voltage or positive voltage with respect to the outside Negative This will attract the sodium ions into the cell So both the concentration forces and the electrical forces are working to attract sodium into the cell and a huge amount of sodium is going to ood into the sodium channels that will depolarize membrane even further Other sodium channels that are close by are going to see how depolarized it is and open so its a positive feedback effect more the sodium channels openthe more the sodium comes into the cellthe more the cell is depolarizedmore the sodium channels are going to open Within a millisecond or less all the sodium channels open and then they all close right away so that you have this pulse of sodium into the cell which generates the upstroke of the action potential like other action potentials that we studiedWhat cause the repolarization You have a bunch of K channels potassium channels in the cell Theres on set of those channels which are very lazy and they will see that the voltage has gone up and stayed up but it takes time before they open So then the K channels open and since the concentration of K inside the cell is much higher 140mmol and outside its lower around 5mmol So potassium leaves the cell and this causes the cell to become more negative it will hyperpolarize and that generates the repolarization of the membraneIn the second graph were plotting the permeability to an ion potassium sodium and calcium The permeability means how easy can the membrane get through the ion If you have a membrane with no channels in it the permeability is zero for all ions Since the membrane is made of lipids and ions dont like to be inside a fatty environment you need these ion channels Theres a bit of an exception to this how can an ion get from one side of the membrane to the other without a channel The sodium potassium pump In cardiac cells you have a sodiumcalcium exchanger that does the same thing So you 1
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