Lecture 1B - Experiments done to determine the role of K in...

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Experiments done to determine the role of K + in generating resting membrane potential. Experiment: used the giant axon of squid (100x larger in diameter than some axons in mammalian species). Table of ion concentration: overall concentration of ions in squid neuron both intracellularly & extracellularly is much larger than mammalian neuron, but the relative distribution of ions across membrane is approximately the same for squid & mammalian neuron. Hodgkin & Katz discovered that the resting membrane potential of squid neuron is -58mV, which is roughly equal to E K+ . It indicates that the resting membrane potential may be highly related to relative [K + ]. To find out whether any of the other ion species (Na + , Cl - , or Ca ++ ) have any impact on resting membrane potential, they manipulated the extracellular [K + ] and measured resting V m after the manipulation. Prediction: if V m was solely driven by K + , then when they plotted the membrane potential of K + outside the cell, they would get a straight line (black line = slope for E K+ ) First use a very low extracellular concentration low resting membrane potential; higher level resting membrane potential increases. Experimental observations vary enough from prediction. Conclusion: there’s some weak contribution from Na + , Cl - , and Ca ++ , among some other minor species. [K + ] has such a high impact in real squid neurons on resting membrane potential b/c there’s a number of K + leak channels which are chronically open channels that allow flow of K + . At rest, tons of Na + channels (& Cl - and Ca ++ channels) are closed Have a combination of high number of opened K + channels & huge disparity in intracellular vs. extracellular [K + ]. The potential from difference in [Na + ] has no impact on resting membrane potential. This graph indicates that there’re some contribution of Na + , Cl - , and Ca ++ channels that are opened at rest, but the majority of open channels are K + channels. Resting membrane potential is not solely driven by K + .
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Membrane potential depends on relative concentration of ions & permeability (how many ion channels are opened). Direction of Cl - is flipped b/c it’s a negative ion.
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Hodgkin & Kats predicted that at peak of an AP, the membrane potential of neuron should be very similar to E Na+ . At peak of AP, the permeability of Na + is at its highest point. Using the same equation, if manipulated extracellular concentration of Na + , it would get a corresponding shift in magnitude of peak of AP. Experiment: they varied external [Na + Prediction: the graph would follow black line, for which the slope is the E Na+ . Observations matched prediction: indicated that at peak of AP, neurons are most permeable to Na
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This note was uploaded on 11/03/2010 for the course BIPN BIPN 142 taught by Professor Wang during the Summer '09 term at UCSD.

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Lecture 1B - Experiments done to determine the role of K in...

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