Lecture_09_29_11 - I.D.1.b. Different permeabilities to...

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I.D.1.b. Different permeabilities to each ion In the GHK equation, the P ion terms are permeabilities to specific ions that are due to differences in the number of open, selectively permeable, protein channels embedded within the cell membrane. The GHK is different from the Nernst equation in several respects 1) The GHK equation is used when a membrane is permeable to multiple ions whereas the Nernst equation applies to membranes permeable to a single ion. 2) The GHK equation includes permeabilities to multiple ions that can traverse the membrane whereas the Nernst equation does not account for permeabilities . In fact, you should be able to show that when the cell membrane is only permeable to one ion the GHK equation simplifies to the Nernst equation!! The GHK is similar to the Nernst equation as both equations use concentrations differences . We cannot calculate V m (the resting membrane potential) using the GHK unless the permeabilities are known . Note: Vm represents the voltage across the cell membrane E k is a one ion theoretical condition representing the voltage a single ion will create across a membrane The GHK equation is used to calculate (or predict) the voltage across a cell membrane due to (1) multiple ions with (2) different concentrations across the membrane and (3) different permeability. the GHK equation @ 17° is Note: the terms are flipped because chloride is a negative ion. [ ] [ ][ ] [] [ ] [ ] + + + + = + + + + out cl in na in k in cl out na out k m Cl P Na P K P Cl P Na P K P V 10 log 5 . 57 Note When applying this equation, impermeable ions are irrelevant. Often in class we will talk about changing the concentration of a permeable ion. For example, “we double the sodium concentration outside of the neuron”. When we do this we are also adding a non- permeable anion which is not mentioned but implied. This is because the non-permeable ion is irrelevant with respect to the membrane potential
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I.D.1.c. What happens if the permeability to one ion suddenly changes?
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This note was uploaded on 11/16/2011 for the course NPB NPB 100 taught by Professor Campbell during the Spring '10 term at UC Davis.

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Lecture_09_29_11 - I.D.1.b. Different permeabilities to...

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