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Medical Why Does Permeability Alter Membrane Potential?

  1. Sep 11, 2011 #1
    I've just finished reading through the generally well-written Wikipedia article on membrane potential:

    http://en.wikipedia.org/wiki/Membrane_potential

    I found almost everything to be clear, but one question is nagging at me. Why do changes in membrane permeability directly affect membrane potential? The Wikipedia article says:

    I can understand why membrane permeabilities affect the equilibrium potential, but why should they affect the membrane potential instantaneously? Isn't the potential totally determined by the concentrations of charge inside and outside the membrane? Changing permeabilities by opening ion channels should shift the equilibrium potential and cause a gradual change in membrane potential in the direction of equilibrium, but they should not change the membrane potential immediately. Am I misunderstanding something? Thanks.
     
  2. jcsd
  3. Sep 11, 2011 #2

    Pythagorean

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    "Immediately" is a relative term. Voltage-gated channels do not require us to wait on the charged particles to spread down the axon. Instead, we have a lot of channels open up in series down the axon and at each a point, a new current comes into the axon through its own gate. So no, it's not immediate, but it's faster than if the charged particles only diffused classically down the axon (much faster).
     
  4. Sep 12, 2011 #3

    Andy Resnick

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    I use sodium transport as an experimental readout by measuring the equivalent short-circuit current across a sample of epithelial tissue, and this is due to an apical sodium channel (ENaC) and a few baso-lateral transporters (Na-K-ATPase and RomK) which result in a vectoral transport of sodium.

    I can verify that the measured voltage is in fact due to sodium transport (as opposed to some other ion) by plugging ENaC with amiloride. The transepithelial current drops by 90% over a period of seconds when this happens, which should give you an idea of the timescale.
     
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