Action Potential

  1. This is about voltage clamping:

    When Hodgkin and Huxley voltaged clamped a nerve - when hyperpolarised there was a small inward current, why is that?

    and when they used a depolarising clamp, there is firstly a brief capacitative current - this current is outwards, why?:confused:
     
  2. jcsd
  3. Hyperpolarize is a term used by convention to mean that the inside of the membrane is more negative then the outside. I know it is confusing when your first learning it since it could always go either way. So the cell membrane is more negative:
    (1) ask yourself what channels are being opened when the membrane is hyperpolarized or made more negative inside then the outside.

    (2)Of those channel(s) what ions are they permeable to?

    (3)What are the [ion] inside and outside the neuron.

    That should be enough information. If they gave you the a voltage set by the actual voltage clamp then you probably have to calculate the Nersnt Potential or more generally the reversal potential.
     
  4. When they depolarize the cell making it the membrane more positive with respect to the outside, there is charge that is "charging" the membrane (like a capacitor).
     
  5. somasimple

    somasimple 716
    Gold Member

    IMHO, it is less positive than the outside. (just conventions)
    It's the traditional way for calculation but free/mobile positive ions (Na, K) aren't arranged in the whole cell but more closer to the membrane (Huxley, Mackinnon...). It's the fate of Gauss and Coulomb's laws. (But they weren't included in these early studies).

    It may be seen as a capacitor but it's only the ions moving under the electrical influence of circuit.
     
  6. Is the inward current chloride ions?

    Does the direction of "current" take into account of the charge of the ion?

    e.g. when you say inward current of Sodium - sodium ion is positive?
     
  7. somasimple

    somasimple 716
    Gold Member

    That is the real problem when you mix electricity (electrical circuit) and ions mobility. You're quickly lost!

    Na and K ions are ever positive. The membrane isn't polarized at all but ions are stuck on it on both sides. If ions crosses membrane thu ions channel, it removes a charge from a side (it decreases the positive charges of this one, it becomes more negative), but the other side becomes more positive.

    So your question is not complete because you may have an inward negative current or a positive one. It depends of the tranported charges.

    An inward current of sodium depolarizes the external side (becomes less positive) and the internal side becomes less negative (or more positive).
     

  8. Inward positive ions or outward negative ions leads to a net positive current into the cell.
    Outward positive and inward negative leads to a net negative current into the cell.

    In your question earlier, the neuron is being hyperpolarized meaning lowering the membrane potential below the resting potential. (Just in general to determine the potential of the cell, you must analyze the permeable ions and their respective concentrations inside and outside the cell.) So at resting potential the channels that are opened are the potassium (K+) channels. When you hyperpolarize the neuron no other channels will open. This is because Na+ channels open when the neuron is depolarized to a potential higher then the resting potential. So all you have is the K+ channels opened so everything should stay the same right? Nope this is because although there is no change in the [K+] the membrane potential is more negative and so in the "eyes" of the K+ ion a more negative membrane potential leads to more K+ flow through the K+ channels that are opened. Hope that helps.
     
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