kpaulc at earthlink.net
Mon Nov 15 01:23:22 EST 1999
Thank You for your discussion. i'd not associated 'voltage clamp' with the
resolution of NA+ & K+ roles in the action potential, most likely, because,
other than a sheep's brain disection, i've never set foot in a Neuro lab, except
in looking for people.
i've worked strictly from the published results of others.
K. P. Collins
Richard Norman wrote:
> Mark Fehr <wolf at brandeis.edu> wrote in message
> news:80l1qh$aum$1 at new-news.cc.brandeis.edu...
> > I'm an aspiring neuroscientist, a junior in college, and was wondering
> > if someone could help me out with a concept in my intro to neuroscience
> > class. Can someone explain how the voltage clamp exactly works, and what
> > read from it? I have a vague idea, and would like it to me more grounded.
> How much technical detail do you want? Better, indicate what neuroscience
> text you are using and that will give us a better idea of what level of
> detail to provide.
> Here is a non-quantitative answer that does not depend on interpreting just
> how the electronic circuit works:
> First, you must understand the Hodgkin cycle, the feedback system that
> drives the action potential.
> 1) Depolarization of the membrane causes Na channels to open (the
> definition of electrical excitability)
> 2) Opening of Na channels causes Na to enter the cell, i.e., an electric
> 3) The entry of positive charge causes the membrane to depolarize further,
> closing the cycle.
> There is a second negative feedback system that turns it off
> 1) Depolarization causes K channels to open
> 2) Opening K channels causes K to leave, another electric current
> 3) The exiting of positive charge causes the membrane to repolarize
> (hyperpolarize, actually)
> In both cases, the argument is circular, a change in voltage causes
> channels to change causes a current to flow which causes further changes in
> voltage which ....
> The voltage clamp circuit is a trick to break open this feedback loop. The
> electronics are arranged so that step 3 of the loop is broken. When the ion
> channels open and the current flows, the current flows through the
> experimental equipment and does not result in a further change in the
> membrane potential. That is, the membrane potential is held fixed, it is
> "clamped" at the value commanded by the experimenter.
> Under "normal" conditions, you stimulate a cell with a pulse of current and
> measure the resulting voltage, the action potenial. Under voltage clamp
> conditions, you stimulate a cell with a voltage step and measure the
> resulting current. The real value is that the current through a channel
> obeys the equation ix = gx(V - Ex), where ix is the current carried by ion
> "x" (x = Na or K or whatever), gx is the conductance of the channel (as
> channels open, gx increases, as channels close, gx decreases), V is the
> actual membrane potential, and Ex is the Nernst potential for ion x,
> representing the diffusion "force" causing ion flow. Under non-clamped
> conditions, there is a constant interaction between i and V, both changing
> all the time. Under the voltage clamp, V is known, and is usually held
> constant, Ex is known, and i is measured. Then, knowing i vs time allows
> you to calculate g vs time indicating just how the ion channels change.
> The real trick is to separate the total current measured into the current
> components, iNa and Ik, but your text should go into that.
> The patch clamp is just a refinement of the technique, applying the voltage
> clamp to a very small piece of membrane so that only a few ion channels are
> present. In this circumstance, you can actually see the individual channels
> opening and closing.
> And make sure to give credit to KC Cole for developing the voltage clamp
> idea! Hodgkin and Huxley certainly made good use of the technique to deduce
> the mechanism of the action potential, but they didn't invent it.
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