Voltage Clamp

Richard Norman rsnorman at mediaone.net
Sun Nov 14 10:18:24 EST 1999


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
I
> 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
current.
 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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