An electrophysiology quesiton

r norman rsn_ at _comcast.net
Wed Feb 11 15:48:52 EST 2004


On Wed, 11 Feb 2004 12:22:12 +0000, Xiaoshen Li <xli6 at gmu.edu> wrote:

>r norman wrote:

>> Voltage clamping is much more difficult to do.  There is nothing to a
>> "current clamp".  Just stick an electrode into a cell (which isn't
>> that easy) and record! To do a voltage clamp, the techniques are far
>> more elaborate.  So, unless the information you seek absolutely
>> requires voltage clamp data (currents under very controlled
>> potential), you just record the potential and interpret it in terms of
>> the channels opening and the currents flowing.  
>> 
>> You eventually get used to all the switching.  Well, you don't get
>> used to it, but eventually you learn to deal with it!
>> 
>Dr. Norman,
>
>It is nice to hear from you. Your post is short, so it is very unclear. 
>I disagree with you on several things based on what I guess you are 
>trying to say. First, V-clamp is not harder than I-clamp to do. There is 
>a switch on the machine to choose V-clamp or I-clamp. It is just a 
>matter of click. In terms of clamping error which Dr. Matt Jones 
>mentioned is another issue.(Maybe this is the issue you hinted). That is 
>for the synapse far from soma while one is patching on the soma. If 
>under I-clamp mode, the amplifier doesn't care how much current flow 
>in/out of the neuron membrane(I am very surprised with this information 
>and I have asked again for confirmation/explanation in my previous post, 
>because if so, I-clamp is not clamping anything), clamping error doesn't 
>exist.
>I have never heard that I-clamp data is less convincing than V-clamp. My 
>impression is that they are with same rigorous. V-clamp and I-clamp are 
>two different techniques to measure different things. They are not just 
>different names one has to get used to it.
>
>Sincerely,
>Xiaoshen
>

<I "corrected" your top posting to the style I prefer.>

IF you already have the much more expensive equipment that allows
voltage clamping AND IF you are recording from a cell where you can
readily pass enough current through the recording electrode AND IF
your electrodes are such that it is relatively easy to accurately
record potential while simultaneously passing relatively large
currents through the same electrode THEN (and only then) is voltage
clamp as "easy" as current clamp.  Of course I assume you are using
intracellular glass microelectrodes, not patch clamp.  Patch clamp is
a whole other level of dfficulty over microelectrode recording. Then
there is the clamp error which only applies to voltage clamp. You are
correct, current clamp is not a clamp at all, just the absence of
voltage clamp.

I did not mean to suggest that epsp's and epsc's are just different
names for the same thing.  They are two different techniqes that
record very different aspects of the same phenomenon.  It is jumping
back and forth between mind sets that one has to get used to, not just
change in terminology.  In fact, having to do exactly that back and
forth is what prompted you to ask about this subject in the first
place.

Both techniques provide data that must be interpreted to be useful --
neither is more or less convincing.  However, if you really are
interested in channel gating, then seeing currents change under
constant voltage conditons is direct evidence that there are
conductance changes.  Seeing voltages change under unclamped
conditions means there is probably a conductance change somewhere that
is causing a membrane current to produce the potential but the
evidence is certainly more indirect. 

 It could be that the authors of the original paper used current clamp
data (epsp) when those results were suitable for the question under
consideration but switched to voltage clamp (epsc) to collect data to
answer specific questions where that type of data was necessary.
Could you cite the specific paper so I could check it out?






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