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Na pump or leakage K

Matthew Larkum larkum at optolab.unibe.ch
Tue Jun 28 08:52:20 EST 1994


In article 3tn at eldborg.rhi.hi.is, thoreys at rhi.hi.is (Thor Eysteinsson) writes:
>In <2ubis9$d6m at aragorn.unibe.ch> larkum at optolab.unibe.ch (Matthew Larkum) writes:
>
>>It strikes me that a study of membrane potential would be hard
>>because one would introduce a variable leak just by impaling a neuron
>>and the average would always be skewed in the positive direction, never
>>the more hyperpolarized direction. 
>
>This I don«t think is a problem if the methods are consistent. My experience
>with intracellular recordings of Vm (retinal neurons) with impalement is
>that if you get consistently good electrodes, and the set up is in shape,
>then the Vm is very consistent from cell to cell.  This also would be a
>problem that should be independent of embryogenesis (or not?)

I agree.  If the recordings are consistently at the same Vm then the worst
case is that one is introducing a constant leak with each impalement.  My
experience is that one tends to "throw away" results from cells which look
"doubtful".  I.e. a Vm of, say, -40 mV on entering the cell which then
proceeds to tend towards 0.  Maybe I've come across a bad crop of
scientists, but you're the first person I heard of that never misses an
impalment.

> (By the way, I find I consistently
>>get a slightly better Vm by patching rather than impaling a neuron).
>
>What do you mean by "better"? More stable? 

I shouldn't have used "better".  I should have said lower (more 
negative), although there's no reason to call this better - I guess 
this stems for calling impalements "bad" when then cell is obviously 
injured by the process leading to a more positive membrane potential.
Being more stable is also a criterion.

>It should be pointed that 
>if you are getting your cells for whole-cell clamping by enzymatic
>dissociation, then there is the danger that that treatment may change
>membrane properties (e.g. reveal "new" channels!).  So, for this 
>project I would prefer as intact a preparation as possible and
>of course then you would have to impale.  Recordings that are unstable
>are suspect and should be discarded. A good impalement means you can
>hold the cell and record a stable Vm for at least an hour.

I find that recordings with patch electrodes give a much higher
value for input resistance which implies that the sharp electrodes
introduce some leak greater than the patch electrodes.  Even very
stable recordings (longer than 2 hours) with microelectrodes seem
to have smaller values for input resistance.

>>Also, if the dendrites are slightly depolarized (or hyperpolarized)
>>relative to the soma due to constant synaptic activity, then the Vm at
>>the soma might be dependent on the dendritic structure of the neuron
>>which might also undergo changes during maturation (although, then
>>I, at least, would have expected Vm to increase not decrease).
>
>But according to the original post (above) it DID increase with
>embryogenesis.  But these points are still important.

I meant that I expected Vm would go from a more negative value to
a more positive value.  The original poster measured it going in
the other direction.

Matthew.
larkum at optolab.unibe.ch





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