[Neuroscience] Re: Explaination of the direction of field potentials

r norman NotMyRealEmail at _comcast.net
Thu Jun 8 17:19:58 EST 2006


On 8 Jun 2006 15:06:57 -0700, "Bill" <connelly.bill at gmail.com> wrote:

>
>Matthew Kirkcaldie wrote:
>> It's positive ions entering the cell.  If you're near the dendrite, but
>> not in it, since dendrites are tiny, the charges are leaving the
>> electrode region to enter the dendrite, so the electrode region is
>> negative. If you're in the soma, the ions are entering the region around
>> the electrode, so it looks positive.
>>
>> However I would love someone with a better answer to chime in.
>>
>>          MK.
>
>So are you saying that the EPSP invading the soma is sensed by the
>extracellular electrode (even though there are 100s of mega-Ohm
>resistors between that voltage change and the electrode, i.e. the cell
>membrane), or are you saying the extracellular electrode is sensing the
>leakage of current out of the cell (probably potassium efflux)?

Extracellular field electrodes really measure the current density in
their immediate vicinity.  The polarity of the potential depends on
the direction of current flow (with respect to the location of the
"indifferent" electrode or an equivalent ground volume.  You have to
know the specific geometric configuration of the cell and the
electrode to interpret the potentials.   Basically what you ordinarily
see is the difference between inward current across the membrane and
outward current.  Just what causes the current flow is another story
-- it can be an active response (opening membrane channels) right at
the electrode site or the completion of a local current flow due to an
active response at a more distant site.  You have to remember that
inward current at an active site is outward current at other
locations.


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