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

Matthew Kirkcaldie m.kirkcaldie at removethis.unsw.edu.au
Sat Jul 8 03:31:44 EST 2006


In article <1152306173.975278.4450 at s13g2000cwa.googlegroups.com>,
 "jonesmat" <jonesmat at physiology.wisc.edu> wrote:

> So it's this deficit of negative charge
> (excess of positive charge) near your pipette, caused by the capacitive
> charging of the membrane by the propagating EPSP, that makes the
> somatic fEPSP go up.
> 
> Well, what d'ya all think? Do you buy it?

I woulda thought the amount of charge involved in charging up the 
membrane would be minuscule, and would be drawn most from nearby 
solution, and would drop off with the square of distance at least, 
meaning that charging the membrane would be nigh-impossible to detect as 
a movement of ions away / toward an electrode sitting off the surface.

The amount of charge as a rough guess would be q=CV, where V is around 
50mV,  C would be the capacitance of, say, a square micron of membrane 
(the nearby bit of the dendrite). I pulled a value of 0.9 µF / cm^2 from 
this article (involving Greg Stuart and John Clements, so they know what 
they're doing):

http://www.biophysj.org/cgi/content/full/79/1/314

which would make the square micron have 9 x 10^-9 µF capacitance. So the 
charge involved would be 9 x 10^-9 x 50 x 10^-3, or 450 picocoulombs, 
drawn from a sphere of the surrounding ECF.

It doesn't sound like it's going to add up to much current at the 
recording electrode ... !

I would reckon minis involve a much greater migration of ions than 
charging up the membrane, and yet you have to be patched on with a good 
strong seal to see them.

I'm still thinking it's ion movement through VGICs and / or LGICs we're 
seeing near the dendrite.  Don't forget dendrites have spikes as well, 
voltage gated sodium and calcium channels can turn a minor EPSP into a 
more serious current event, even before an AP is triggered.

I suspect I'm teaching my grandmother to suck eggs here ... !  But if 
so, let me know whether this sounds reasonable.

      Cheers, MK.


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