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

Imre Vida Imre.Vida at anat.uni-freiburg.de
Sat Jul 8 01:06:04 EST 2006


Hi Matt

> The synapse is activated in the dendrites, and (ignoring the spike
> because we're just talking about the EPSP now) this is where most
> positive charge is flowing into the cell. IF you could record this
> charge flow from your pipette near the SOMA, it would go downward (look
> like a negative wave) because charge is flowing toward the dendrite
> away from your pipette at the soma.  > But this isn't what you see, so 
>  this dendritic sink isn't what makes the fEPSP at the soma. The
> conductance of the dendrite is relatively low, except out where the
> EPSC occurred, so I expect a relatively small contribution (but not
> zero) from resistive currents as the EPSP propagates toward you down
> the dendrite. Instead, where I expect a strong contribution is from the
> capacitive component. As the EPSP travels down the dendrite, it
> depolarizes each local region of dendrite. That is, it "charges the
> capacitance' by causing positive charge to line up against the *inside*
> of the membrane. This occurs exactly simultaneously with negative
> charge lining up against the *outside* of the membrane (by definition
> of capacitance).
yes, that was my point: the dominant component of the transmembrane current 
further  away from the synapses is the capacitive current, but of course 
as the membrane charges up there will be an increase in the resistive 
component ("leak") as well 
And locally, at the synapses it is the synaptic conductance that
dominants, but here too there will be a capacitive component 
(and an associated resistive component) that has the opposite 
direction reducing the net trasnmembrane current density 


imre



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