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

[Imre Vida]

Hi,

i agree with the explanation of Strula concerning the 
polarity of the signals. However i have some problems 
with the ions movements:

> Field potentials caused by dendritic exitiation (f-EPSPs) are
> *posiitive* in stratum pyramidale. What you record then is the *return
> current* from the dendritic EPSC. I.e. as sodium ions flows into
> dendrites, potassium ions flows out of the soma. This has to do with
> Kirchoff's law of preservation of electric current.
Yes, ions (primarily Na+) flow into the cell at the excitatory synapses, 
the membrane  will be charged locally but also more distally. 
Locally the  net transmembrane current will be dominated by the inward 
synaptic current (active "sink" for the extracellular space), 
distally the transmembrane current is an outward current (passive "source") 
consisting of a capacitive and a resistive component. 
This latter component is largely but not exclusively K+ current, as 
a "leak" current. 

When the synaptic current decays, the charged membrane will be discharged 
through the "local" membrane conductance (leak) resulting in no net transmembrane 
current - i.e. the time course of extracellular potential
correspond to the synaptic current (EPSC) and not the intracellular potential
(EPSP).

In addition to the excitatory response, inhibitory interneurons are also 
activated and produce an active "source" in the somatic layer - Cl- flow 
into the somata of the neurons - with passive sinks in the dendritic layers.
These passive and active sinks and sources overlap spatially and
temporally.



imre