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

r norman NotMyRealEmail at _comcast.net
Sun Jun 11 18:16:18 EST 2006


On Sun, 11 Jun 2006 23:13:52 +0200, Imre Vida
<Imre.Vida at anat.uni-freiburg.de> wrote:

>
>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.

Field potentials don't care what carries the current nor what causes
it.  They only care about the current density in their vicinity and
its direction.  So whether the transmembrane current is active (caused
by ions moving through newly opened channels) or passive (caused by
ions moving through already open channels  including "leak" current)
or capacitative (caused by a changing electric field across the
membrane but no ion translocation) doesn't matter.  In the
extracellular space, the current is always a mixture of primarily Na+
and secondarily Cl- moving the opposite way.  These are the dominant
mobile charge carriers in the medium.

During the decay phase of the synaptic potential, current densities
are very low for a large number of reasons, hence field potentials
will be minimal.  The field potential primarily reflects the active
phase of current flow, when either the synaptic channels or the action
potential channels are open and current densities are high.




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