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

sturlamolden sturlamolden at yahoo.no
Sun Jun 11 13:37:04 EST 2006

Bill wrote:

> This may seem like a very simply question, but I've heard two different
> electrophysiologists (both of whom publish in fancy journals as a
> matter of course) give two different explanations as to why field
> potentials of action potentials recorded from a cell body region (e.g.
> Stratum Pyramidale of CA1) are positive, while field potentials caused
> by dendritic excitation (e.g. Stratum Radiatum of CA1) are negative.
> What is your understanding of this?

Extracellular spikes recorded in stratum pyramidale are *negative*.
This is the result of positive current flowing into the cell, i.e. away
from the electrode. During the initial phase of the somatic action
potential, sodium ions flows into the cell soma, and this can be
recorded extracelluarly as a negative voltage "spike" . This accounts
for single unit spikes, as well as an evoked population spike.

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.

Sturla Molden, Ph.D.

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