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current sinks and sources in hippocampal neurons

Venkatesh Murthy venk at stein.u.washington.edu
Thu Feb 11 12:39:45 EST 1993

In article <12763 at bonzo.ed.ac.uk> kate at cogsci.ed.ac.uk (Kate Jeffery) writes:
>When you record extracellularly from the dentate granule cell body
>layer in the hippocampus, there is a positive-going EPSP on which is
>superimposed a negative population spike (summed action potentials).
>We're having a debate in our lab. about why the spike is upside down
>with respect to the EPSP: I thought I understood why this was but now
>I'm not so sure. Can anybody give a good explanation?
>Kate Jeffery, Dept. of Pharmacology, University of edinburgh, Scotland UK

I should think about this more and check out the references before I
write....... but we don't always speak AFTER we think :-)  So here goes.

The EPSP is usually due to synaptic currents flowing INTO the dendrites - in 
other words, the sink is more distal to the soma (and your recording
electrode in the cell body layer.  So at the location you are recording
from there is in effect a source.  When the population spike occurs,
presumably many of the cells are spiking.  This causes a great big sink
to occur near/at the some since by far most of the action potential
currents are dominant at the soma (if not by density of channels, by 
sheer number).  So, your recording electrode is seeing a sink and it 
records a "negative" potential.  

Can someone else confirm or refute this?  Bill Calvin, maybe?

-Venki Murthy
Physiology & Biophysics, SJ-40
University of Washington
Seattle, WA 98195
(venk at u.washington.edu)

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