current sinks and sources in hippocampal neurons

[Kate Jeffery]

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?

Thanks,

Kate 
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Kate Jeffery, Dept. of Pharmacology, University of edinburgh, Scotland UK
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