question- field potential & population spike

Hannah Dvorak hdvorak at
Fri Sep 22 14:15:51 EST 1995

In article <43s5fl$hpj at>, Matt Jones <jonesmat at> says:
>In article <43qei5$9vm at> Lisa Olson, lisao at
>>Can someone please explain what these things are, when recording
>>from brain areas? I know they have something to do with neurons'
>The "field" part means that the potentials are being recorded from a
>field or group or population of neurons. These recordings are made
>extracellularly (without actually making contact with or impaling a
>cell). Because the potential changes at individual neuronal membranes are
>hard to detect at a distance, it's much easier if you can get a whole
>field of neurons to respond at the same time so that the signals from the
>whole group add up together. Usually, a bundle of axon fibers that send
>synapses to a whole bunch of neurons is electrically stimulated, and
>causes the synchronized synaptic release of transmitter (usually
>glutamate). This activates receptors in all of the target neurons, giving
>rise to a "population excitatory postsynaptic potential" (pEPSP or field
>EPSP).  In recordings, these field EPSPs are usually shown as upward
>waves of potential. They're upward because positive charge is being moved
>toward the recording electrode, increasing the local positive potential.
>I'm a bit confused as to why the EPSP (which involves positive charge
>moving *into* the neurons) should also cause positive charge to move
>toward the electrode. 

I think this is just a matter of convention in deciding whether 
positive-going potentials are depicted as moving up or down.  In the
vast majority of papers I've read, EPSPs are shown as downward
deflections, and the pop. spikes go upwards.  Electrically speaking,
the extracellular field at the dendritic site of an EPSP is definitely
negative with respect to ground at a distant point in the bath.

[rest deleted]

Hannah Dvorak			hdvorak at
Division of Biology 216-76
California Institute of Technology, Pasadena, CA 91125

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