question- field potential & population spike

Matt Jones jonesmat at
Thu Sep 21 11:56:21 EST 1995

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 it has to do with positive ions at the
surface membrane being repelled during depolarization. People often use
the size or the rising slope, of the pEPSP to judge the strength of the
synaptic excitation. The long lasting increase in synaptic strength is
"long-term potentiation".

Pop Spikes are the synchronous action potentials of the field of neurons,
triggered by the EPSP. These are usually sharp downward spikes,
superimposed on the upward pEPSP. Presumably, they're downward because a
huge number of sodium ions are moving into the neurons during the action
potential (away from the recording electrode). Sometimes people use pop
spike amplitude as a measure of synaptic strength too, but that's
probably not the best measure because there are a lot of other factors
contributing to action potential generation than just synaptic strength. 

-Matt Jones

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