question- field potential & population spike

Hannah Dvorak hdvorak at cns.caltech.edu
Thu Sep 21 13:20:25 EST 1995


In article <43qei5$9vm at dingo.cc.uq.oz.au>, lisao at psy.uq.oz.au (Lisa Olson) says:
>
>Hi all ..
>
>Can someone please explain what these things are, when recording
>from brain areas? I know they have something to do with neurons'
>action potentials, but not exactly what they are. The books I've
>read try to explain them in too-complicated fashion for a beginner.
>I'm writing a literature review of long-term potentiation in the
>hippocampus (for an assignment), and they use these measures to
>assess enhanced synaptic efficacy.
>
>Thanks for the help!
>
>Lisa

I'm not sure how much of a beginner you are, but a fairly good 
explanation can be found in chapter 14 of a textbook by Johnston
and Wu, I think it's called "Foundations of Cellular Neurophysiology";
it just came out this year.  However, let me see if I can explain
fields and pop. spikes to you myself.

If you record intracellularly from a neuron while stimulating its
presynaptic afferents, you will see a postsynaptic potential (PSP)
in the neuron.  (For simplicity, let's assume we're only looking
at excitatory synapses, so the PSP will be an EPSP, for excitatory
postsynaptic potential.)  During the EPSP, there is a net influx
of positive ions into the postsynaptic cell through the ligand-gated
channels at the synapse; this inward current causes a depolarization
of the cell membrane at the site of the synapse.  Now, current must 
always flow in a complete loop; so the current passes through the 
interior of the cell, eventually leaving the cell across the cell
membrane again and flowing through the extracellular medium back 
to the synaptic site.  Because the extracellular medium has a finite
resistance, the extracellular current flow will also cause a change
in potential that can be recorded extracellularly (V=IR).  This 
extracellular signal is orders of magnitude smaller than the 
intracellular signal (i.e. microvolts rather than millivolts).  
It is also of opposite sign to the intracellular signal; you can 
imagine that at the site of an excitatory synapse, you're at a 
current sink, and so you'll see a negative potential with respect 
to ground at a distant site in the bath.

The extracellular signal from a single neuron is extremely small 
and thus next to impossible to record.  However, in some areas of
the brain, such as the hippocampus, neurons are arranged in such 
a way that they all receive synaptic inputs in the same area.  
Because these neurons are in the same orientation, the extracellular
signals from synaptic excitation don't cancel out, but rather add
up to give a signal that can easily be recorded with a field electrode.
This extracellular signal recorded from a population of neurons
is the field potential.  If you're reading about hippocampal LTP,
you've seen the figures that show the field EPSP in stratum radiatum
of CA1 in response to Schaffer collateral stimulation.  So, this
is the signal seen by an extracellular electrode placed in the layer
of apical dendrites of CA1 pyramidal neurons.  The Schaffer collaterals
make excitatory synapses onto these dendrites, and so when they are 
activated, there is a current sink in stratum radiatum: the field EPSP.

When an EPSP is sufficiently large, the postsynaptic neuron will
reach threshold and fire an action potential, which is the result
of lots of sodium current entering the cell at the spike initiation
zone at the base of the axon.  Again, the current loop is completed
by current flowing through the cell, out across the membrane, and
back to the site of the current sink.  During an action potential,
an electrode positioned near the dendrites will therefore see a 
current source (because the sink is at the soma) and hence a positively
going potential.  The signal seen when a large population of neurons
fires simultaneously is the population spike.

I hope this helps; feel free to email me if this explanation is unclear.

- Hannah

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



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