current sinks and sources in hippocampal neurons

gcarre at gcarre at
Thu Feb 11 09:50:10 EST 1993

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

First a brief description of an evoked potential recorded
in the demtate gyrus. Single stimulation of the perforant path evokes a
glutamate-mediated monosynaptic field potential with at
least two major components, the population EPSP and the
population spike.
  The population EPSP is mainly a recording of the current
flow into the dendrites (sink) of the granule cells
subsequent to their synaptic activation by perforant path
fibres synapsing at the outer two-thirds of the molecular
layer. At any location, the field potential depends on the
linear sum of potentials from each of the current sources
and sinks, weighted according to distance and the
extracellular conductivity. If measured at the
dendrites, this potential is seen as a negative deflection.
As the electrode descends to the cell body the field
potential "flips" at a reversal point, and the population
EPSP is now seen as a positive wave.
  The population spike is mainly a recording of the current
flow into the axon hillock as a result of granule cell
action potentials generated by a sufficient accumulation of
dendritic EPSPs. Measured at the granule cell layer, this
wave is seen as a negative deflection superimposed onto the
positive wave generated by the dendritic EPSPs.
 The population spike reflects an averaged potential change
of neurons in the vicinity of the recording electrode, the
amplitude of which is dependent on the number of granule
cells that discharge in synchrony. Similarly,
the amplitude or slope of the population EPSP reflects the
number of perforant path fibres activated by the stimulus,
and the efficacy of the synaptic process involving such
determinants as the amount of neurotransmitter release and
the sensitivity of postsynaptic receptors.

Now, to answer your question, lets look at the EPSP alone.

       |   |            __          _____
    ---+-> |     A        \        /
   ^   | | |                \ ___/
   |   | | |
   |   | | |
   |   | | |     B       _______________
   |   | | |
   |  /  |  \
   <-+---    |   C            ___
      \_____/               /     \
      granule           __/         \_____

 Synaptic currents flow in a closed loop. Assuming excitation at the distal
dendrites causes distal dendritic depolarization. This makes the
intracellular potential at distal dendrites more postive than at other
parts of the membrane. As a consequence, an intracellular current flows from
high to low membrane potential, away from the distal dendrites. Using
the current loop principal, the intracellular current has to complete an
extracellular loop. This assumption requires that the the main extracellular
current flow in a direction opposite to the intracellular current. The
extracellular potential at point C will be positive, whereas that at point
A will be negative. Somewhere between point A and C, at point B, a zero
potential point may be encountered. Therefore, in the extracellular medium,
there exist both positive and negative potentials (a dipole field).
 When the dendritic potential is strong enough, action potentials will be
generated causing a reverse of the current flow shown above (depolarization
now occurs at the axon hillock). Hence, at point C, this will be seen as
a negative going potential (population spike). (see Leung,L-W.S.
Neuromethods, Vol 15: Neurophysiological techniques: Applications to
Neural Systems. Eds. Boulton,A.A., Baker,G.B. and Vanderwolf,C.H. Humana
Press, 277-312, 1990)

  It has been suggested that a third component contributes
to the shape of the perforant path-evoked field potential
{2783}. Adding to the second positive wave that follows a
population spike, it reflects an outward current across the
soma membrane as a result of inhibitory synaptic activity
that has been recorded intracellularly in granule cells.
 These IPSPs presumably arise from feedforward and/or
feedback activation of inhibitory local circuit neurons, the
majority of which contain GABA. Chloride ions rushing in as
a result of GABA-A receptor activation would represent an
outward current, that, when measured at the granule cell
layer, should add a positive deflection to the field
potential (as in figure above). A significant contribution
of soma inhibitory postsynaptic potentials (IPSPs) to the shape of an evoked
field potential is, however, equivocal. Bicuculline, a GABA
receptor antagonist, has been found to either abolish (study 1),
or have no effect upon (study 2), the size and shape of the
second positive wave.

Hope this helps

geoff carre   GCARRE at LEIF.UCS.MUN.CA

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