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[Neuroscience] Re: Electrophysiology: monosynaptic vs heterosynaptic transmission.

Michael Grammer mkgrammer at gmail.com
Thu Feb 23 11:04:46 EST 2006

The last post on this subject seems quite good.  It's my understanding as
well that, in extracellular field recordings, the initial slope of the
response is the best estimate of the monosynaptic current (perhaps also in
pscs, but I commonly see pscs reported in amplitude).  It is also my
understanding that the amplitude of the psp can be contaminated or
influenced by the population spike and, to a lesser extent, by inhibitory
activity which shapes the falling phase of the response.  Regarding the
Hodgkin and Katz reference, they probably used the slope of the ap as an
estimate of the na current rather than the amplitude due to the
contamination of the ap peak by k current(?).
Best regards,

On 2/22/06, r norman <NotMyRealEmail at _comcast.net> wrote:
> On Wed, 22 Feb 2006 19:29:36 +0100, SJM Guzman
> <jose.guzman at medizin-uni-leipzig.de> wrote:
> >Hi R. Norman:
> >
> >Your explanation was fabulous. I was kind of confuse regarding the
> >terminology homosynaptic/monosynpatic. Now is clear to me. Again thank
> >you very much.
> >
> >Regarding the field potential question.... To access the monosynaptic
> >component, some laboratories analyze EPSP slopes using the least-squares
> >regression. Why not simply to analyze the amplitude?
> >
> >Here some examples of publications...
> >
> >"For EPSP analysis, The initial rising slope was measured, (1 msec
> >period form its onset, in milivolts per millisencond), which contains
> >only a monosynaptic component )..."
> >
> >"To minimize the contribution of voltage-dependent conductances, initial
> >slopes of EPSPs were calculated..."
> >
> >I would appreciate some literature about it. I miss some about PSC/Ps,
> >because traditional electrophysiological books (i.e Hille, Neher &
> >Sakmann), don't tell much about it.
> >
> >Thank you very much again!
> It is very easy to find a lot of papers that refer to PSP slopes,
> whether measured as field potentials or with intracellular electrodes.
> However I have not been able to find any web sites that explain just
> why this measurement is used.
> Here is my interpretation, but this is really conjecture on my part.
> Somebody jump in here if it is wrong!
> The really proper direct measure of synaptic response is channel
> opening.  However that can't be seen directly.  Channel opening is
> directly related to membrane conductance.  However measuring
> conductance is often technically impossible, is disruptive to normal
> cell function, and averages conductance changes over too large an area
> of membrane.  The next most direct response is synaptic current.  If
> the membrane is voltage clamped to a fixed value, the current through
> any one synapse will be directly proportional to the channel openings
> (conductance changes) assuming the reversal potential (ion
> concentrations) do not change significantly. The most indirect
> response is  synaptic potential itself.  The amplitude of the psp will
> depend greatly on all sorts of disturbing influences going on in the
> cell.
> So measuring synaptic current is a far better way of measuring
> synaptic function than measuring synaptic potential. Unfortunately,
> directly measuring current usually means doing a voltage clamp which
> is often technically impossible.  However, the slope of the rising
> phase of the psp is a very good measure of synaptic current.  During
> this time, the current through the synaptic channels completes its
> closed loop by flowing across the membrane in the form of capacitative
> current which obeys the law I = C dV/dt.  Since C is essentially
> constant, dV/dt is a direct measure of I.  The falling slope is very
> different since the synaptic channels are then closed and that slope
> depends on the cable properties of the cell, especially the way that
> the current distributes longitudinally down the dendrites and the
> membrane time constant.  Note: the "effective time constant" during
> the rising phase of the psp is very different from that during the
> falling phase because of the changes in membrane conductance.
> I don't know just where all this is explained.  I do know that Hodgkin
> and Katz (I think it was their 1949 paper, I don't have my texts with
> me on vacation) used the slope of the rising phase of the action
> potential as a far better measurement of the sodium current inrush
> than the actual amplitude of the action potential.  They explained why
> in an appendix to the paper.  Just as the action potential is caused
> by sodium currents flowing through open sodium channels, the synaptic
> potential is caused by ion currents flowing through open synaptic
> channels and the same logic applies.
> If I recall correctly (again, I don't have the books with me) Neher
> and Sakmann concentrate (as to be expected) on patch clamp data and I
> seem to recall Hille doing similarly.  Many texts go into the
> molecular machinery of ion channels and receptor binding and so on but
> short change the biophysics of the electrical details.
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Michael Grammer, Ph.D.
Biological Sciences
University of Southern California
Los Angeles, CA  90089-2520
(213) 740-9189

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