High Resolution Intracellular Recordings?

r norman rsn_ at _comcast.net
Thu Sep 4 08:03:36 EST 2003

On 3 Sep 2003 16:48:24 -0700, y.k.y at lycos.com (yan king yin) wrote:

>Hi Everyone =)
>I'm trying to find intracellular recordings of the soma of
>in vivo neurons, with *temporal* resolutions in the sub-ms
>range (the finer the better). What I want to see is how the
>spatial and temporal integration actually take place in action.
>Any web page, paper etc...?
>I've read from Koch's "Biophysics of Computation" that it
>takes about 64 EPSP inputs *at* the soma close together to
>generate an action potential. (The threshold being about 16mV
>above resting potential and each EPSP typically around 0.3mV).
>Is it possible to actually discern individual contributions
>of EPSPs from such recordings? What about background noise
>that is not from EPSPs?
>Thanks a lot!
>Yan King Yin
>My Home Page -- http://www.geocities.com/Softuploading/Index.htm

Matthew Kirkcaldie has already give you some valuable information.
But here is some other stuff you should realize.

First, the cell membrane has a relatively large capacitance resulting
in a time constant at least several msec in duration.  That is, the
membrane potential is not easily changed in times much shorter than
this. I am talking about "passive" potentials, here which includes the
spread and summation of psp's.  Active potentials are different. In
the production of an action or a synaptic potential, open channels
produce a high conductance or low resistance membrane, hence a fast
time constant.  In other words, the passive properties of the membrane
act as a low-pass filter and events in the sub-millisecond range are
drastically reduced in amplitude.  No such activity spreads very far
down a dendrite.  Note:  this whole discussion ignores the active
responses of dendrites that Matthew talks about.  Still, it is a good
first approximation to what happens.

Second, intracellular recording technique with microelectrodes
introduces its own limitations. With an electrode resistance in the
tens of megohms, a stray capacitance of a few picofarads produces
another time constant and a low pass filter so that the signal is
limited in bandwidth.  In addition, the high electrode resistance
produces random "thermal" noise that obscures small signals.  In other
words, it is very, very difficult to see small, fast signals, even if
they really are there.  Patch clamping, again described by Matthew,
uses very different electrodes and has very different constraints, but
often the type of data you want is best seen by the older
intracellular microelectrode.

Then you have the problem of actually seeing individual events in the
smear of membrane potential.  The "classical" view of synaptic
integration is a "simple" spatial/temporal summation of potentials on
a distribution R-C cable.   Again, Matthew shows why this is
inadequate, but again it is a good first approximation.  The
properties of the cable equation (the low-pass filtering of the
membrane I referred to earlier) means that the psp's produced by
distant synapses reach the soma as a blur.  The potentials are
drastically reduced in ampltude an drastically slowed and spread out
in duration.  It is not at all possible to separate out individual
contributions.  And the cell may not even care about individual
potentials -- in may be the "mass effect" in changing the general
excitability of the cell that is the "purpose" of this wiring pattern.

You may have in mind a model of synaptic integration that works like a
logic equation:
  Fire an action potential IF (A AND B) OR (C AND D AND E) BUT NOT (F)
or something of the kind but with hundreds of terms.  You want to see
the data with enough resolution to work out each individual term.  It
doesn't work that way. 

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