up-to-date values for dendrite model

Richard Norman rsnorman at mediaone.net
Thu Sep 16 08:04:58 EST 1999


The problem you are looking at, how the time course of the synaptic
potential varies as it spreads down the dendrite, is quite complex.

The membrane capacitance is constant, at about 1.2 uF/cm2 because
the lipid portion of all membranes is very similar. However, the
resistance, perhaps 5kohm-cm2 can vary tremendously because it
depends on the number of open ion channels.  That means it depends
on the channel density in the membrane, and on the kind of channels,
and the state of activation of those channels.  And this can change from
cell to cell, along different regions of one cell, and from time to time at
any one location.

In dendrites, this can cause considerable complications.  For example,
the spread of one psp is strongly modified by the changes in membrane
conductance caused by other synaptic activations.  Essentially, the psp
is "short circuited" by the other synapse, whether the second interfering
synapse is excitatory or inhibitory!

Another major complication is the fact the the geometry of the dendrite may
not be constant -- changes in the membrane area can cause local changes
in capacitace and conductance.  Again the geometry is not a simple cylinder,
but varies along the dendrite and can even change over time.

The shape of the psp is a result of the usually brief time course of the
currents produced by the synaptic channel activation interacting with
the cable properties of the dendrite.  Very definitely, the shape changes
as the potential spreads any distance on the order of a space constant
or more.

Another major complication is the fact that some synapses may produce
a complex sequence of channel activation events with complex time events,
so that, yes, the original shape of the synaptic potential can vary.

For all these reasons, overly precise computations of synaptic events are
usually not too useful unless you know a tremendous amount of specific
information about the particular preparation you are studying.  Simulations
are valuable, but only up to a point, and do not substitute for direct
experimental measurements (which are difficult at best).

And don't worry about your English.  Just be grateful that I am not
responding
in French!

Seguier wrote in message <37E0A94F.8E52D7CC at supelec.fr>...
>Hy,
>
>I'm searching for latest value about the membrane time constant,
>membrane capacitance and membrane resistance. I
>heard that those values change in 90th with patch clamp technique.
>My problem is to know if we are sure that spatial sommation exist in the
>dendrite.
>Are the shapes of the PSP very different one from an other ?
>Are those differences coming from different location of synapses on the
>dendritic tree ? But if Rm is very high : the
>neurone
>is very compact so the shape of a PSP just on the location of the
>synapse is the same as the shape of this PSP on the
>soma.
>Maybe the neurone is effectively very compact, but the shape of
>different PSP can also be different because of the
>synapses can create different shape of PSP ?
>
>Sorry for those very innocent questions, but Thanks a Lot for the hoped
>replied !
>
>PS. Sorry for my very very very poor English ...
>
>





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