Need help on synapses.

Bill Skaggs bill at nsma.arizona.edu
Thu Dec 14 13:36:20 EST 1995

tzusheng at cs.utk.edu (Tzusheng Pei) writes:

   > My training is largely in computer science, and I know little
   > about neuroscience.  My questions might be easy (or just
   > commonsense) for you neuroscientist.  Please take a few minutes
   > to help me with the following questions.

If it is important to you to understand these things, read the first
few chapters of *Principles of Neural Science* by Kandel, Schwartz,
and Jessel.  The book is widely available.

   > Suppose that we have two neurons with synapses in between the two
   > neurons.  We feed an electric pulse into the presynaptic side of
   > a synapse, and we get a pulse from the postsynaptic side. 

This is an oversimplified picture of how synapses work.  The simplest
types of synapses evoke a conductance change in the postsynaptic
neuron; some other types have much more complicated and long-lasting

   > I need to know if all the synapses in between the two neurons
   > have the same, similar, or quite different response (postsynaptic
   > pulse) with the same input pulse at presynaptic side. 

All synapses between any given pair of neurons evoke the same type of
conductance change, but this can give rise to different postsynaptic
pulses, because different synapses are located at different points in
the circuit.  (You probably know that the current flowing through a
resistor depends on the applied voltage and also on the other elements
of the circuit that are in series and in parallel with it.)  Even so,
in most cases the differences are relatively minor.

   > If we take out different pairs of neurons from a part of the
   > brain, such as hippocampal neurons from a rat, I need to know if
   > synapses from different pair of neurons are different. 

They can be very greatly different.  Some synapses are excitatory,
others are inhibitory, others have indirect effects.

   > So the question can be reduced to: Are the synapses in the same
   > functional region of the brain homogeneous ?  If not, how
   > different are they ?  It seems to me that if they are quite the
   > same, they don't process information much.  I might be wrong.
   > Thanks. 

As I've already said, synapses within a given region are certainly not
homogeneous.  It is probably an error, though, to conclude that
information processing depends on these variations.  Different
synapses, even if they are of the same type, can play distinctive
roles because they connect different pairs of cells, whose firing
represents different items of information.  In many connectionist
models, only a single type of synapse is used, and these models
certainly do process information.

Why, then, are there so many different types of synapses?  Most likely
for purposes of control.  In the hippocampus, for example, there are
literally dozens of different types of synapses, but the great
majority, probably around 99%, are of one specific type (excitatory
synapses from pyramidal cells to pyramidal cells), and certainly most
of the information processing in the system is done by these synapses.
Of the other types, most are inhibitory, and serve to constrain the
amount of activity in the system, or to shape the activity pattern in
more specific ways.  

Thus, the diversity of synaptic types in the brain probably exists in
order to permit diverse mechanisms of control, not to permit the
ability to represent information.

	-- Bill

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