Need help on synapses.

Matt Jones jonesmat at ohsu.edu
Fri Dec 15 13:38:19 EST 1995


In article <4aoe2oINNe60 at duncan.cs.utk.edu> Tzusheng, tzusheng at cs.utk.edu
writes:
>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.

Your question has sparked quite a wide variety of responses. I think this
is probably an indication that neuroscience doesn't have a very solid
answer yet for the issues you've raised. Maybe it would be helpful to
list the things we know for sure, and those that we don't:

It Is KNOWN (or at least suspected by many of us):
In even simple brain structures (e.g., the hippocampus), synapses are not
homogenous. There are several different transmitters at work (although
most synapses use either glutamate or GABA), and even considering just
one class of synapse (e.g., glutamatergic) there's considerable variation
in the size of the response at each site, the proportions of different
receptor types that have very different functions, the probability that
transmitter will be released after an action potential, the ease with
which the signal is carried to the cell body from the dendrites, etc.
Even in studying a SINGLE SYNAPTIC SITE it now seems likely that the
properties of that site can change with time, often in response to things
happening elswhere in the hippocampus (LTP, LTD, neuromodulation,
etc...). 

It Is NOT KNOWN (at least not by me):
How or why the diversity among synapses gives rise to the behavior of the
system. In his post Bill Skaggs suggested that the diversity is probably
there for control, rather than being the main workhorse of information
processing. He's probably right, but it's never been exactly clear to me
what people mean by "information processing". People often say that
information is encoded in the firing rate of a neuron. To me, that's sort
of the same thing as saying that a song is encoded in the grooves of an
album. It's obviously true, but it doesn't really capture what we have in
mind when we think of a song. The firing rate of a neuron contains a
little bit of information (maybe) about what got that neuron to fire. But
the actual information that we're interested in (perhaps a memory, in the
case of the hippocampus) is a really, really complicated thing that
probably involves an awful lot of neurons and synapses and transmitters.
So (humor me for a minute) in that sense, maybe all that diversity is
actually important in generating this really complex behavior (encoding a
memory). On the other hand, maybe it's there just because biology is a
little imprecise, and not every synapse gets constructed to exactly the
same specifications every time. This seems just as likely. 

-Matt



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