Synaptic modification rules ?
m.kirkcaldie at removethis.unsw.edu.au
Sun May 16 21:06:26 EST 2004
In article <ec29a509.0405161715.46916f1f at posting.google.com>,
nettron2000 at aol.com wrote:
> Ive bin recently reading about a synaptic modification rule discovered
> by Donald Hebb ( Im assuming this is related to the Pavlovian
> conditioning experiments?) in which a synapse is modified depending on
> whether a pre-synaptic spike occurs before or after a post-synaptic
> spike ( still somewhat unclear about that one), but are there other
> "rules" that govern synaptic modification ?
Hebbian learning isn't a rule - it was a concept Hebb thought up to
suggest how synapses might be changed according to the activity of the
cells sending and receiving them, in order that experience would shape
the connections between neurons. The idea is if two cells are usually
active at the same time, this activity would cause the synapses between
them to become stronger. If their activity occurred at different times,
the connection would become weaker. Conceptually, he showed that this
was enough to explain some kinds of behaviour and learning, so he
guessed that a process like this might operate in the nervous system,
without knowing what that process was.
The nearest known physiological processes to Hebbian learning are
long-term potentiation and long-term depression, which are effects on
synaptic strength caused by patterns of firing and the biochemical
processes which these patterns trigger. LTP and LTD are studied very
widely around the world in all sorts of systems, and are understood
moderately well in terms of receptors moving to and from the synapse
according to activity. There are all kinds of reviews of LTP and LTD
ranging from the conceptual to the severely technical - if you can
indicate what you'd like to know, myself and wiser heads here could make
As far as "rules" go, there are no rules, just consequences of
particular firing patterns for cells which have particular membrane
properties and biochemistry. The people trying to understand these
processes give them names and descriptions, but they're for our
convenience - there's nothing in a neuron which says "well, conditions A
and B are met, so this synapse will be altered." It's more like inputs
A and B trigger events inside the cell, and the interaction of those
events might cause side effects which modify the strength of the synapse.
Recently a very interesting mechanism has begun to be unravelled,
whereby activity at a synapse can cause the synapse to "capture" the
connection by causing DNA to be transcribed in the nucleus to make RNA,
but this RNA only becomes new protein at the synapse which was active.
So that's like another "rule" in that specific patterns of events can
trigger it, such as the receipt of a puff of the transmitter serotonin
at the right time. Other recent studies have looked at how signalling
between presynaptic and postsynaptic membrane can maintain the physical
structure, and the role that glia have in allowing the synapse to exist
instead of pushing in to separate the cells, and how long synapses
typically last (minutes? days? years? nobody knows for sure).
Anyway - nobody really knows how all our synapses are made and
maintained. But that's what makes it all interesting.
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