Synaptic modification rules ?

Matthew Kirkcaldie 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 
a recommendation.

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.

      Cheers,

         Matthew.



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