long term potentiation

Sturla Molden stumol at stud.ntnu.no
Tue Oct 20 11:04:19 EST 1998

F. Frank LeFever <flefever at ix.netcom.com> wrote:

: re LTP and Alzheimer's: LTP is just one of MANY neural activities which
: may be impaired in Alzheimer's, but just because both have to do very
: loosely with "memory" that does not mean that LTP is of central
: importance in Alzheimer's.

There is no evidence that LTP is a naturally occuring
phenomenon. It araises from repeated tetanisation of
specific neural pathways, eg. in the hippocampus. It is 
also not a homogenous phenomenon, there are several 
sub-types of LTP depending on different receptors
(eg the NMDA receptor, opioid receptors, mGluRs)
and with different durations.

LTP depend on the same mechanisms that seem to be
involved in memory formation, but there is no evidence
that LTP is the substrate for memory.

Theoretical models of associative memory are very 
robust with respect to the choise of plasticity
mechanism, and a number of different mechanisms 
will do the job equally well. Neurons do also 
express different types of plasticities, not just
linear scaling of synaptic weights.

There is no reason to believe that Altzheimer patients
have impaired memory because they lack LTP. If they
do lack LTP, one should rather say that they are
impaired on synaptic plasticity.

The hypothesis LTP = memory is far from proven. 
It is supported by virtually no experimental
evidence. I do think it is more likely that LTP 
is an emergent phenomenon araising from massive 
recruitment of neural plasticity apparatuses 
due to the teanising stimuli; the real mechanisms 
of memory - which is the natural output from 
the cellular pasticity apparatuses - are small
and inconspicous.

And as long as single-unit studies show several
mechanisms of plsticity in single cortical neurons
and the theoreticl models work equally well with
a number o diffferent forms of plsticity, there 
is no reason to suspect that Altzheimer's
patients are impaired on memory because they are
impaired on LTP. It is just as possible that
they are impaired on LTP because they are
impaired on memory (neuronal plsticity).

Sturla Molden




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