"Brian" <zhil at online.no> wrote in message news:<57%08.191$0W1.3295 at news4.ulv.nextra.no>...
> "mat" <mats_trash at hotmail.com> skrev i melding
> news:43525ce3.0201141354.5d0a47f2 at posting.google.com...> > New synapses would very unlikely be grown from the soma, you'd first
> > have to grow another axon etc.
>> Yes yes, I wasn't implicating anything else.
>> > New synapses can be formed by the
> > numerous final divisions of the axonal branches, but these are long
> > term changes. Changing the efficency of a synapse by long/short term
> > depression/potentiation is a form of plasticity, and certainly very
> > important in memory formation as shown by hippocampal studies and
> > knockout mice lacking important proteins for LTP and LTD.
>> Well, why call it plasticity, wouldn't it be far better to term it "synaptic efficiency"
> or "synaptic enhancement" ?
> Plasticity is centered on the mold, to form something new out of something unfinished.
The powers that be deemed it to be called plasticity. There's nothing
wrong with the term and in a functional sense it is plasticity - the
temporal modulation of the efficency of the synapse. As long as you
are explicit about what type of thing you're talking about its ok.
Long/Short term potentiation/depression (STP, STD, LTP, LTD) are the
terms usually used to describe changes in synaptic efficency.
Plasticity is a word for the general phenomenon of synaptic change.
Over the past year there have been some really good articles in Neuron
and Nature Neuroscience concerning how spike coincidences effect
changes in synpatic strength.
> Synapses are not very changeable, but their efficiency, effectiveness or enhancement
> could be improved.
yep. But I thought you said you were looking into the 'not very
changeable' aspects? Depending on how you view the way brain function
arises from its structure then the change in synaptic efficency may be
the most important form of plasticity (for instance in computational
theories) Changing the spatial distribution of synapses seems
intuitively to be too slow a mechanism to account for the
computational aspects of brain function. Depression and potentiation
of synapses can occur over seconds and smaller timescales.
> > Cells exhibiting plasticity of the form you describe were recently
> > photographed doing so in a paper in the journal Cell, probably worth a
> > look (Principles of Neural Science is only an intro!).
>> Ahhh, now this looks interesting.
It was, they were grown on a silicon wafer and selectively stimulated.
The synapses were videoed changing.
> Anyway, Yan did a report on this during REM-sleep.
Really? Synaptic plasticity in vivo? A study demonstrating the growth
of new synapses during sleep? If thats true then that would be a
remarkable paper. The reason that the Cell paper I mentioned was so
important is that though it has always been a prediction, seeing
synapses do their thing has never been accomplished until now
> You could ask for the post, as I lost it in a harddisk crash.
> It was fairly interesting, even as unexperienced as I am.
>> > But again I
> > would be weary of the direct associations you make. It is by no means
> > certain that the cognitive functions into which we have divided our
> > minds map isomorphically to singular neural processes, in fact I would
> > tentatively suggest that it is highly unlikely.
>> Thanks for your input.
> I was thinking along Dr.Calvin's idea of Darwin machines, and was thinking how this
> could fit in biologically.
> He described it in "The Celebral Code".
Though the cerebral code is undoubtably an interesting idea, and I
know that Calvin makes it reasonably explicit that we shouldn't expect
to actually see hexagons pattening the cortex his theory is a bit
questionable. One of the main premises is this idea that pyramidal
cortical neurones in layer 4 consistently synapse on to similar cells
approximately 0.5mm away. I've seen quite a lot of evidence to
suggest that holding this to be true over the whole cortex is a gross
generalisation and highly simplistic. However, perhaps the most
valuable thing that Calvin contributes is not the theory itself but
demonstrating how we might go about constructing one based on more
sound (and inherently more complex) neurobiological principles