Mark Zarella <zarellam at nospam.twcny.nospam.rr.com>
> > The fact remains that distal EPSPs are small when they arrive at
> > the soma as compared to those from more proximal sites. This
> > accounts for what's called 'synaptic strengths'. From what I read,
> > a synapse on the soma can result in an EPSP ~5mV high, versus
> > a distal EPSP will attenuate to less than 0.5mV at the soma, ie
> > 10 times. So this means synapses do have different strengths
> > (though I'm not sure what's the maximum range of synaptic
> > strengths). There's a theory that some kind of auto-regulation
> > will result in equalization of synaptic weights because only
> > those synapses that cause firing will remain in the long term.
>> Only those synapses that cause firing? You don't mean alone I presume.
> So how would this process work exactly?
Actually I should say those synapses that fire concomitantly
with the target neuron are retained whereas others are gradually
weakened / eliminated. It's the Hebbian doctrine, expanded to
include synapse elimination.
> > But this does not take away the fact that synaptic weights take
> > a *range* of values. And that's probably the basis of memory.
>> The basis of memory? How so?
The Hebbian theory is widely used in neural network modeling
to account for learning. In fact I don't know of any other
kind of NN models that use entirely non-Hebbian mechanisms
to learn. There are many minor variations to the Hebb rule though.
> > So my point is that if we look at the sub-threshold membrane
> > voltage at the soma we can tell what kind of dendritic inputs
> > are being recieved at a time slightly earlier.
>> How did you arrive at this conclusion from what you've said above?
It'll take a lot of space to explain.. but what I say above is
not different from what you'd find in the standard texts.
> > You can't see
> > that with low sampling rates (ms range) but maybe with sub-ms
> > resolution it's possible. This is clearly doable with current
> > techniques but maybe no one has bothered to do that.
>> I'm not so sure the sampling rate is the limiting factor, but rather the
> impedances, no?
In order to extract information about dendritic integration by
recording at the soma, I think a higher sampling rate may help.
This despite the blurring of individual PSPs. As for impedance,
I think it depends a lot on the specific configuration (circuit).
If I'm not wrong I think it's possible to use insulated metal
electrodes to achieve very accurate recordings. Impedance is a
problem with glass electrodes because its resistance is big. The
impedance is more or less characterised by "RC", so if R is
close to zero the impedance will be negligible. Well, I just
learned this stuff recently so correct me if I'm wrong =)