Increased blood flow detected by fMRI scans?

John H. John at faraway.com.au
Fri Oct 26 00:25:17 EST 2001


"Richard Norman" <rsnorman at mediaone.net> wrote in message
news:jjagtto38k73lrbkhc21bsajitet6e234j at 4ax.com...
> On Fri, 26 Oct 2001 00:59:55 +1000, "John H." <John at faraway.com.au>
> wrote:
>
> >
> >"Brian" <zhil at online.no> wrote in message
> >news:ApkA7.4295$Dv2.94840 at news1.oke.nextra.no...
> >>
> >> "Richard Norman" <rsnorman at mediaone.net> skrev i melding
> >> news:lmd3ttgv1s43jeda5eqjforg38t8e4ilh4 at 4ax.com...
> >> > >> They say "These
> >> > >> findings suggest that the BOLD contrast mechanism reflects the
input
> >> > >> and intracortical processing of a given area rather than its
spiking
> >> > >> output."
> >> > >
> >> > >In other words, they said that the energy-usage by the cell reflects
> >the
> >> > >oxygen-flow through the blood, while spike(s) are _not_(no big
surprise
> >> > >there, I would be much more surprised if the oxygen-level correlated
> >with
> >> > >the spikes...).
> >> > >
> >> > >Brian
> >> >
> >> > Yes, it is a tautology that oxygen use correlates with metabolism.
> >> > The question is what cellular processes of the neuron consume the
most
> >> > energy?  Is it making action potentials?  Or is it synaptic
> >> > transmitter synthesis (and recycling)?  Or is it all the cell
> >> > processes that go along with neuromodulation, including up- and
> >> > down-regulation of membrane proteins?  Or just what?
> >>
> >> Yes, it would be nice if I could answer these questions, but I'd say
that
> >as
> >> they were measuring in monkeys that were unconscious, they would not
have
> >> measure the real energy usage.
> >> Protein production for growth of the synapses are probably the greatest
> >user
> >> of oxygenated blood.
> >
> >No, my bet is is ion channel function(for cellular ion homeostasis and
> >transmission) is the greatest user of resources in neurons. Eg. various
> >calcium pumps are continually pumping against a substantial gradient.
They
> >don't just switch on for a while, more probably are maintaining a basal
> >level of function. I don't like the term 'electrical signals' because in
> >neurons transmission is a very demanding process, not just some flow of
> >electrons but the continuous synchronous activities of a plethora of ion
> >channel types. The energy to create new synapses in no way compares to
the
> >amount of energy required to maintain normal ion homeostasis because new
> >synapses are relatively rare. Synapses come and go, degraded and
recreated,
> >probably even in long term memory, but even a resting brain is
metabolically
> >expensive. 2% bodyweight, 20% of all resources.
> >
> >If fMRI was measuring synaptic formation activity, then in the cases of
long
> >term memory at least there should remain activity in the region after the
> >individual has stopped observing the stimulus. The area should remain
> >activated as indicative of ongoing synaptic formation. That does not
happen,
> >or at least I don't know of it, my understanding that fMRI relates to
> >immediate cognitive activities.
> >
>
> I know the traditional answer for why nervous tissue has such a high
> metabolism is that it mostly goes for ion pumping -- the Na/K and the
> Ca systems.  But I wonder--- In an area packed with synapses, there
> are othe large energy costs:
>
>     Synaptic transmitter recycling (which certainly does involve
>     presynaptic calcium, but also other costs)
>
>     Metabotropic post-synaptic events --all that phosphorylation
>
>     Upregulating and downregulating membrane proteins
>
>     Transporting everything up and down the cell
>
> Could these, perhaps, outweight the ion pump cost?


In localised regions that seems possible. I find it very difficult to think
about such matters, quantification is a hassle here. Recently read an
article (sorry, can't reference it, just a browse), which suggested that
synapses are not 'constant', that a synapse will degrade but another
immediately or concurrently arises to 'take its place'. It does seem
plausible that even in a 'non-learning' CNS region there is a certain amount
of synaptic restructuring etc taking place. It may be the case that an
activated CNS region experiences a certain number of synapse 'deaths' as a
natural consequence of activity, so maintenance of synaptic integrity(and I
imagine this requires transcription) at high levels of activity could
outweight ion pump cost? Interesting question, thanks because I know bugger
all about synaptic formation.

John H.





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