IUBio

Increased blood flow detected by fMRI scans?

John H. John at faraway.com.au
Thu Oct 25 09:59:55 EST 2001


"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.

> Recycling (as they said) would in part be accounted for by anaerobic
burning
> of glucose.
> The rest would in be used by the cell for homeostasis.
> As for action potentials, they would be the small increase of the
oxygenated
> blood.
> That is; recycling from cyclic AMP to ATP.
> I'd guess that the phosphorylation should be the next step, although I
have
> no idea how they would accomplish that.
> But for percentages...............that is the main question.
>
> Brian
>
>







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