IUBio

Increased blood flow detected by fMRI scans?

Richard Norman rsnorman at mediaone.net
Thu Oct 25 10:17:20 EST 2001


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?




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