Increased blood flow detected by fMRI scans?

John H. John at faraway.com.au
Fri Oct 26 00:43:49 EST 2001


"Brian" <zhil at online.no> wrote in message
news:KR_B7.1697$kV1.25255 at news1.oke.nextra.no...
>
> "John H." <John at faraway.com.au> skrev i melding
> news:_KVB7.1322$Fi4.73149 at ozemail.com.au...
> > > 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.
>
> Ok, thanks for the input.
> And I didn't add the extra usage by the new synaptic connections.
> More synapses, more energy-usage.
> It probably follows an exponatial rate, as there's an upper limit of how
> many synapses a neuron can handle.........

I believe it is speculated that dystonia is caused by the gradual increase
in glutamate receptors in the relevant region of the motor cortex leading to
a hyper sensitive motor activity. (Eg. musicians)So sustained usage of a
region can increase synaptic density, but from a behavioural perspective at
least this density can become bad. There must be an upper limit, but I
vaguely remember reading something about mossy fibres in the cerebellum
having up to 10,000 synapses!

Imaging generally:

Recently read this, would appreciate comments because it confuses me. Just
what is measured by an EEG and how is it that simulated neurons can generate
theta waves? I haven't been able to look at the studies below but am hoping
someone here can shed some light on this for me.



Frontiers of Complexity, Peter Coveney and Roger Highfield  pp325

The Dream Machine

"We are a long way from running computer simulations of the human brain in
its full glory. Yet surprises continue to emerge in even highly simplified
simulations. As one example, a computer model of the hippocampus, quite
distinct from that of Treves and Rolls, was developed by Roger Traub at IBM
in a collaboration with Columbia University to study the brain's electrical
rhythms. The model connect 10,000 simulated neurons, each one described in
considerable microscopic detail so that it would respond in a manner close
to that of the real thing. It was a bottom-up approach to complexity simliar
in spirit to what we encountered in Denis Noble's work on the heart. What
resulted was unexpected: Traub's network produced electrical waves simliar
to those generated in large populations of brain cells that can be detected
by electroencepthalography. One emergent behavior in the model was the
"theta rhythm," which occurs during dream sleep." [note; hippocampal theta
waves are always present in learning???]"

Norway? You lucky chap, I've always wanted to visit that place, probably
because one of my old school friends was a Norwegian immigrant to sunny Aus.


John H.






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