Brain energy expenses

Richard L. Hall rhall at webmail.uvi.edu
Sat Sep 2 09:03:26 EST 2000


Of course there is some glycogen available, but very, very little. 
The "marked increase in brain glycogen content" may be significant as 
a percent increase, but are the increases significant in absolute 
terms?  Large changes in small quantities are still small quantities.

The types of metabolic adaptations discussed in the glial glycogen 
metabolism studies you have referenced are very interesting and 
clearly relevant, but do not appear to scale energy consumption into 
the realms implied in posts by other contributors to this string. 
The results confirm a line of reasoning based on a CNS operating with 
very marginal energy reserves.  The mammalian brain is NOT well 
suited nor well adapted for dramatic increases in energy consumption. 
Dredging up a recent post from 8/25/00  Re: neurotransmitter storage 
(all or one?):

[the graded synaptic interactions and energy expenditures of a 
functioning brain appear to be effectively...]

"   averaging the rates of energy consumption so you do not have problems like
      running out of fuel just when you need maximal computing power.

The brain has virtually no energy reserves and without this 
adaptation, a sudden increase in energy demand would be fatal."

rlh

>
>"Richard L. Hall" wrote
>
>>
>>
>>In normal function brain energy consumption must be essentially
>>constant.  Why?  There are no energy stores in the brain and
>>consequently the brain extracts energy moment by moment from the
>>blood.
>>
>
>Just my humble contribution to the trend, as I read that glycogen 
>could serve as an energy reservoir in the brain. But this may be 
>only unverified hypotheses :"
>Physiologic coupling of glial glycogen metabolism to neuronal 
>activity in brain. Can J Physiol Pharmacol 1992;70 Suppl:S138-44
>Brain glycogen is localized almost exclusively to glia, where it 
>undergoes continuous utilization and resynthesis. We have shown that 
>glycogen utilization increases during tactile stimulation of the rat 
>face and vibrissae. Conversely, decreased neuronal activity during 
>hibernation and anesthesia is accompanied by a marked increase in 
>brain glycogen content. These observations support a link between 
>neuronal activity and glial glycogen metabolism. The energetics of 
>glycogen metabolism suggest that glial glycogen is mobilized to meet 
>increased metabolic demands of glia rather than to serve as a 
>substrate for neuronal activity. An advantage to the use of glycogen 
>may
>  be the potentially faster generation of ATP from glycogen than from 
>glucose. Alternatively, glycogen could be utilized if glucose supply 
>is transiently insufficient during the onset of increased metabolic 
>activity. Brain glycogen may have a dynamic role as a buffer between 
>the abrupt increases in focal metabolic demands that occur during 
>normal brain activity and the compensatory changes in focal cerebral 
>blood flow or oxidative metabolism." End of quote
>
>  And also:"
>
>Acta Physiol Scand 1999 Dec;167(4):275-84. The role of astrocytes 
>and noradrenaline in neuronal glucose metabolism
>In the classical model the energy requirements during neuronal 
>activation are provided by the delivery of additional glucose 
>directly into the extracellular compartment that results from the 
>increase in local cerebral blood flow (rCBF). The present review 
>proposes that astrocytes play a key role in the response to neuronal 
>activation. Arginine for the synthesis of NO, which has a major role 
>in the increase in rCBF, is released from astrocytes in response to 
>stimulation of astrocytic glutamate receptors. The increased 
>delivery of glucose by the blood stream enters astrocytes, where 
>some of it is converted to glycogen. During neuronal activation 
>there is a decrease in extracellular glucose owing to increased 
>utilization followed by a delayed increase; this results from 
>stimulation of astrocytic beta-adrenergic receptors, which leads to 
>a breakdown of glycogen and the export of glucose." End of quote
>
>Alexandre Blais
>
>

Richard L. Hall, Ph.D.
Comparative Animal Physiologist

University of the Virgin Islands
2 John Brewers Bay
St. Thomas, U.S.V.I. 00802

340-693-1386
340-693-1385 FAX

rhall at uvi.edu

"Live life on the edge...the view is always better"  rlh
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