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Brain surface

r norman rsn_ at _comcast.net
Thu Nov 25 07:32:45 EST 2004

On Thu, 25 Nov 2004 12:44:23 +1100, Matthew Kirkcaldie
<m.kirkcaldie at removethis.unsw.edu.au> wrote:

>In article <u24op0her5t9ss5tblv8kgei6nr7df5q0b at 4ax.com>,
> r norman <rsn_ at _comcast.net> wrote:
>> The traditional mediators of increased local circulation include
>> "metabolites" like decreased pO2, increased pCO2,  increased
>> extracellular [K+] and adenosine. Any number of cellular metabolic
>> processes produce the first two: nerve activity resulting in increased
>> Na/K pump activity is certainly a leading candidate.  The same nerve
>> activity will also increase K+.  These factors would readily diffuse
>> over distances up to 1 mm.  The pO2 and pCO2 changes would readily
>> cross glial cells, probably a pH change could also, although the
>> astrocytes may shield arterioles from K+ changes.  However, you could
>> easily imagine that whatever metabolic effect controls arteriolar
>> dilation would be something that the astrocytes were adapted to pass
>> through rather than shield.
>> Using up ATP most definitely does mean a change in the metabolic rate
>> of the cells -- the oxygen consumption -- since brain ATP is
>> essentially produced completely by aerobic metabolism.  Even if the
>> energy is derived from local stores, you still need oxygen and that
>> means blood flow.
>These things are true in principle, of course, but are you really saying 
>that the picomoles of K+ crossing the neural membrane during an AP are 
>going to make a detectable dint in the millimolar K+ extracellular 
>concentration a millimetre away?  I'm not pretending superior knowledge 
>here, I'd be stupid to, but that really surprises me.

You are right to be sceptical.  In teaching about the action potential
I always emphasize that the tiny amounts of Na and K that cross the
membrane during a single  action potential are so small as to cause
virtually no change in intracellular or extracellular concentrations.
However the situation changes when you consider cells making thousands
of action potentials (twenty/second for a minute) and when you
consider thousands of cells all active in a small volume.  The
extracellular space is also quite confined.

Here is a paper described some experimental data:

  A Chvatal, P Jendelova, N Kriz, and E Sykova
  Stimulation-evoked Changes in Extracellular pH,
     Calcium and Potassium Activity in the Frog Spinal Cord.
   Physiol. Bohemoslov., 37 (3): 203-212, 1988.

or, for a review 

E Sykova
Extracellular K+ accumulation in the central nervous system.
Prog Biophys Mol Biol. 1983;42(2-3):135-89.


GG Somjen
Extracellular Potassium in the Mammalian Central Nervous System
Ann Rev Physiol 41:159-177 (1979)

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