IUBio

Myelin capacitance measurement

G K GRAY gord at homostudy.win-uk.net
Sat May 18 07:18:53 EST 1996


 
In article <483eh316 at neubio.sld.ar>, Administrador del Nodo (Postmaster at neubio.sld.ar) writes:
>Hello all!
>
>            We have plenty of brains with homologous
>myelinized and demyelinized areas (even a lot of tho-
>se brains still in use!), but the global measurement
>of  capacitance per tissue volume seems me too uncer-
>tain to assess absolute values per axon length.  So
>I would like proposing to heuristically putting memo-
>ry-issues aside and searching for capacitance values
>of myelin in gray matter.  How could we measure them?
>
>             Just as almost everybody else, I believe
>that myelin-sheath capacitance is low.  An undeniable
>merit of Gordon Gray is making us to realize that the
>measurements grounding our belief are not a daily ma-
>tter. I find useless discussing it theoretically whi-
>le at least some measurement reports are not indicated,
>so as to ascertain what was really measured, and then
>criticizing the design. So I would like to pray everybo-
>dy for directing us to published measurements, hopefu-
>lly commenting them in the net.
> 
>               ---------------
>
>               Now due to the good will of Gordon Gray
>I have seven copies of his paper though I am unable of
>decoding them, and a paper copy of Figure 3 (sent by
>surface). So I cannot completely understand his expla-
>nations of today, with which he attempts to cope with
>R. Norman's indication of a crucial difference with the
>conventional lore.  In particular I would like to ask
>Gordon (to grasp his views) for the following:
>
>1) What is that which Gordon calls nodal "edges"?
>   Can "in parallel with the nodal edges" means "in
>   series on other accounts"?  (I cannot draw the cir-
>   cuit on Gordon's depiction; see below a mention
>   on the morphology of those "edges"). So,

For much of what I recognise I have not yet been able to find
accepted terminology, "Nodal edges" (of the lamellae - NE) being a
case in point. I will try to construct an acceptible diagram for
this: 

____________________                  ____________________
___________________ \     Node of    / ___________________
______________ \   \ \    Ranvier   / /   / ______________
_____________ \ \   \ \            / /   / /   
_______ \    \ \ \   \ \
       \ \    \ \ \ NE \ \           &c to match in post-nodal
        \ \ NE | |\ \   | |                   sheath structure
  EF     \ \___/ / \ \__/ /      Ne includes glial membrane
          \_____/   \____/       and contents
______________________________________________________________
______________________________________________________________
..............................................................
..............................................................
                          Axon

This  channel in the Nodal Edge appears at entry and exit paranodes
of the one sheath coil. Both act as conductors which put the
internal lamellar films in parallel. EF, the External Fluid
performs the same function for the fluids external to axon and
glial cells. Ergo the full capacitance of the sheet is maintained
(see questions 2 and 4) 
                          
>2) What is the circuitry assumed by Gordon to keep
>   the high capacitance of myelin as a sheet, when it 
>   infolds?  How does such a circuit differ from a
>   single rectangular sheet of deflated cell?
>
>3) Up to my knowledge, each infolded layer is made by
>     four constituents:
>
>     a) interstitial extracellular fluid 40 A thick;
>     b) membrane bilayer;
>     c) reducing cytoplasm about 200 A thick;
>     d) membrane bylayer
>
>(Please correct me if my thickness values are old).
>
>Is Gordon considering any transmembrane difference 
>among both electrolytes to account for such anomalous
>scale effect as he proposes? I.e., making oxidizing
>the interstitial fluid?  In such a case, with which
>pH values can such anomalous scale effect be done?
>And, in such a case: 
>
>4) "The ends of the lamellar turns", Gordon writes,
>   "are connected in parallel". By what?  Am I mistaken
>   in suppossing that those ends overlap yet remain loo-
>   se in extracellular fluid? Or is this last electroly-
>   te what is taken as making the connexion, among the
>   double tiles made by the deflated cell at the nodal
>   end of every turn? (= Norman's "very high resistance
>   thin layer of electrolyte" as connector).   
>
>5) How does Gordon understand the role of the cytoplasm
>   in the non-deflated part of the myelin cell regarding
>   the capacitance of the sheet(s) to which it is connec-
>   ted?  As a source or as a sink? Ionic charge carriers
>   cannot circulate fast enough to make myelin a kind of
>   Leiden flasks (bottles, reservoirs: I know not the pre-
>   cise word) making Eniac-like memories.  So, what role
>   could the cytoplasm in the non-deflated part of the
>   myelin cell play? 
        I see the function of oligodendrocytic myelin is to act as
part of a system for registering rapid changes in both external and
internal environments, those changes which are most likely to need
immediate attention. This means that the sheath capacitor(s)
around an axon that has just fired hold a charge that prolongs the
RRP of the axon, An adjacent axon was *not* fired at that time so
it is in its resting state. A second pulse arrives to trigger
both, one still in its RRP, the other fully rested. A coincidence
detector (Converse of lateral inhibition) at the afferent synapses
reads the start of timing. Lateral inhibition at efferent synapses
signals a recent change which deserves attention. Look for these
circuits from, say, the LGN to the Visual areas.
        A Leiden Flask was probably the earliest form of *capacitor*
to be invented. I contend that Natural Selection got there before
us with myelin.GKG 
>
>All these questions are not meant to replace the most di-
>rect measurement of capacitance values.  Nor are them ob-
>jections but sincere wonderings.  It might be that former
>measurements could have been done regarding sectors of the
>morphology irrelevant for the possibility of the effect
>that Gordon tries to point to us. So I dare to press the
>issue in this way; I find it valuable.

On the count of earlier observations that should be re-tried,
hopefully with more precise techniques, Chiu cited an observation
by     of variation in myelin interstitial film thickness during 
repeated firing of the axon.

>
>                Cheers,
>                Mariela
>NB: Please Gordon dispose of that horrendous zipping program!
>Cheers again, M.
>=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=
>       Prof. Mariela Szirko,
>       <postmaster at neubio.sld.ar> 
>                            
>       Centro de Investig. Neurobiologicas, Ministry of Health 
>& Welfare, Argentine Republic; and 
>       Lab. of Electroneurobiological Res., Neuropsychiatric
>Hospital "Dr. Jose Tiburcio Borda", Municipality of Buenos Aires,
>       Office:  Phone/Fax (54 1) 306 -7314
>                e-mail <postmaster at neubio.gov.ar>
>       Standard disclaimer: Las opiniones de este mensaje son
>personales y no comprometen las dependencias a cargo de la firmante
>  Reply to THIS message,  ONLY to: <postmaster at neubio.sld.ar> 
>=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=
>
It's past midnight! I must get some sleep. More later.

CHeers! Gord




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