Reply to gord at homostudy.win-uk.net
In one response to my hypothesis about a myelin capacitor R
Norman wrote:
"Yes the membrane does have substantial capacitance and the myelin
sheath has an exceptionally large surface area, providing an
enormous capacitance along the axon.
"However, a real capacitor made of wound foil has a very highly
conducting metal layer connecting the pieces, whereas the sheath
has a very high resistance thin layer of electrolyte connecting
the *concentric* (my emphasis GKG) layers. In essence, these layers
are capacitors in series, making the total capacitance very small,
not very large. Since the membrane also has an an extremely low
conductance (and also consists of many layers in series
1 The "layers" (lamellae) are not concentric in the same
sense that circles can be concentric, but each turn, exactly as in a
wound-foil-capacitor, is *continuous* with the next, forming in
effect a *single* curve - viz. a *spiral*which appears very clearly
in electron-micrographic longitudinal and transverse sections (See
figs 2 a & b p327 Kuffler, Nicholls & Martin - "From Neuron to
Brain" 2nd edition, and elsewhere). Hence the lamellae are in fact
in parallel from the nodal edges, not in series. Moreover, inside
the sheath membrane the film of water and electrolyte opens out
into a spacious tapered channel which connects directly to the
cytoplasm of the Schwann Cell or Oligodendrocyte.
2 Can anyone who still accepts the view that myelin is an
insulator "pure and simple", not a capacitor, explain how a Schwann
Precursor or an Oligodendrocyte might grow an appendage that is in
effect a chain of series-connected capacitors?
The only well-known engineered electrical component with
this sort of structure can be seen on the high voltage terminals of
Transformers in High-Tension switch-yards, but the ends of the
concentric conducting tubes are not connected together in parallel
as are the ends of the lamellar turns. This would completely defeat
their object, which is to prevent "tracking" (carbonisation of the
insulation)
3 To perform its function as a capacitor the membrane of
the sheath does not need to be a "perfect" insulator, only to be
"improved" by natural selection or humans. The history of radio
engineering proves this in the case of electrolytic capacitors
which were originally used a *rectifiers*, i.e. "lopsided"
conductors.
In order to serve the needs of metabolism there must be
"leaks" in any cell membrane, which means that in the case of
the myelin capacitor, their concentration in the membrane must be
reduced in relation to that found in most cell membranes. Chiu in
his studies of oligodendrocytic myelin reports that this is
exactly the case.
Any questions? Gord