help me understand myelin

K C Cheng kccheng at postoffice.idirect.com
Mon Mar 30 18:33:31 EST 1998


Vincent Canter wrote:
> =

> I have a couple questions about the function of myelin. As I have not r=
ead
> this group before, let me first apologize if these are questions that h=
ave
> already been gone over.
> =

> The explanation I have always got is that myelin is an insulator, i.e.,=
 is
> non-conductive, and the axonal membrane is a comparatively good conduct=
or.
> Non-myelinated nerves are not especially fast at transmitting signals. =
A
> spike potential would take several seconds to go one meter down a
> non-myelinated axon. But in a myelinated axon, the signal somehow "jump=
s"
> from one gap between the myelin bands to the next gap, because the myel=
in
> bands insulate that part of the axonal membrane. That eliminates
> most of the time necessary for the signal to propagate down the length =
of
> the axon.
> =

> I apologize again if this is over-simplistic. I'm only presenting it he=
re
> because this is the explanation I've received from more than one
> accomplished and well-esteemed professor. I'll now relate my own
> explanation, and you can tell me if I'm way off.
> =

> The "spike" is a potential difference between the interior and exterior=
 of
> the axonal membrane, as well as a difference between one end of the axo=
n
> and the other. Therefore, a spike started at one end will not naturally=

> travel to the other end, it will disperse into the surrounding tissue.
> The speed of transmission through the axon and the surrounding tissue i=
s
> actually incredibly fast, but the signal is so quickly dispersed that i=
t
> never gets very far along the length of the axon before it is too small=
 to
> register.
> =

> For that reason, the spike, the difference between the interior and
> exterior of the axon, has to be regenerated every so often, probably ev=
ery
> 0.1 mm or so. There has to be a switch for all the ion channels, which
> opens when it is time to regenerate the inside-outside potential.
> =

> Here's the important part-this is what I want to know about- It is the
> constant switching of these channels, not the actual transmission of th=
e
> action potential which make the transmission of the signal slow. The
> actual voltage can be transmitted almost instantaneously, but it is not=

> the voltage from one end of the axon to the other that is relevant. It =
is
> the inside-outside voltage that is relevant, and so it is the constant
> switching that makes the propagation slow.
> =

> The intermittent insulation afforded by the myelin, then, does not allo=
w
> the actual voltage to propagate faster. It performs the same function t=
hat
> rubber insulation on copper wires performs. It allows the signal to got=
 a
> little further before it has to be amplified, and so the little ion
> channel switches simply don't have to be turned on and off as often as
> they do in non-myelinated nerves. Ideally, the entire length of the axo=
n
> would be myelinated, but because the resistance/ unit length of the axo=
n
> is so high, it can only be insulated for a short length (maybe a few mm=
).
> =

> If anyone out there can tell me if I am correct about this, I would
> appreciate it.
> =

> Thanks in advance,
> =

> Vince
Re the above:
What you say is logically sound.  However,  in fact this is not the
precise reason or mechanism.   There is an actual space between the
myelin  and its ensheathed  axon.   Electrons easily go through this
space from one node to the next.   Since electronic flow has been
proven  to occur in the propagating action potentials,  this jump from
node to node further proves electronic propagation in nerve impulses.   =

These are proven in my =

The Electromagnetism of Memory, Mentation and Behaviour,  vol.  1R
which I  hope to complete in the near future.  Some preliminary work at

http://webhome.idirect.com/~kccheng =

-- =

kccheng:=B9=BF=C2=F7=F7=F7=F7=F7=A5=BB=BC=A7



More information about the Neur-sci mailing list