help me understand myelin

Vincent Canter cvcanter at gsbux1.uchicago.edu
Thu Mar 26 16:47:21 EST 1998


I have a couple questions about the function of myelin. As I have not read
this group before, let me first apologize if these are questions that have
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 conductor.
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 "jumps"
from one gap between the myelin bands to the next gap, because the myelin 
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 here
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 axon
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 is
actually incredibly fast, but the signal is so quickly dispersed that it
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 every
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 the
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 allow
the actual voltage to propagate faster. It performs the same function that
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 axon
would be myelinated, but because the resistance/ unit length of the axon
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




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