help me understand myelin

JValla jvalla at mail.utexas.edu
Fri Apr 3 16:01:00 EST 1998


Try this one for a <straight> answer:

>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).

Because of phenomena usually termed "cable properties," the electrical signal 
degrades as moves along the axon (as does a signal in an uninsulated electrical 
wire.  Myelin prevents this degradation and speeds transmission at the same 
time.  The signal starts at the hillock with a spike, then begins degrading as 
it travels down the axon.  Just when it has just enough potential left to open 
enough channels to regenerate the spike (about 1 mm), it comes to a node and is 
allowed to do so; then the whole thing starts over.  There are all kinds of 
formulas for figuring out the exact potentials and how much degradation there 
will be at which point, and so on.
The nodes of Ranvier are spaced so that as few spikes as possible are required, 
presumably to eliminate that "switching" time.  So, a completely myelinated 
axon would be unable to conduct as the signal would degenerate and be unable to 
 renew itself.  Conduction is sped up because the potential traveling through 
the axon from node to node travels at (or near) the speed of light.  
Unmyelinated axons (almost all very short) are slower because they require 
continuous renewal of the spike.

If you really want, email me, and I can dig out the facts and formulas from my 
old notes.

J. Valla
Behavioral Neuroscience
UT-Austin




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