Re: [HAPS-L] Myelination

[amagid@xxxxxxxxx]

Ken-

For any given diameter from 1 micron and up, myelinated fibers conduct more rapidly than unmyelinated fibers 
of the same diameter. For myelinated fibers, conduction velocity is linear with diameter, whereas for 
unmyelinated fibers it goes up as the square root of diameter. I can't show in this format the mathematics 
behind this, and the scaling argument only flies when channel density and conductance scale with membrane 
surface area. 

OTOH, there are no myelinated fibers below 1 micron (and very few below 2 microns). So, to get a high density 
of innervation where top conduction speed is functionally not important, tiny unmyelinated fibers makes sense. 
This probably underlies why most autonomic (sympathetic) postganglionin efferents fall in the C fiber class (as 
tiny as 0.1 microns). They conduct at only a few tenths of a meter/sec, but fast enough for the purpose of 
peripheral autonomic control. Don't forget the effectors they control are quite sluggish compared to skeletal 
muscle (glands, vascular smooth muscle).

BTW, the C fiber class also includes the sensory afferents that conduct pain. There slow conduction velocity 
accounts largely for the fact of "slow" pain, although mulitsynaptic central pathways slow things too.

-Alan

----- Original Message -----
From: Ken Saladin <ksaladin@xxxxxxxxxx>
Date: Sunday, February 18, 2007 8:49 am
Subject: Re: [HAPS-L] Myelination
To: HAPS-L@xxxxxxxxxxx

> 
> >Why are the postganglionic nerve fibers in the autonomic NS 
> unmyelinated? 
> >Is this some type of evolutionary adaptation? Is this so that the 
> signals 
> >don't get to the organs too quickly?
> >
> >I appreciate your time and insights.
> >
> >Regards,
> >Darren
> 
> 
> Maybe to save space where speed of response is not crucial. 
> Somewhere or 
> other, I read that myelination or nonmyelination is a tradeoff 
> between 
> nerve fiber bulk and conduction speed. Why have large bulky fibers 
> (and 
> their metabolic maintenance costs) if it isn't necessary to get a 
> response 
> within mere milliseconds? But there may be other answers, and I 
> look 
> forward to hearing some.
> 
> Ken
> 
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