Re: [HAPS-L] Myelination (errata)

[Alan Magid <amagid@xxxxxxxxx>]

In this earlier post, I apologize for two spelling errors. In paragraph 2,
it's "postganglionic" and in the last paragraph it's "their", not "there".

Too early to compose, I guess.

-Alan


> From: <amagid@xxxxxxxxx>
> Reply-To: <HAPS-L@xxxxxxxxxxx>
> Date: Sun, 18 Feb 2007 09:34:51 -0500
> To: <HAPS-L@xxxxxxxxxxx>
> Subject: Re: [HAPS-L] Myelination
> 
> 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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