In answer to Nancy's query about the existence of unmyelinated
fibers, on Tue, 16 Jan 2001, David Wheeler wrote:
> Some have none at all. Myelin in generally found on axons
> that cover great distances. I would be surprised to find
> myelin on an interneuron with an axon that was less than a
> millimeter long. Yes, there are many neurons in that
> category.
I didn't know there was a relation to length. However, I agree
that both myelinated and unmyelinated fibers exist. For example,
the standard statement about pathways carrying pain information
is that they consist of two kinds--the smallest of the
mylelinated fibers (the A-delta fibers), and an even smaller
unmyelinated class of fibers (the C-fibers). And as a specific
cite, Caterina and Julius (1999), in a recent paper on the
neurophysiology of pain, note:
"Nociceptors can be broadly divided into two classes: one group
has small-diameter cell bodies and slowly conducting,
unmyelinated axons (known as C fibers), whereas the other has
medium-diameter cell bodies and faster conducting lightly
myelinated axons (known as A-delta fibers)."
Myelinated fibers get their myelin from two kinds of cells
(oligodendrocytes in the CNS, and Schwann cells in the peripheral
nervous system), which wrap themselves repeatedly (10-160 times)
around the nerve fiber. Each wrap provides two layers of their
myelin-containing membrane. So I assume that the mention of
"lightly-myelinated fibers" in the above quote refers to axons
that have a lesser number of wrappings.
I've never come across a discussion concerning how number of
wraps (degree of myelination) might be related to function.
However, I would guess that the more myelination, the faster and
more efficient the conduction. This is because more wrapping
would increase the separation of charges inside from outside,
producing less capacitance (tendency of the axon to soak up
charge like a battery rather than send it down the axon).
And Pat Cabe queried:
> Can someone clarify a question about myelin for me? Schiffman's
> Sensation & Perception book (5th ed., 2000, p. 13) says:
> "Myelin is made of protective and nutritive cells (called glial
> cells)...."
I've always defined glial cells as all the other cells in the
brain that are not neurons, so the cells providing myelination
would fall into that category. Sure enough, Rozenzweig (1999)
says in his textbook, "The third and fourth types of glial
cells--oligodendrocytes and Schwann cells--perform a a vital
function for nerve cells, as the next section describes".
But if Schiffman really said it that way, it's a rather confused
statement. Myelin is not made of cells, but cells are made of
myelin (their membranes contain it, anyway). Myelin itself is
a substance consisting of both protein and lipid.
-Stephen
References
Rozenzweig, M. et al (1999). Biological Psychology, 2nd ed.
Caterina, m., & Julius, D. (1999). Sense and specificity [cute
title, eh?]: a molecular identity for nociceptors. Current
Opinion in Neurobiology, 9, 525-530.
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Department of Psychology fax: (819) 822-9661
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