Nerves bundled?

Bill Browning bbrownin at nospambellatlantic.net
Fri Sep 15 16:04:09 EST 2000


   Thanks for your informative response.
     Referring to peripheral nerves, I expected that evolutionary selection
would have
bundled the related axons to reduce bulk.  Other components of legs and arms
are
slimmed down in a way that reduces weight and improves mobility.  Having
separate
myelin sheaths around each axon in the sciatic nerve must increase its
diameter
enormously and reduce the mobility of the hip joint over what it could have
been if
they were bundled.  I suppose the evolutionary path had passed the point of
no
return before the consequences began to affect survival.
     I don't blame you; besides it's too late to change it now.
Bill B.
---------------------------------------------
Bill B. had asked:
As I understand it, a nerve signal is carried by multiple nerve fibers, not
just by a
single axon.  Is the myelin sheath around single axons; or is it around
groups of fibers
which carry the same signal?
     I visualize a nerve structure like a stranded electrical wire inside an
insulating
jacket.
Bill B.
And the following reply ensued:
From: Richard Norman <rsnorman at mediaone.net>
Subject: Re: Nerves bundled?
Date: Sunday, September 03, 2000 10:43 PM

This depends to some extent on what you mean by nerve "signal".

Individual neurons carry information down individual axons, although
the axon can branch in a complex pattern.  Axons in the peripheral
system are bundled into nerves, like the multiple wires in a telephone
cable.  However, the separate nerve fibers seem to function without
any significant interaction, even if they are quite close together.

The myelin that wraps single axons of vertebrate axons is more than
an insulator, it dramatically alters the speed of conduction.  One
oligodendrocyte in the CNS can wrap an average of some 15 different
axons, while the Schwann cells in the periphery only wrap single axons.
However, I have never seen any suggestion that the group of cells
surrounded by a single oligodendrocyte are functionally related in any
way except to the extent that neighboring cells are likely to have similar
function.  The myelin "interconnection" has no functional significance.

All the above indicates that the neurons are distinct individuals, each
with a different signal.  However, in a broader sense, there is usually
a population of neurons that serves any particular function especially
in the vertebrate animals  A group of several to several hundred
or more cells all participate in the same general function and each
carries a slightly different aspect of the information.  In that sense,
the nerve signal is really carried by the population of cells.  For example
intensity of a signal can be carried by the frequency of firing of an
individual cell or by the total number of cells in the population that
are active.  Look up the terms "labeled line" and "across fiber" coding
in any good neurophysiology text to see more complex examples of
how a population of cells can transmit information.








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