The diameter and mass of peripheral nerve is small compared with
that of the skeletal, muscular, and circulatory apparatus in the
appendages. So there would be no evolutionary disadvantage
in the way you suggest. Of course there always is an advantage
to cutting down on biomass wherever possible -- cells need quite
a lot of metabolic maintenance just to keep themselves alive.
The myelin sheath around each nerve in one sense enormously
increases the diameter over that of the naked axon inside. However,
that increase is trivial in comparison to the advantage gained in
conduction velocity. And large vertebrates need that extra
speed for rapid response and coordination.
The conduction velocity in unmyelinated axons is proportional to
the square root of diameter, in myelinated axons it is roughly
proportional to the diameter itself. A 1 um unmyelinated cell
conducts at about 1 m/s, a 1 um myelinated mammalian cell at
about 5 m/s (the warmer temperature also contributes). As a
result, a 16 um myelinated cell might have a conduction velocity of
80 m/s while a similar size unmyelinated cell would be 20 times
slower. So the bulk of myelin actually provides for high conduction
velocity with relatively much less cell volume than the unmyelinated
case. That, combined with the fact that myelin itself is relatively
inert metabolically, gives an enormous advantage to vertebrates.
The axons are wrapped in a connective tissue sheath for strength
and mechanical protection. Bundling the axons into nerves
significantly reduces the amount of connective tissue needed to
wrap the ensemble, as opposed to wrapping the individual fibers.
Having the nerves travel together throughout most of their course
might also be important in the way they develop and find their
way to the proper destination. This tendency for nerve fibers
to collect in ensembles rather than being diffusely scattered in
a nerve plexus is generally called "centralization", and is clearly
seen in the difference between, say, the CNS of a flatworm and
that of annelids, arthropods, or mollusks.
"Bill Browning" <bbrownin at nospambellatlantic.net> wrote in message
news:jgww5.2070$T6.54682 at typhoon2.ba-dsg.net...
> 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.
>>>>>
