Text 1153, 130 rader
Skriven 2004-12-21 22:08:00 av Tim Tyler (1:278/230)
Ärende: Re: Question regarding th
=================================


r norman <rsn_@_comcast.net> wrote or quoted:
> On Mon, 20 Dec 2004 21:39:01 +0000 (UTC), Tim Tyler <tim@tt1lock.org>
> wrote:
> 
> >Tim Tyler <tim@tt1lock.org> wrote or quoted:
> >
> >[origin of nerve crossover]
> >
> >> I can also think of a theory based on vision inversion - along
> >> somewhat-related lines to the one I previously cited: [...]
> >> 
> >> The most obvious other theory it that the feature was never an
> >> adaptation in the first place - and is simply the result of
> >> some sort of developmental accident.  I don't give that theory
> >> much weight either.
> >
> >Another theory occurs to me: that the phenomena arose from
> >a system that governed a creature's gait.
> >
> >Early vertebrates would have been swimmers - and one way to
> >generate a sinusoidal motion along a spine is to connect
> >stretch receptors on one side of the body through amplifiers
> >to motor fibres on the other which are a bit further down the body.
> >
> >I haven't done a sophisticated simulation of this - but the
> >midline crossover means you only have to feed one signal in
> >at the top to drive the system - and may have some other
> >helpful properties as well.
> >
> >The theory shares an important feature with the "defense 
> >against attack" theory - in that nerve fibres on one side
> >of the body connect to motor units on the other.
> 
> You totally fail to recognize the true problems of the crossover in
> the vertebrate nervous system.

?
 
> First, you seem not to understand that there is no crossover at the
> spinal level.  Sensory organs on one side connect predominantly to
> interneurons and motor neurons on the same side.  Motorneurons on one
> side connect to muscles on the same side.  There are a significant
> number of contralateral connections, but the basic wiring of the
> peripheral system is ipsilateral.  All your arguments about crossover
> seem to apply primarily to spinal circuits.  Hence they fail.

Surely not so.  The question is why crossover exists in the
first place.  The proposed explanation is that - in some ancestor -
it was adaptive for connections on one side of the body to be
linked to motor units on the other.

We know that - in most of the descendants - the opposite more
frequently applies - and both sensory and motor systems cross -
but selection on descendants can be different from that on the
common ancestor - and very likely was in this case - if there's
an adaptive explanation at all.

> Second, you seem not to understand that the CNS has crossover in both
> the sensory and in the motor pathways.

An incorrect impression.  I don't know where you got it from :-(

> Sensory activity is sent to the contralateral brain.  There it 
> activates motor pathways which send their outputs to motorneurons on 
> the contralateral spinal cord.  As a result, the muscle which get 
> activated are on the SAME SIDE as the sensory stimulus!  Hence your 
> arguments fail once again.

I - and the page I originally cited - argued that the crossover was
an *ancestral* feature - and that modern organisms exhibit it
as a consquence of common descent.  In the common *ancestor*, it is 
proposed that sensory and motor systems did *not* cross - and that's
the origin of the lateral swapover.

The fact that complex descendants have arisen with different selection 
pressures acting on them that cause motor nerves to cross as well
doesn't seem valid as a counter-argument to this idea.

> How do you explain that sensory activity on the right side of the body
> connects to the left sensory cortex and that the left motor cortex
> connects to motorneurons on the right side of the body that control
> muscles on the right side of the body?

Because - long after the crossed nerves had evolved - same-side links
were subsequently what was selected for.

Selection pressures favouring this are easy to imagine - when you
see something to the left, you contract muscles on the left - and
turn to the left.

As to why the motor fibres swapped back - rather then the sensory fibres,
I note that that is what would happen if the change happened gradually -
and the demand for motor functions on the same side as the senses
that activate them increased gradually.  Any one of a number of
other asymmetries between sensory and motor nerves could account
for why motor fibres could swap - while nerve fibres were less
likely to change sides in response to selection.

The account offers no explanation for why a spinal crossover
doesn't also remain.

I'm not sure how much of an effect there is there - since many spinal 
nerves do cross the mid-line - but there are a number of possible 
explanations if the observation is significant - for example,
selection for same-side structures may be more powerful in the case of 
reflexes - since they are time critical.  Alternatively, structures
in the spine may be simpler and less locked in by developmental 
constraints.  Either would mean that traces of the crossover in
the spine would have been more rapidly kicked over.

Another interesting clue is the fact that not all senses
participate in the cross.  The sense of smell doesn't cross
over - for example.  It seems that this is unlikely to be due
to use of cranial nerves - since the optic nerve still manages
to cross over.  It may be a developmental consequence of a
more primitive development path - perhaps - or maybe any original
selection favouring a cross had very little to do with the sense
of smell.
-- 
__________
 |im |yler  http://timtyler.org/  tim@tt1lock.org  Remove lock to reply.
---
ū RIMEGate(tm)/RGXPost V1.14 at BBSWORLD * Info@bbsworld.com

---
 * RIMEGate(tm)V10.2á˙* RelayNet(tm) NNTP Gateway * MoonDog BBS
 * RgateImp.MoonDog.BBS at 12/21/04 10:08:09 PM
 * Origin: MoonDog BBS, Brooklyn,NY, 718 692-2498, 1:278/230 (1:278/230)