Text 1086, 133 rader
Skriven 2004-12-18 16:23:00 av Tim Tyler (1:278/230)
Ärende: Re: Question regarding th
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r norman <rsn_@_comcast.net> wrote or quoted:
> On Fri, 17 Dec 2004 23:14:03 +0000 (UTC), "Curious in Minneapolis"

> >The December 11 issue of The Economist magazine carried an article
> >about lefthandedness.  It suggested that the fact that the left side of
> >the brain controls the right side of the body, and vice versa arose
> >because "long ago in the evolutionary past, an ancestor of humans (and
> >all other vertebrate animals) underwent a contortion that twisted its
> >head around 180 degrees relative to its body."
> >
> >I am intrigued by this claim, which I have never heard before.  Could
> >someone offer more details about it?  What empirical evidence is there
> >for it? What do biologists suggest could have been the reason for this
> >"contortion" or was it purely an accident? Is it possible that it never
> >really took place, that there is actually some advantage to having this
> >left-right crossover in the brain?
> >
> >I would be grateful if you could direct me to any good sources
> >understandable to a layman regarding this phenomenon. Thank you.
> 
> This "contortion" is the remnant of a very old (and discredited)
> hypothesis to explain why many invertebrates have no crossing, between
> right and left brain vs right and left sensory/motor function,  a
> ventral nervous system, and a dorsal heart whereas vertebrates show
> the crossing and have a dorsal nervous system and a ventral heart.
> However, it is quite likely that there was an early switch in the
> developmental genes that determine body symmetry and differentiate the
> left vs. the right sides.  If the brain "chose" one set of criteria to
> define which is left vs. which is right but the peripheral system
> "chose" the opposite way, then things would cross.  
> 
> Whatever the cause, it is not true that somewhere in early evolution
> the head of an ancestral vertebrate got twisted around 180 degrees on
> the body axis.
> 
> I don't know of any reasonable explanation for the crossover as an
> adaptation or advantage.  There have been some lame brained
> explanations, but nothing that captures the enthusiasm of the majority
> of scientists.

http://publish.uwo.ca/~jkiernan/anfound.htm

....offers information on the subject:

``Comparative neuroanatomists cite decussations as an example 
  of the continued exploitation of a structural feature that 
  helped our lowly ancestors escape from predators more 
  efficiently than their even more lowly competitors. Natural 
  selection would not allow the loss of a decussating pathway 
  if this were an advantage in a world full of other edible 
  animals with non-decussating neural connections. In order to 
  have left and right sides an animal must have different 
  dorsal and ventral surfaces. The struggle for survival is 
  supposed to have been among animals that lived where 
  "dorsal" and "ventral" were significantly related to he 
  surroundings (on the ocean floor,in shallow water, or on 
  land). Even the most primitive nervous systems include motor 
  and sensory neurons. A potentially fatal stimulus should 
  evoke a movement of withdrawal, so that the attacked 
  individual may survive and reproduce itself. The animal is 
  more likely to escape by moving away from the assaulted 
  side, especially if the predator is not smart enough to 
  predict such a response. The fastest neuronal circuit for 
  stimulating withdrawal to the other side of the midline is a 
  monosynaptic reflex: a sensory neuron has an axon that 
  crosses the midline and contacts motor neurons that make 
  nearby muscle fibers contract. Such an arrangement makes a 
  worm-like creature bend away from the attacked side. [...]''

It goes on to give an explanation in terms of the left-right
inverting property of camera lenses (appended).
Unfortunately, I cannot see how this explanation makes any sense.

I appreciate that sections of the optical cortex associated
with near objects needed to be adjacent in the brain - so
depth-processing of 3D objects could be performed by exchanging
signals between nearby cells - but this does not need 
decussating pathways - you can produce that result by simply
rotating the image through 180 degrees.

I'm inclined to favour the first explanation: in ancestral
creatures sensory fibres crossed and motor fibres did not (for
adaptive reasons) - and we inherit the cross from them.

Such a crossover can get permanently "locked in" - even if
individual nerve fibres can cross back, it doesn't "pay" for
them to do so.

``Decussating pathways in vertebrates allow for the formation 
  of congruent representation in the brain of images in the 
  visual fields of the two eyes. The camera-type eye of 
  vertebrate animals projects an inverted image onto its 
  retina, so that events in the left half of the visual field 
  of the left eye trigger neural signals that arise in the 
  right half of its retina. If these signals were sent to the 
  right side of the brain, the inverted projection would be a 
  mirror image of the equivalent projection from the right 
  half of the visual field (Fig. 16 -Uncrossed visual 
  pathway). 
  
  A decussating projection from the retina to the brain 
  assures that the central topographic representations of the 
  visual fields are correctly adjacent. In most vertebrates, 
  the eyes see separate visual fields, and all the fibers of 
  the optic nerve cross the midline (Fig. 17 - Completely 
  decussating visual pathway). Some mammals (including man) 
  have forward-facing eyes with overlapping visual fields. In 
  this case the decussation of only the fibers from the medial 
  half of the retina provides a correctly aligned 
  topographical projection to the brain (Fig. 18 - Partial 
  visual decussation). 
  
  A visually guided movement is most likely to be needed on 
  side from which the visual signal originates. Projection of 
  the left and right visual fields to the contralateral tectum 
  or cerebral hemisphere provides for rapid communication 
  between the visual pathway and the motor neurons that work 
  the muscles of the opposite side of the body. The visuomotor 
  connections are ipsilateral in the brain but require a 
  compensating decussation in the tracts that descend to the 
  motor neurons (Fig. 17).''
-- 
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