Text 1109, 183 rader
Skriven 2004-12-20 06:18:00 av Tim Tyler (1:278/230)
Ärende: Re: Question regarding th
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r norman <rsn_@_comcast.net> wrote or quoted:
> On Sat, 18 Dec 2004 18:50:53 +0000 (UTC), Tim Tyler <tim@tt1lock.org>
> >r norman <rsn_@_comcast.net> wrote or quoted:
> >> On Fri, 17 Dec 2004 23:14:03 +0000 (UTC), "Curious in Minneapolis"

[brain crossover -> head reversal?!?]

> >> 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. [...]''

> As I said, there have been some lame-brained explanations, but none
> that have caught the enthusiasm of the majority of scientists.
> 
> The information you cite claims that very rapid escape mechanisms with
> monosynaptic connections would be most adaptive with a crossed system.
> Unfortunately, in the vertebrate spinal cord, the only monosynaptic
> connections (the stretch reflex) are distinctly ipsilateral -- the
> motor neuron is on the same side as the sensory neuron.  The flexor
> reflex and associated crossed extensor reflex, a response especially
> to extremely strong and painful stimuli, is multisynaptic.

The argument was targetted at a primitive "worm like creature".

Presumably we are witenssing only its distant descentants.

There's likely to have been some scope for wiring changes in
the interim.

> If an escape reflex must cross the midline from receptor to effector,
> it doesn't matter whether the sensory system crosses or the motor
> system crosses.

The hypothesis made little distinction between the systems - and
simply suggested that nerve fibres ran - somehow or another - from 
"attack" sensors on one side of the body to "escape" motor units on
the other.

> However in the vertebrate system, if the escape
> reflex must control the same side as the stimulus, then either both
> sensory and motor must cross the midline, a system that takes longer
> pathways and hence takes longer time, or neither does, a system with
> shorter pathways requiring less time.  Vertebrates use the former.

The idea is that both systems crossing is an offspring feature,
derived from the original ancestral cross by differing selection
pressures when the organism was more complicated than a worm.

> This whole argument makes no real sense when examined closely.  One
> would expect arthropods to be subject to exactly the same evolutionary
> pressures as vertebrates, yet only vertebrates have the crossover.

I would have to look at the common ancestor of both species before
making that judgement.  It appears that the argument depends critically
on the frequency of attack from one side.

Maybe the vertebrate ancestor was comparatively long and thin - making 
attack from the side more common (and perhaps simultaneously requiring 
structural segments along its length).

I don't think you can just assume that the common ancestors of both
groups were under similar selection pressure relating to the frequency
of attack from one side - without good supporting evidence.

> Once you start talking about brain connections, the argument still
> makes no sense.  Given the multiplicity of CNS interneurons involved,
> any "computed" response through the brain requires typically several
> hundred milliseconds to perform.  Suppose the response latency is only
> several tens of milliseconds.  At, say, 10 meters/second = 10 mm/msec
> conduction velocity for myleniated neurons in lower vertebrates, and
> given a brain even as large as one centimeter = 10 mm  , it would take
> only 1 msec to go the extra distance of crossing.  I am using very
> conservative values -- a relatively large brain for a protovertebrate
> and a relative slow conduction velocity for a myelinated axon.  No,
> the argument about selective advantage for escape or for computational
> efficiency simply doesn't hold.

The argument refers to a primitive creature.  The responses under
discussion will typically be travelling through its middle -
without going through its brain at all.

If some of your left legs are eaten, you need to jerk away with
your right legs.  If a bite is taken from one side of you, you
may need to contract the other side, moving your head and tail
away from the attacker - and then anchor them to pull your
midsection away.

Only once a pattern of connections across the body has been 
established will the crossover become locked in, and spread
through other systems, including eventually the brain.

> Most of the arguments presented would make even better sense if
> neither the sensory nor the motor systems crossed.

The /main/ argument is that a primitive long, thin creature needed the
crossover for surval purposes - most likely escaping from predators
attacking from one side.

It suggests a reason why vertebrates have this feature - their common 
ancestor was segmented, long and thin - and was thus more likely to be 
attacked from the side than most other organisms.

> So, as I said to start with, I still don't see any good explanations
> that bear close examination.

IMO, this explanation /does/ bear fairly close examination.

I can also think of a theory based on vision inversion - along
somewhat-related lines to the one I previously cited:

The retinal inversion means that nearby objects are projected
onto distant points on the retina.  To bring these together (
e.g. to facilitate calculations to determine distance), the retinal 
pathways have to either cross, hemispheres, cross internally - or rotate 
through 180 degrees.  Under a  model where connections between points
that are involved in such calculations form and then shorten, the
images could often be pulled through one another - rather than
rotating through 180 degrees.

However, I suspect this theory is not correct as an explanation
of the sensory crossover.

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.
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