This pattern of crossing over (decussation) is seen to varying
degrees in bilaterally symmetrical animals.
Many motor pathways also cross in vertebrates and to some extent in
the invertebrates as well. For example, in decapod crustaceans such
as crayfish, there are five tonic motor fibers leaving segmental
third roots and innervating superficial flexor muscles of the
abdomen. Four of the fibers are ipsilateral (the cell body and axon
are on one side) and one fiber is contralateral (the axon crosses to
the other side).
Quite a few reflexes involve crossing over. For example the crossed
extensor reflex in mammals. Auditory triggered startle reflexes
decussate. In contrast, some afferent input into the cerebellum
seems to be pretty much ipsilateral.
Programmed cell death could explain these phenomena. Some animals
undergo torsion (twisting) during development which may also
contribute to crossing over patterns of innervation and neural
projections.
rlh
>"Karl Self" <karl.self at gmx.net> wrote in message
>news:9dkml6$insp0$1 at ID-34153.news.dfncis.de...>> Thanks for your speedy reply. As always, new knowledge spawns new
>> questions ... (see below).
>>>> "James Teo" <james at teoth.fsnet.co.uk> wrote:
>> ...
>> > Nobody knows. Good luck for your nobel prize.
>>>> Ta, but I can't right now. I'm too busy trying to win me the Field
>> Medal and few Pullitzers.
>>>> ...
>> > All sensory input crossover in humans before entering the hemispheres.
>>>> I hope I do not sound smart - arsed asking this: so vision and
>hearing
>> are cross wired, but I guess taste isn't because it is not stereoscopic
>(or
>> do we have two tongue hemispheres)? What about nostrils -- is smell
>> stereoscopic (can we smell left from right, and if not, what is the point
>of
>> having two nostrils rather than one (other than aesthetic reasons, maybe),
>> and are the nostrils cross - wired as well?
>> Sorry to be so inflationary in my questioning. I promise to be
>equally
>> inflationary in my appreciation of any forthcoming answers.
>>The vertebrate brain is crossed like you describe, but invertebrate brains
>are usually wired "correctly" -- the left side of the body is connected to
>the left side of the CNS. There is really no point to the crossover -- it
>probably is an anomaly of the developmental genes controlling the
>bilateral body, distinguishing anterior from posterior, dorsal from ventral,
>and left from right. Somewhere along the line, the left-right axis in the
>nervous system got mismatched.
>>I am not sure whether taste and smell are crossed -- I believe the
>special "visceral" senses are wired somewhat differently from the
>special "somatic" ones. But there are indeed many animals that
>can accurately locate food sources by detecting the direction of
>taste and smell. The "distance" sense of smell is more complex
>because odors are carried by air or water currents. Often, on
>detecting an attractive weak odor, an animal will respond simply
>by swimming or flying upstream. But then when the odor is strong
>enough, many animals can detect the gradient and definitely move
>towards to source. Similarly, many animals use the contact
>chemical sense (the barbels of fish, for example) to explore their
>environment and to locate food.
--
Richard L. Hall, Ph.D.
Comparative Animal Physiologist
University of the Virgin Islands
2 John Brewers Bay
St. Thomas, U.S.V.I. 00802
340-693-1386
340-693-1385 FAX
rhall at uvi.edu
"Live life on the edge...the view is always better" rlh
---
