Why is the nervous system crossed over?

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1styrbscphysio

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Why is the nervous system crossed over?
« on: 27/11/2010 19:30:03 »
Michelle Huntley  asked the Naked Scientists:
   
We have a question about our spinal columns.

Last week in our physiology class our tutor could not answer why spinal tracts cross from one side to the other to synapse - it all seems a bit unnecessary and overcomplicated. Is there a evolutionary theory that answers this?

We would love to know....

1st year BSc Physio Students at Brighton uni        

What do you think?
« Last Edit: 27/11/2010 19:30:03 by _system »

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SteveFish

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Why is the nervous system crossed over?
« Reply #1 on: 27/11/2010 20:14:35 »
I don't think that there is any good answer to your question, except the surprising fact that evolution is not an intelligent designer (look at the reversed vertebrate retina). Evolutionary design is opportunistic. There are several hypotheses regarding the vertebrate contralateral design but no way to support any one strongly. They usually are something like a very primitive ancestral vertebrate survived best with a reflex that caused it to bend away from a touch stimulus. Bending away requires contraction of muscles on the side opposite the stimulus, so the fastest reflex response, with the fewest connections, would be a crossed sensory input to the muscles on the opposite side.

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Offline granpa

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Why is the nervous system crossed over?
« Reply #2 on: 27/11/2010 22:40:29 »
not only is it crossed over but the head is where the tail should be.

its shared by all vertebrates (not sure about echinoderms though) but not present in molluscs or arthropods


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SteveFish

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Why is the nervous system crossed over?
« Reply #3 on: 28/11/2010 01:15:49 »
Granpa, I am pretty sure that there are quite a few humanoid vertebrates with their head in the wrong place, but otherwise please explain. Steve


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SteveFish

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Why is the nervous system crossed over?
« Reply #5 on: 28/11/2010 16:10:59 »
RD, thanks for the link. I think the binocular vision hypothesis is very weak and Roger Keynes doesn't inspire trust in his opinion because he is incorrect in his explanation of retinal optics. Using early vision (not binocular) as an explanation is a little more logical and similar to the touch hypothesis I mentioned (from Sarnat and Netsky, I think), but both have problems. I think that somebody will eventually figure this out. Steve

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Offline chris

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Why is the nervous system crossed over?
« Reply #6 on: 28/11/2010 16:14:43 »
Roger Keynes doesn't inspire trust in his opinion because he is incorrect in his explanation of retinal optics.

Hi Steve

Are you referring to the hemifields example given by Roger?  I'll need to check back to whether this was an audio transcription error.

Chris
« Last Edit: 28/11/2010 16:17:51 by chris »
I never forget a face, but in your case I'll make an exception - Groucho Marx

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SteveFish

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Why is the nervous system crossed over?
« Reply #7 on: 28/11/2010 16:30:39 »
Chris:

Yes, in the area of binocular overlap an object in the left visual field should be projected on the right retinas of both eyes. Steve

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Offline chris

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Why is the nervous system crossed over?
« Reply #8 on: 05/12/2010 11:25:37 »
Yes, I spotted that, but only more recently when going through the transcript. Despite that error, I think he makes a good point about the original need to evolve a mechanism for crossing over of neurological connections; how an why this became applied to the rest of the nervous system, however, I haven't a clue!

Chris
I never forget a face, but in your case I'll make an exception - Groucho Marx

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SteveFish

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Why is the nervous system crossed over?
« Reply #9 on: 05/12/2010 18:33:01 »
Chris:

A clue? Neither do either.

Steve