Neurological Question

Brown

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There is no doubt that squid and octopus nerves and axons regenerate (like most invertebrates) . Work by Young in the 70s and recently Packard on the nerve tract that contains the giant axons and chromatophore nerves shows that functional regeneration after a nerve section completes in 40- or so days depending on the time of year and temperature (degeneration occurs first of course). As to toughness, an isolated axon will survive for a couple of days but they don't survive stretching or bending well.
 

Brown

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What determines conduction velocity?

Conduction velocity in a 450 micron diameter axon would be about 18 m/s at 20 degrees. The range in nervous systems is from a few cm /s (pain fibres) 100 m/s myelinated nerve. There are several ways to 'speed up' conduction velocity (speaking in terms of evolution and biophysics). The two main ones are …

1. Decreasing the longitudinal axonal resistance. As the axoplasm resistance is constant this improvement can only be achieved by increasing the diameter of the axon (in the way that a length of copper of large diameter is of lower resistance than one of small diameter). If you like equations, this means that conduction velocity is proportional to the square root of axonal diameter. Selection pressure for high conduction velocities has resulted in the appearance of giant axons.

2. Increasing the transmembrane resistance. This is achieved by wrapping the axon with more than one glial membrane= myelination. Mostly in vertebrate (but not all vertebrate nerve see slow conducting pain fibres) ns. Is also present in some invertebrates (but not cephs). Leads to salutatory conduction and high conduction velocities with small diameter fibres.

hope this helps
 

Steve O'Shea

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Looks like we have ourselves a new, resident neurological expert online; thanks for those words of wisdom (that went right over my head); I'm sure John (and others) will appreciate them.

Welcome to Tonmo, Dr Brown; I'm sure that you have expertise in other areas also.

Steve
 
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i know physciatrists and neurosurgeons that wouldnt understand that where did you get the degree ?!!!!! also doctor steve thats a bit of an understatment ! hail prof.brown :notworth:
 
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Brown said:
1. Decreasing the longitudinal axonal resistance. As the axoplasm resistance is constant this improvement can only be achieved by increasing the diameter of the axon (in the way that a length of copper of large diameter is of lower resistance than one of small diameter). If you like equations, this means that conduction velocity is proportional to the square root of axonal diameter. Selection pressure for high conduction velocities has resulted in the appearance of giant axons.

Brown,

Thanks a lot! Actually, that does help quite a bit. My physiology class has moved past neurons and action potentials, but my instructor has encouraged me to gather more information on ceph neurology (he's a bat person). He thought that they (giant cephs) might solve the issue of conduction velocity by means of larger overall axonal radii.

Now to figure out why they are immune to serin! :smile:

Sushi and Sake,

Fujisawa... AKA John of the Great Pacific Northwest

P.S. Welcome to TONMO!
 
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em youve explained the system perfectly acording to this gargantuin book ive got on my lap about marine biology and anatomy..but do they sense the world around them more like us or fish is what im still not clear on


:goldfish: :read: :|
 
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mcatee123 said:
em but do they sense the world around them more like us or fish is what im still not clear on

mcatee,

Heh heh, you've hit the nail right on its proverbial noggin! A lot of my questions on this and other boards deal with how cephs look at the world around them. My guess (not yet being a marine biologist anyway) is that we have a lot of work to do and a lot of research to accomplish before we can truly answer these questions.

Maybe you should get into marine biology! :biggrin2: I would love to work with a fellow TONMO'er someday in research!

John
 

Brown

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em youve explained the system perfectly acording to this gargantuin book ive got on my lap about marine biology and anatomy..but do they sense the world around them more like us or fish is what im still not clear on


Good question.. ... there is no clear answer to this one as yet. The reason is that although they have acute visual and tactile senses (as good a ours), we have no idea how this is represented in the brain. So for example, in your CNS there is a representitive physical map of your sensory fields. This helps you to 'know' where you arm is positioned is space with your eyes shut. This does not seem to be the case for Octopus. For example from the recent the work of Hochner+ co on arm control it seems that the arm is 'cast ' (like a fly fisherman) towards an object and there seems to be no cns control other than a go signal. More work required though before a firm conclusion can be reached...
 
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mcAtee,

Say, you mentioned the you have a large book on marine biology and anatomy... Could I have the title and author please? I would really appreciate it.

Brown,

I have a textbook Biopsychology: 5th Edition by Pinel, and Dr. Pinel makes reference to the type of CNS sensory mapping done by the brain. What I find interesting is the idea that an invertebrate has come up with similar adaptations due to convergent evolution of sorts.

I wonder if Hochner is right? Even then, there seems to be an overall reduction in number of arms over the evolutionary history of the cephalopoda. This has been addressed here, but if such a reduction has occured in the Class, maybe its due to the increasing complexity of the arms as sensory systems. It's "evolutionary streamlining", if you will, but I would think it would be less neurologically taxing to only control eight to ten appendages rather than a much larger number.

Do you know of any other spatial orientation or sensory tests done on cephs?

Thanks for your earlier information, btw

John
 

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