Re: (sigh)sauropod necks again--long! -Reply

["tons" <tons@logicsouth.com>]

I won't comment on this part of the thread, but I would like to add some
comments on some of the physiology of defying gravity.  I haven't been
keeping up much lately so if I go over old ground please forgive me.

First, I'm assuming part of the thread is that sauropods are deprived of
blood and therefore O2 when the head is raised substanially over the heart.

I will make an assumption that sauropods had enough blood and therefore O2
delivered to the brain when arising.  If they were able to "stand up" then
we can probably assume there was a neurocardioregulatory mechanism of some
sort.  Getting up would place a much greater demand on this system than
raising the head because the volume of blood pooling would be far greater
with both the body and the head/neck involved than the neck itself.

I do not know the neurocardioregulatory mechanism in reptiles and suspect
that it is not as sophisticated in today's reptiles as it was in some
dinosaurs because no modern reptile stands up very high.  I'm just guessing
this though.

In erect mammals there is a very sophisticated mechanism for maintaining
blood to the brain on arising.  To simplify, there are baroreptors in the
neck that sense diminished flow/volume and send messages to the heart to
speed up its rate and to beat harder therefore ejecting more blood per
stroke.  This increase in stroke volume (SV) and heart rate (HR) increases
blood pressure (BP) and cardiac output (CO).  The autonomic nervous system
also cause vasoconstriction to prevent pooling of blood and decreased return
to the heart as much as possible.  In some animals the system does not work
properly.  If the HR, SV, and CO do not match needs the animal feels bad on
arising or actually passes out.  Sometimes the system overreacts and CO goes
to high.  Neuroreceptors and cardiac receptors decide there is too much CO
and it drops.  Sometimes it drops too far and syncope occurs.  Lower and
higher centers in the brain can also alter CO.

Since sauropods were so successful for so long, we have to assume they could
arise from the ground, drink, and probably raise there heads to feed.  I'm
certain there was some sort of neurocardioregulatory mechanism as well as
anatomical considerations to allow them to do that.


----- Original Message -----
From: Mickey P. Rowe <mrowe@indiana.edu>
To: <dinosaur@usc.edu>
Sent: Tuesday, May 25, 1999 5:57 PM
Subject: Re: (sigh)sauropod necks again--long! -Reply


>
> "Augustus  Toby  White" <toby.white@chamberlainlaw.com> claimed:
>
> > Although it would not take much oxygen to fully supply the brain, a
> > sauropod brain is presumably made of pretty much the same stuff as a
> > giraffe's.
>
> In case the rearing of the head of the rearing of the sauropods thread
> wasn't bad enough, I'm going to risk starting off another ugly
> exchange by mentioning that the above isn't necessarily true if you
> add in nuances about thermoregulatory physiology.  As others have
> noted time and again, mammalian neurons are generally more energy
> intensive than reptilian neurons because their membranes are leakier
> to electrolytes.  Mammalian neurons need relatively large amounts of
> oxygen to maintain the appropriate intracellular and extracellular
> concentrations of sodium, potassium and chloride.
>
> In other words:
>
> > There's no reason to think that the sauropod brain could withstand
> > oxygen deprivation any longer or better than a giraffe,
>
> is wrong -- I've just given a potential reason.  It might also be
> instructive to analyze how hypoxia effects the brains of different
> animals.  During the last ten years it's become increasingly clear
> that the primary mechanism of brain damage (in the short term) for
> humans experiencing a stroke is excitotoxicity.  That is, if a region
> of the brain is deprived of oxygen, many of the neurons in that area
> suddenly become excessively active, and then they die.  Although I
> don't recall what all of them are off the top of my head, many of the
> steps in this physiological reaction are fairly well understood.
> Physiological adaptations could make this reaction less likely if an
> animal's normal behavior might lead to significant periods of cerebral
> anoxia.  I think most work on this area has been performed on
> mammalian brains since preventing damage to humans suffering from
> strokes is the main reason people have looked into this topic.  But I
> suspect that others have also looked into the manner in which hypoxia
> affects the brains of other animals more closely related to dinosaurs.
> Can anyone else shed any light on such research?
>
> --
> Mickey Rowe     (mrowe@indiana.edu)
>