Question
Take your time. No pressure.

-------------------------------------------
The text above is a follow-up to ...

-----Question-----
Hi:

I've read about injuries to the brain and spinal cord, with respect to autonomic functions. It seems that such injuries only decrease the sympathetic activity. Parasympathetic activity is not decreased. Why is this? I would expect that such injuries would also impair parasympathetic function. How is it that these injuries can decrease sympathetic activities to the point of a fatal shock [neurogenic shock] but leave the parasympathetic nervous system unscathed -- or perhaps even stimulate the parasympathetic system??


Thanks,

Green
-----Answer-----
Hi Green- sorry I've been so slow in responding.  I wasn't sure about how to answer your question so I left a message for a friend of mine who is a physical therapist and is very well versed in physiology.   Unfortunately we have been playing phone tag for several days now...

I don't want to not answer your question, but currently I don't have the information to answer it appropriately.  If you can give me a couple of days and ask a follow-up then I will have spoken to my friend and will be able to tell you what you want to know.  I'm sorry to put you off like this, but I'm just trying to give your question better consideration.
thank you, and I hope you will give me a second chance in a day or two-
leslie

Answer
hey Green- ok, finally got a hold of my physical therapist friend.  i will quote her answer, then add some stuff.

"I think parasympathetics are wired completely differently than sympathetics.  Sympathetics are deep, and are concerned more with the reactions of fight or flight.  I may not be correct, but I think they receive input within the spinal cord itself.
Parasympathetics are wired throughout the body and outside the spinal cord in ganglia.  They actually run along the spinal cord in a strange little network. They are extensive, and I think that when the body is injured they can sense these things, and send signals to the brain.  When they are not counteracted by the sympathetics, or when they do not reach the brain possibly due to an cord injury, they might go unchecked.  Possibly causing the huge over reaction of the body to the point where it can even shut down.
That is my guess as to why this happens, but I am not
an expert according to the site, and I AM just
speculating.  It is a good question, and I like to
think about them and try and guess what is happening."

That is her answer, and when I thought about it some more something else made sense about it being related to the location the 2 systems stem from:  autonomic dysreflexia.  The activation of both at once, due to pain in part of the body that you can't feel.  Crazy high blood pressure, headache, can kill you if you don't stop the painful stimulus, right?  Well, that happens when the Sympathetic system senses pain in the body below the injury level (where apparently it is uninjured and behaving as usual?) in the body- it prepares to fight or flee by constricting blood vessels, which increases to dangerous levels because of the fact that the parasympathetic system- though it sees the sympathetic going out of control (via nerves near the aorta that sense changes in blood pressure), and it sends out the message to slow heartbeat and stop constricting- can't communicate because of the damaged cord.  The weird part is that it apparently CAN slow the heart rate, but it can't talk to the blood vessels in the rest of the body that are constricted.
It is just this confusing to live with SCI- like a big science experiment is being conducted on your everyday self.

wait a minute- that's the opposite of what you originally spoke of (this is an example where sympa is fine, parasymp can't communicate with body, or at least parts of it)....

back to the drawing board, eh?  i think that something lies in the location of the 2 systems and in the route of their communication with the body.  i am still thinking about it, though, as is my friend, and we both enjoy doing so-- so if you'd like updates, ask as many follow-ups as you like.  i'll tell you when we get to the end of the line.  or perhaps you can figure it out and tell us??

i'll keep working on it-
leslie