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In my story, anyone who uses magic has to shut down their autonomic nervous system in order to gain enough control of their bodies and minds. What are the autonomic processes most immediately necessary for survival that a mage needs to consciously control while casting magic? Is there anything beyond breathing and heart beating that is absolutely necessary for moment-to-moment survival?

Edit: Casting doesn't take much longer than a few seconds to a few minutes. Some mages alter their bodies or are born into bodies specifically built for casting magic. Tech level is mostly at around late medieval period. In the story, magic is something that was discovered after humanity had already reached godlike, Type 4 Civilization status. However, resulting war destroyed almost all traces of that civilization and its technology.

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  • $\begingroup$ They obviously need to control all of them, at least to maintain the current state--it is a real-time system. This does not mean that they need to expend a lot of computing power, just that they need to dedicate a certain fraction of their computing power to those functions. $\endgroup$ – AlexP Jan 11 '17 at 2:32
  • $\begingroup$ How long of a time period are they casting? You can turn off breathing for 30 seconds easily. Also, do you count the pacemaker structures in the heart as part of the autonomic nervous system? They're a little unique -- the heart will beat without a brain. $\endgroup$ – Cort Ammon Jan 11 '17 at 6:08
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    $\begingroup$ Is it just me or do I find working with magic heart stopping? I had to admit it is a grizzly bear who taught it to me when I was trekking along a river. $\endgroup$ – user6760 Jan 11 '17 at 7:31
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    $\begingroup$ Note that there is a list here. $\endgroup$ – JDługosz Jan 11 '17 at 7:36
  • $\begingroup$ What's the time of casting a spell? And by the way, what's the tech level and biology level? $\endgroup$ – Mołot Jan 11 '17 at 8:06
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If you are in a very cold or very hot location, regulating the body temperature would also be essential for survival. Irreversible frostbite damage could happen within minutes if the mage neglected the peripheral circulation, and hypothermia could set in quite rapidly if the mage didn't properly constrict the blood vessels in the skin to reduce heat loss.

The autonomic nervous system also controls the core skeletal muscles, so if your mage wasn't paying attention, they might just stop being vertical. It also controls the reflex where when you tighten a muscle (eg quadriceps), the opposing muscle (in this case, hamstring) relaxes. Your mage might find themselves tearing their own muscles whenever they moved.

You wouldn't be able to focus your eyes without the autonomic nervous system. Your head wouldn't automatically turn toward a sound. Your eyes wouldn't stay fixed on an object when your head moved (think wobbly-cam and motion sickness, though the motion sickness is also autonomic).

You wouldn't reflexively pull away from hot things. Your pupils wouldn't respond to increased light by getting smaller.

Given that the mage can only focus on 7 plus or minus 2 items consciously, the chances of some kind of temporary pain, minor injury, or even permanent damage would be almost 100% unless the mage was motionless in a cotton-wool lined temperature-controlled coffin with their eyes closed.

But it wouldn't actually kill them, most likely, if they kept their heart beating and their lungs moving.

The autonomic nervous system is also responsible for the "fight or flight" response, so the mage would need to manage all the components of that manually, too.

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    $\begingroup$ The "automatic nervous system" or ANS designates a specific part of the peripheral nervous system regulated by the hypothalamus and does not just mean all the automated processes of your body... Some of the things you've mentioned, while indeed initiated by a different part of the peripheral nervous system are not managed by the ANS. Namely muscle reflexes and pain induced reflexes. $\endgroup$ – AngelPray Jan 11 '17 at 23:53
  • $\begingroup$ Given that the OP mentioned breathing, which is voluntarily controllable, it wasn't clear that they were distinguishing between "autonomic" and "automatic". $\endgroup$ – Jnani Jenny Hale Jan 13 '17 at 0:54
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Well unless they were careful they might shit and piss themselves as the automatic nervous system controls amongst other things the anal sphincter and detrusor muscle. Avoiding such a misfortune is for many (including myself) absolutely necessary for moment-to-moment survival.

Jokes aside, yes besides being in control of respiration and cardiac regulation the ANS also initiates vasoconstriction in the event of the body being wounded. Particularily when haemorrhaging is severe (as one might expect from a injury sustained from a fight involving magic for example) immediate vasoconstriction can be the difference between life and death (death from acute blood loss).

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