system
Given the following post, respond as if you're commenting on Reddit:
human
Why do we lose bladder/bowel control in response to acute fear?
gpt
Fear causes activity of the autonomic nervous system. Deep brain centers like the hypothalamus and the limbic system govern whether the autonomic nerves will tell the organs of the body to operate in a way that is conducive to "rest and digest" behavior (parasympathetic tone) or "fight or flight" behavior (sympathetic tone). 

If well regulated, a fear stimulus should generate a large sympathetic outflow - that is a "fight or flight" response. However, a very disorganized emotional stimulus (as one would get if you were freaking the f*ck out under pressure instead of fighting or fleeing like a sensible person) would include a large component of parasympathetic outflow as well. 

The detrussor muscle of the bladder is the muscle used to squeeze out urine, and it contracts under stimuli from the parasympathetic nervous system. That is why you sort of have to relax your mind to pee, in addition to simply bearing down. The parasympathetic nervous system also causes bowel motility (peristalsis of the small bowel and segmentation of the colon) that move feces along. The final act of pooping is ultimately voluntary, but having the autonomic nervous system deliver a big payload of poop to the rectum sure makes it easier to soil yourself!

Therefore peeing and pooping yourself is aided by a big disorganized stimulus from parasympathetic nerves. It's a maladaptive and atypical response seen only in people who have a disorganized emotional response to a fear stimulus, which should realy produce only a "fight or flight" sympathetic response. 

TLDR: The parasympathetic NS influencing the detrussor muscle (peeing) and the smooth muscle of the colon (pooping).