Question that stumped my anatomy professor wife

September 21, 2008 2:17 PM

My wife is an anatomy professor and needs help with this question: Most anatomy textbooks say that urination and defecation are controlled by the parasympathetic nervous system (aka "rest and digest system") Then why is it that some lose bladder and bowel control in frightening situations? Isn't the opposing sympathetic "fight or flight" system supposed to in charge?
posted by up in the old hotel to science & nature (17 comments total)

My understanding is that the "pooping yourself when afraid" response is a paradoxical simultaneous stimulation of both branches of the nervous system.
posted by rxrfrx at 2:40 PM on September 21, 2008

I thought it was because being soiled would make you unpalatable to eat, thus increasing your chance of surviving if you were being attacked by a predator.
posted by Class Goat at 2:51 PM on September 21, 2008 [4 favorites]

If you've got to run or enter combat, it's more efficient for you to maneuver with less body mass. That's why animals poop and pee when faced with a fight or flight situation.
posted by infinitewindow at 2:57 PM on September 21, 2008

I agree with infinitewindow, I always thought it was to reduce your body weight, making it easier to escape.
posted by Admira at 3:13 PM on September 21, 2008

The in a situation where the flight or fight response comes into play a massive amount of epinephrine (adrenaline) is dumped into the body causing all kinds of crazy effects.
posted by Science! at 3:19 PM on September 21, 2008

It's easier to GTFO when you don't have an abdomen full of gunk.
posted by turgid dahlia at 3:19 PM on September 21, 2008

As far as the "bowel dumping to make you lighter" idea. It kind of makes sense, but if the prey was being chased by any predator with a long range chase ability they're really just leaving a trail to make themselves easier to find. Predators would have adapted and become able to track an animal by the trail of feces left behind them along with all the other normal scents. If you've ever chased a goose with a car (true story, but not me) they don't always shoot way up into the air. Sometimes they stay just a few feet off the ground and in front of the car (predator) defecating as they go, and thus leaving a trail until you stop and they land a couple of dozen yards away where they can be stalked again.
posted by Science! at 3:23 PM on September 21, 2008

I believe it's because the sympathetic system is in charge. In a life-threatening situation, all resources are commandeered by the sympathetic system and resources that are devoted to non-essential functions (such as bowel control) are temporarily diverted to dealing with the crisis at hand.

In short, if you have to make a choice between do not die and do not poo, you want the do not die department to be the one making the decisions, even if the do not poo department has to be briefly taken offline.
posted by stefanie at 3:40 PM on September 21, 2008 [12 favorites]

Just an additional theory to consider...maintaining bowel and bladder control takes some subconscious consideration. Which is why when people die they void them. In a situation where your brain thinks it's in a dangerous situation, it may be a matter of priority. Divert all resources to fight or flight in order to survive, or do waste management. Sometimes our bodies are smarter than we realize.
posted by hungrysquirrels at 3:45 PM on September 21, 2008

Or what stefanie said...gotta learn to preview.
posted by hungrysquirrels at 3:46 PM on September 21, 2008

You know who would know about this? A neurologist. Now if I were an anatomy professor, I would ask one of my colleagues in the neurology department about this rather than having my spouse ask the Green.

That said, I think that stefanie's answer sounds right, but I am not a neurologist--I just have a big ol' fangirl crush on V. S. Ramachandran.
posted by Sidhedevil at 4:21 PM on September 21, 2008 [1 favorite]

Try this explanation (Google Books) I found by Googling "scared parasympathetic defecate".
posted by scrump at 5:02 PM on September 21, 2008

From chapter 50 of (p. 993) Principles of Neural Science (Fourth Ed. ) Erik Kandel et al.

Figure 50-9 (my crude summary of a very nice diagram showing the neural pathways of a of scared mouse experiencing an electric shock)

FEAR -> Central nucleus of the amygdala -> Dorsal motor nucleus of vagus nerve -> Parasympathetic activation -> ulcers, urination, defecation, bradycardia

You and your girlfriend may also be interested in the fear-response work of Joseph LeDoux. His band the Amygdaloids play around NYC every so often.
posted by abirae at 6:18 PM on September 21, 2008

Shit surely doesn't weigh that much?
posted by A189Nut at 2:32 AM on September 22, 2008

Some observations from working with mice:

If I try to pick one out of a cage, or chase one that has escaped onto the floor, it will try like hell to get away from me. I'm sure in this situation, these mice are undergoing a "flight-or-fight" response as befits the usual understanding of the paradigm. And in this situation, they're never urinating or defecating.

However, if I pick up the mouse, it will spontaneously do both (mice really don't like being restrained). In this case, the mouse is far more scared (and powerless), and let everything loose.

I wouldn't try to look for any evolutionary advantages in this response (as fun as they are to try and concoct), I think it surpasses the "normal" physiological response.
posted by kisch mokusch at 3:54 AM on September 22, 2008 [1 favorite]

I always thought that they muscles that suddenly clenched put pressure on the diaphragm, which would move and cause the accident. But, I never thought too hard about it.
posted by Citrus at 10:29 AM on September 22, 2008

Some predators/annoyances might be distracted by the chaff being dropped. But in humans, I always took it more as a joke, except for vomiting.
posted by Goofyy at 10:36 AM on September 22, 2008