Editor's note: This is one part of a three-part Science Scene in celebration of Halloween. To see the other two parts, click here.

Few clowns, zombies or witches could make jaded college students scared enough to lose control over their bowels and bladders, even on Halloween. However, the potential for humiliation is still there, hidden deep within our brains.

Our bladders and intestines empty in different biological processes, but both organs are associated with the sympathetic nervous system, which activates when people are exposed to fear or stress. This system prepares our bodies for the
“fight-or-flight” response. 

The human brainstem is constantly monitoring the basic processes that keep us alive, including those of the heart, lungs and bladder. But the brainstem is not responsible for causing people to actually urinate — otherwise, the bladder would empty every time the brainstem registered that it was full, leading to problematic slip-n-slide situations. 

The prefrontal lobe controls urinary function. During times of extreme stress or fear, however, the limbic system of the brain is activated, distracting the brainstem from the prefrontal lobe. This causes people to urinate more frequently during times when they are stressed or anxious, such as during a high-pressure test. 

It takes more than a test for a college student to be scared enough to soil their pants. During a bad scare, the hormones epinephrine and norepinephrine are secreted by glands on top of the kidneys during a bad scare. Epinephrine activates the sympathetic nervous system. The heartbeat increases, muscles quickly convert food into energy, smooth muscle relaxes so that breathing is easier, and water transport to the intestines increases. When there is more water in the intestines, they are more likely to empty, possibly causing disaster and humiliation. 

The bacteria that causes the disease cholera, which lives in the gut, produces toxins that imitate this process. They stop the body’s transport of water to the intestines after epinephrine signaling. Consequently, more and more water is rushed into the intestines, causing severe diarrhea and dehydration in the rest of the body. This reaction spreads the disease in places with
improper sanitation. 

The caffeine in coffee also imitates the actions of the sympathetic nervous system, and not just the water transport system. After drinking coffee, people have faster heart rates, higher blood pressures and an increased need to use the facilities, according to a study in the journal Gut.

While these physiological processes are well understood, biologists still debate about why animals evolved to urinate and defecate when they are afraid. One possible evolutionary explanation is that the animal in distress needed to drop unnecessary weight as quickly as possible, while other scientists think it may be a method for animals to mark their territories or a way to confuse predators. 

Whatever evolutionary purpose these adaptations once served, they are less than useless today. Students may find the possibility of wetting their pants scarier than any haunted house or elaborate costume. 

“Possibly when I was younger, that [uncontrolled urination] happened,” biology junior Robert Cochran said. “Now I don’t expose myself to scary situations because I’m too much of a wimp.”