To most people, pulling into a highway rest stop is a profoundly mundane experience. But not to neuroscientist Rita Valentino, who has studied how the brain senses, interprets, and acts on the bladder’s signals. She’s fascinated by the brain’s ability to take in sensations from the bladder, combine them with signals from outside of the body, like the sights and sounds of the road, then use that information to act—in this scenario, to find a safe, socially appropriate place to pee. “To me, it’s really an example of one of the beautiful things that the brain does,” she says.

Scientists used to think that our bladders were ruled by a relatively straightforward reflex—an “on-off” switch between storing urine and letting it go. “Now we realize it’s much more complex than that,” says Valentino, now director of the division of neuroscience and behavior at the National Institute of Drug Abuse. An intricate network of brain regions that contribute to functions like decision-making, social interactions, and awareness of our body’s internal state, also called interoception, participates in making the call.

In addition to being mind-bogglingly complex, the system is also delicate. //

The human bladder is, at the most basic level, a stretchy bag. To fill to capacity—a volume of 400 to 500 milliliters (about 2 cups) of urine in most healthy adults—it must undergo one of the most extreme expansions of any organ in the human body, expanding roughly sixfold from its wrinkled, empty state.

To stretch that far, the smooth muscle wall that wraps around the bladder, called the detrusor, must relax. Simultaneously, sphincter muscles that surround the bladder’s lower opening, or urethra, must contract, in what scientists call the guarding reflex. //

Filling or full, the bladder spends more than 95 percent of its time in storage mode, allowing us to carry out our daily activities without leaks. At some point—ideally, when we decide it’s time to pee—the organ switches from storage to release mode. For this, the detrusor muscle must contract forcefully to expel urine, while the sphincter muscles surrounding the urethra simultaneously relax to let urine flow out.

For a century, physiologists have puzzled over how the body coordinates the switch between storage and release.