Have you ever taken a gulp of water that goes down the wrong way? Or felt acid reflux bubbling up? You probably emitted a cough or three.

For a long time, scientists didn’t know exactly what caused this. But they knew this quick reaction was important. Water or something else that doesn’t belong in the airway “can cause damage,” explains Laura Seeholzer. She’s a neurophysiologist at the University of California, San Francisco (UCSF). That damage could even halt our breathing.

Seeholzer and a UCSF coworker recently found that certain cells help protect against this. These neuroendocrine (Ner-oh-EN-do-krin) cells can sense water or acid passing through the the larynx and trachea. This passageway runs between the mouth and lungs. Neuroendocrine cells here communicate with the nervous system. That triggers reflexes such as coughing.

The researchers shared their finding April 19 in Science.

“This is not at all what I was expecting [these cells] to be detecting or doing,” Seeholzer says. Neuroendocrine cells usually release hormones that move throughout the body. One place where these cells hang out is the larynx and trachea. But scientists had little idea what information neuroendocrine cells collected here. They also didn’t know whether the cells talked to the nervous system.

Seeholzer and physiologist David Julius grew neuroendocrine cells from mouse airways in a dish. They wanted to see what stimuli caused the cells to become active. To do this, they tracked calcium levels in the cells. These levels point to how much the cells are chatting with their neighbors. The cells responded to water and acid, they found.

Other experiments activated these cells in mice. The neuroendocrine cells made the mice swallow and cough. This may help kick unwelcome substances out of the airway.

A different trigger activates neuroendocrine cells in the lungs. Those cells respond to pressure changes.

“It turns out that there’s a lot of cells that act as basically sensory detectors,” Seeholzer says. “And then they communicate with the nervous system in order to drive behavior.” Other sensors include taste buds and outer skin cells. Together, they help protect us from harmful substances.