Why can't you sneeze on demand?

Years ago, when Barra da Tijuca was still new on the other side of Rio de Janeiro and it took almost two hours to get there in good traffic, I went with friends to the newly opened mall to see a movie. We took a public bus, but it was fancy, with air conditioning.

And I sneezed from one side of the bus to the other during the entire trip. I am grateful to my friends for not throwing me out the window. If I had tried to sneeze that much on purpose, I wouldn't have been able to. In fact, not a single sneeze would have come out.

The reason, as shown in a beautiful study recently published in the journal Cell, conducted by researchers at Washington University School of Medicine in Saint Louis, USA, is that sneezing is exclusively a reflex: either it is caused by irritation of the neurons that monitor the inside of the nose, or there is no sneezing.

The reason is that sneezing, as the study reveals, is an aberration from the normal breathing pattern caused directly by the release of the peptide neuromedin B in cases of irritation of the nasal mucosa. Genetically modified mice without neuromedin B do not transmit the signal of nasal irritation, and can smell pepper at will without sneezing. Not even an electrode inserted in the right place in the brain, which would normally trigger a sneeze, makes these mice sneeze. Meanwhile, their normal breathing continues unaffected, oblivious to what is happening in their nose.

Fortunately, the normal rhythm of breathing is involuntary and independent of will, generated by a network of neurons that activate each other alternately, in a cycle that only ends when the brain dies. Thus, it is possible to suppress or accelerate breathing for a few moments, or to inhale on command (now!), but no one is in danger of forgetting to breathe. For a sneeze to happen, you have to kick the source of the merry-go-round (the center that generates the breathing pattern in the brain stem) and force the sudden and intense contraction of all the muscles that cause the lungs to empty.

This kick, according to the study, is the release of neuromedin B onto the neurons of the ciranda by the neurons of the spinal nucleus of the trigeminal nerve, which in turn happens when, inside the nose, the endings of the neurons of the spinal ganglion of the trigeminal nerve detect irritation, poking, cold, light, or dust. Without neuromedin B, there is no sneeze. Thinking about sneezing even evokes the sensation of irritation from sneezing, courtesy of your cerebral cortex, but the breathing pattern remains there, steady and strong, perfectly normal. To sneeze on demand, just smell pepper.

Extracted from Suzana Herculano-Houzel (2025) Neuroscience of Everyday Life, originally published in Folha de São Paulo in June 2021