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What is behind that sneeze?

By Marla Vacek Broadfoot
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Dr. Matthew Ellison is director of the new sinus and allergy program at Duke Otolaryngology of Raleigh. Here, he explains the science of sneezing. Questions and answers have been edited.

Q: What causes sneezing?

The most common cause of sneezing is something irritating the nasal passage – especially the septum (which divides nostrils). It can be something external (dust, strong odors, smoke, pollen) or something internal (your own mucus during a cold).

In this situation, “irritation” means nerve endings are activated. Once an adequate number of nerve endings are activated, the brain (actually, the brain stem) sends out a command to sneeze. An entirely new cascade of coordinated nerve signals begins. This extremely complex nervous system activity quickly manifests as a physical sneeze.

Q: Why are sneezes sometimes triggered by sunlight?

We don’t know exactly why. When that happens, several reflexes are triggered, like squinting the eyelids (which is partially voluntary) and the pupils contracting (which is completely involuntary). Presumably the bright sun so strongly activates these reflexes that the command signal “overflows” and stimulates other brain stem reflexes, like sneezing or hiccups.

Q: How do sneezes get so powerful?

A strong sneeze involves the contraction of your diaphragm, a large sheet-like muscle that separates your chest (lungs, heart) from your abdomen (stomach, intestines). The contraction causes you to breathe in quickly. The chest and rib muscles then contract, causing the air pressure in your lungs to increase. Normally, this would cause you to exhale and force the air out. But during a sneeze the throat and vocal cords close briefly, causing the pressure in the lungs to go even higher. When the throat and vocal cords do finally relax, the high pressure in your lungs releases very quickly, pushing all the air through whatever is open – be it your nose, your mouth or both! Thus, a sneeze.

Q: What causes double or triple sneezes?

Sneezing is a coordinated activation of several specific parts of your nervous system that follows a central “command” to sneeze. The command signals have usually ended before the sneeze ends. The command signals probably keep going longer in people who are double or triple sneezers. In other words, once the command is triggered it can keep itself going longer – even if the trigger is already gone.

Why don’t we sneeze the entire time we’re exposed to something that irritates our nose? This has to do with a neurologic principle called a negative feedback loop. The nervous system activity after the command to sneeze also includes activity to stop future commands, or to increase the threshold of irritation required to trigger another command to sneeze.
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