|MadSci Network: Medicine
Generally, when people think of sneezing, they think of someone who has a cold, or allergies, or has gotten something irritating in their nose, like pepper. In fact, many, many stimuli can trigger a sneeze. A list of these stimuli would include colds, allergies, cold air, humidity, irritants such as pepper or other smells, exposure to bright sunlight (called the ACHOO syndrome no less!), eating too much, cooling certain parts of the skin, sexual excitment, hair pulling, shivering, and yes, even eyebrow plucking. And that's only some of the triggers. Quite a list, isn't it?
So, why does this happen? I think the first step to answering this question is to briefly describe the sneeze reflex.
We know that for most people, sneezing happens in response to irritation of the nose. (as an aside, have you ever noticed that babies sneeze through their nose, whereas most adults sneeze through their mouth? We seem to modify our sneezes as we get older) In any event, irritation of the nasal passages excites your trigeminal nerve. The impulses travel through the trigeminal ganglion to a set of neurons located in the brainstem which have collectively been termed the "sneezing center". The "sneezing center" sends impulses along the facial nerve back to the nasal passages and face, and causes your nasal passages to secrete fluid and become congested. Sometimes your eyes will water also. This nasal congestion is called the nasal phase of the sneeze.
The "sneezing center" also sends impulses to your respiratory muscles via the spinal cord. These impulses cause the deep inspiration and forceful expiration of the sneeze. This "act of sneezing" is called the repiratory phase.
To make a long story short, the reason all of the stimuli I listed can trigger a sneeze is because they all are able to excite the "sneezing center". Eyebrow plucking excites a branch of the nerve that supplies your nasal passages. Some doctors speculate that even though the impulses aren't coming from the nose, the stimulation generated by eyebrow plucking makes the entire nerve more sensitive, and makes it very easy for enough impulses to reach the "sneezing center" to generate a sneeze. In a similar manner, bright light, odors, and even sensations from your stomach are all able to excite the neurons of the "sneezing center" and so trigger a sneeze.
As a final point, I think I'd like to share a couple of tidbits I came across while researching this. You may have heard the story that we say "God Bless You" to a person after they sneeze because their heart briefly stops during the sneeze (At least I've heard this story before). It is rubbish.
Originally, sneezing was considered good luck. The Hebrew Talmud says that sneezing means that God will answer your prayers, and Hebrews called sneezing the "Pleasure sent from God". The Greeks and Romans likewise considered sneezing a good omen, and wished each other good health or long life when they heard a sneeze.
It was Pope Gregory VII who came up with the current custom of saying "God Bless You". This blessing was a response to the bubonic plague that was decimating Rome at the time. You may have heard the rhyme
Ring around the rosy,
Pocket full of posey,
All fall down!
This rhyme started as a description of the plague. "Ring around the rosy" refers to the appearance of the blackened swellings or "buboes" that appeared on the victims. "Pocket full of posey" refers to the fresh flowers that people would carry around and hold near their face to ward off the horrid odor associated with the plague. "Ashes, ashes" is a corruption of "Achoo, Achoo" and refered to the observation that people with the plague frequently sneezed. I think we all can figure out "all fall down" for ourselves.
Anyway, the Pope proposed that people say "May God Bless You" when a person sneezed, in the hopes that this short prayer would protect the person from death. Of course, neither the Pope nor anyone else understood that getting rid of the fleas which carried and spread the plague would have ended it immediately. The plague ran its course, and we were left with a much different cultural view of the meaning of a sneeze.
Thanks for the interesting question!
If you have any questions or comments, or would like a list of the references I used to construct this answer, please send me an email.
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