biology

the sneeze | 2015-11-25

permafrown

25 Nov 2015 — 3 min read

David Makiri sneezes into a tissue. Germs, dust, are pollen that get inside the nose are no match for the mighty sneeze.

this week’s topic: the sneeze

when something irritates the upper lining of the nasal cavity, the trigeminal nerve (transmitter of all facial sensation to the brain) sends a signal to the medulla (great and mighty controller of autonomic and involuntary functions) telling it to fill the lungs with air, close the eyes, constrict the chest, and expel most the air in the lungs as fast as possible, usually at a rate of around 100mph — this is known as sternutation or sneezing. you’ll note, the medulla does not tell the heart to stop beating for any period of time, despite popular belief, though the sudden constriction of the body’s core and the sudden expulsion of all that air/O2 does affect blood pressure which may cause a temporary adjustment in the heart rate. if the heart stopped beating when people sneezed, we’d see a lot more people passed out on the ground immediately after sneezing which, while awesome, would probably not be safe or helpful.

no one is quite sure why sneezing is a necessary response. theories range from some prehistoric need to expel harmful parasites from plant spores before they take hold and kill you to the development of control by viruses that allows them to make us sneeze so that we send them on to other hosts (yuck). not sure; whatever… people sneeze and they do so because their trigeminal nerve tells them to.

but what about looking at the sun? why does that make you sneeze?

well, in the first place, it doesn’t make everybody sneeze — only some people have the remarkably pleasant experience of gazing at the sun and being overcome by a righteous and violent sternutation. it seems to be a genetic abnormality that is passed down from parent to child and, what’s more, if either parent is prone to the photic sneeze reflex (or photoptarmosis), it is highly likely that the trait will be passed down; looks like this trait is a very dominant gene, indeed. as an aside, sometimes, this trait is referred to as an autosomal-dominant compelling helio-ophthalmic outburst which, conveniently, can be acronym’ed as ‘achoo’… i think sometimes scientists are trying too hard.

but why? well, no one is quite sure why. the general understanding is that sneezing is caused by overloading the trigeminal nerve one way or another. so, you look at the sun and your eyes go “holy crap! that’s super bright!” and send a similarly coded message down the optic nerve. that optic nerve runs really close to the trigeminal nerve, so it seems likely that the trigeminal nerve, which we already know is prone to panic, sees the optic nerve freaking out and goes “well, obviously this is the apocalypse… better trip the sneeze alarm” and, whammy! sternutation!
similarly, plucking an eyebrow hair or a nose hair will cause a pain response in your face and the nervous (no pun intended) trigeminal runs as fast as it can to the big red ‘sneeze’ button and hits it like an astronaut trying to close an air lock while being pursued by xenomorphs. one nerve bugs another nerve and there you go!

interestingly, some people claim they sneeze when they eat particularly pure chocolate, when they think of their beloved family members, or after sex. talk about a set of inexplicable claims!

so far today in trivia. tune in next week for more, yet to be determined, awesomeness.