Anyone who’s ever cut up an onion knows that some smells can actually be painful. Now researchers from Baltimore and Denver are closer to understanding why. Scientists used to think that odors that are irritating, like onions and ammonia, directly activate the trigeminal nerve. That nerve responds to touch, temperature and painful stimulation all over the head, and its fibers extend to the membranes that line the inside of the nose.
 
But the current study suggests that the trigeminal nerve doesn’t act alone. Specialized chemosensory cells that line the nasal cavity are actually the first to detect irritating smells. They pass the news to the trigeminal nerve, which then lets your brain know that it’s time to feel the burn. And to tear, or cough, or gag or just generally back off. The findings are described in the March issue of the Journal of Neurophysiology.
 
Having these special sensory cells as a go-between, the scientists say, might make our noses sensitive to a broader variety of irritating odors. That may not seem like such a bonus. But the system probably evolved to protect us, by giving us a heads up when we run into something noxious. It may also keep us from adding too many onions to the spaghetti sauce.

—Karen Hopkin

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