The Doctor Weighs In

By Dov Michaeli MD, Ph.D

That’s one of those questions that children ask you when the are at the age of “why?” which I’d bet you couldn’t answer. I know I couldn’t. So I was happy to read a paper in the March issue of the Journal of Neurophysiology that, for the first time, provided a complete answer to the vexing issue.

Prior to this work, scientists who study smell and taste thought that irritating odors directly stimulated the trigeminal nerve, which senses touch, temperature and pain throughout the head region, including the delicate membranes that line the inside of the nose.

Weihong Lin, Ph.D., of the University of Colorado Denver School of Medicine and University of Maryland , Baltimore County , led the study which discovered that a particular cell, abundant near the entry of many animal noses, plays a crucial and previously unknown role in transmitting irritating and potentially dangerous odors. Dr. Lin and colleagues from both universities plus the Mount Sinai School of Medicine identified the role of this solitary chemosensory cell in transmitting irritating chemical odors in the noses of mice.

Prior to this work, scientists who study smell and taste thought that irritating odors directly stimulated the trigeminal nerve, which senses touch, temperature and pain throughout the head region, including the delicate membranes that line the inside of the nose. The research team found that solitary chemosensory cells scattered in the epithelium inside the front of the nose respond to high levels of irritating odors and relay signals to trigeminal nerve fibers.

Solitary chemosensory cells on the surface of the nasal cavity are in close contact with trigeminal nerve fibers which end just below the surface. Earlier research revealed that these cells contain bitter taste receptors and that bitter substances applied to the surface of the nasal cavity trigger a trigeminal nerve response. Once stimulated, the trigeminal nerve will convey pain and burning sensations and can trigger protective reflexes such as gagging and coughing. The architecture of nasal tissue with solitary chemosensory cells on the surface and trigeminal nerve fibers just below allows the nose to detect a greater number of irritating odors, the scientists explain. Fortunately, the threshold for triggering a response is high, so exposure to a small amount of an irritating chemical, as might naturally emanate from some kinds of fresh fruit, will not bring on gagging and coughing. For example, lemons contain the volatile chemicals citral and geraniol but at levels too low to trigger a trigeminal response. Only high, potentially dangerous levels of odors will trigger the protective gagging-and-coughing response.

So why is this important? Have you ever seen an animal approach food? It doesn’t taste it first—it smells it. Those chemosensory cells protect us from noxious materials that may be hiding in our food. Have you ever got a strong whiff of ammonia? It literally hurt your face. Ask any fainting lady who summoned the "smelling salts" before conveniently collapsing on a couch. Ammonia is a product of bacterial breakdown of protein; in other words, this is the gas emitted from contaminated and decomposing foods. Without those solitary chemosensory cells we would surely die of food poisoning. And now that Passover is upon us, have you ever loaded up your Matza with horseradish?

I could never understand how my wife could detest watermelon because of its smell. Watermelons smell? But that's OK with me--I always get her portion as well. But she would drive me up the wall with her complaints that the refrigerator smells. For the life of me—I couldn’t smell a thing.

Now I know; she is probably endowed with a lot more chemosensory cells than I am.

Thank God for Science—it saved a marriage.