Every other Friday on NHPR's Morning Edition, the Outside/In team answers a question from a listener about the natural world.
This week, Aubrey asked: "Does it gross you out a little bit to know that every time you smell a thing, a little bit of that thing is in your nose? And then my follow-up question is: Once the molecules are in your nose, do they get absorbed? Where do they go?"
Outside/In's Justine Paradis and Nate Hegyi jumped into a studio to talk it over.
Transcript
This has been edited for length and clarity.
Justine Paradis: Does it, Nate, gross you out that molecules of dog poo are in your nose?
Nate Hegyi: Yes, absolutely! I’ve seen this question and it has just stuck in my brain much like the dog poop is somehow stuck in my olfactory. . . . It’s so gross.
Justine Paradis: You’ve been haunted by this question for months.
Nate Hegyi: I am. But is it true?!
Justine Paradis: It is true, and there’s more to it.
Nate Hegyi: Okay.
Justine Paradis: I’ve been exploring the science of how smell actually physically works, and what I’ve discovered is that research on olfaction, as it’s called, is still a little bit in its infancy because, historically, research into the senses have been way more focused on vision.
Nate Hegyi: Which kind of makes sense. Vision has historically been considered the dominant sense for humans, right?
Justine Paradis: Well, a lot of research suggests that our brains devote more processing power to vision than other senses. But anyway, here’s what we do know about smell. Most of the chemicals we smell are volatile chemicals, and that just means chemicals that evaporate easily.
Nate Hegyi: Okay.
Justine Paradis: Unlike other mammals like mice or dogs, most of the human nose is devoted not to smell, but to breathing. So it warms and filters air before it gets to your lungs. To actually get to the point of detecting a smell —
Bob Datta: There's a long series of events that has to occur.
Justine Paradis: That’s neurobiologist Bob Datta, of the Harvard Medical School. Bob explained that the part that’s actually doing the smelling is deep in the nasal cavity. It’s way up and back, kind of at the level of your eyebrows. It’s called the olfactory bulb, and this is actually a bundle of nervous tissue.
Bob Datta: Inside our nose is a sheet of neurons. These are the same types of cells that live in your brain.
Justine Paradis: But these ones live outside the brain, much more exposed to the world. As protection, they’re covered in little hairs called cilia, plus a layer of mucus.
Bob Datta: And so anything you smell has to float through the air and then be dissolved in the mucus and then diffuse through the mucus and find its way to the tips of these neurons to the cilia.
Justine Paradis: So, a lot’s happening inside that mucus on the way to the neurons. Enzymes are in there metabolizing and changing the chemistry of the odors. And there are proteins too which —
Bob Datta: – kind of act to escort smells from the gas in your nose to the neurons that sense the odor in your nose. . . . And only then do you sense the odor.
Justine Paradis: He also used the word “chaperone” here.
Nate Hegyi: Great. “Chaperone” is the perfect metaphor. Holding the odor’s nose and walking it towards the neurons.
Justine Paradis: Because they can’t be trusted on their own, you know. And so to answer Aubrey’s question: some of those molecules are getting “absorbed,” at least into the mucus. Others are destroyed – and still others get exhaled and never smelled at all.
Nate Hegyi: Okay.
Justine Paradis: I do want to share one more cool thing . That little bundle of nerves inside your nose — they have a kind of superpower. They’re so exposed to the world, to pollutants and smells and everything else, that they’re dying off all the time, but they’re also being regenerated by stem cells, which isn’t the case with any other neurons in your body.
Bob Datta: There's a lot of interest in this just to figure out whether we can't learn about how the nervous system might repair itself.
Justine Paradis: And so they might be important in ways that go well beyond smell.
Nate Hegyi: Very cool!
Submit your question about the natural world
If you’d like to submit a question to the Outside/In team, you can record it as a voice memo on your smartphone and send it to outsidein@nhpr.org. You can also leave a message on our hotline, 1-844-GO-OTTER.
