How does the olfactory sense work?

You breathe in deep, and you might get a whiff of shampoo or maybe stale coffee breath on the person next to you. It’s the kind of information that the brain just registers. So how, exactly, does the sense of smell work?

smell receptor image

Smell receptors. Coloured transmission electron micrograph (TEM) of a section through smell receptors (cilia) projecting from an olfactory neuron (blue). This nerve cell is responsible for detecting smell. The swollen part of the cell is the olfactory vesicle. Information from the receptors is passed to the olfactory bulb in the brain, where it is processed. Image courtesy Steven G Schmeissner.

Our noses are equipped with special receptors located in a small area at the back of the nasal cavity. Each receptor detects a specific chemical, according to the National Institutes of Health website on smell.*

When we breathe in an odor–perhaps a molecule of banana aroma over breakfast–and it lands on the appropriate receptor, the activated receptor fires off a signal to the brain via a specialized nerve alerting us to the smell. There is one olfactory nerve for each smell receptor, so the whole process is very specific.

A second hypothesis about how olfaction works suggests that a molecule’s vibration, rather than shape, activates the olfactory neuron.**

Either way, human olfactory neurons are directly exposed to the environment. Turns out that olfactory neurons are the only part of the nervous system with direct exposure to the outside world.

Two researchers, Richard Axel and Linda Buck, discovered the gene family responsible for the olfactory receptors and improved our understanding of how the sense of smell works. In 2004, they received the  Nobel Prize is Medicine for their work on the sense of smell.

Axel and Buck performed their landmark studies on mice, whose noses contain about one thousand different odorant receptor types. Humans have a smaller number of odorant receptors than mice because some of the genes have been lost during evolution, likely at the expense of improved vision (doi: 10.1371/journal.pbio.0020146). By comparison, the area allocated to olfactory receptors in dogs’ noses is about forty times larger than in humans.

But we’re better at smelling than those data might suggest. Dogs do better on the short chain compounds, like those found in urine, but humans perform as well or slightly better than dogs on the longer chain compounds, like those emitted by flowers. Humans perform significantly better than rats (doi:10.1371/journal.pbio.0020146).

Further Reading:

* Source –
**Note: There is a competing hypothesis advanced by Turin L (1996) in which chemicals don’t bind individual receptors according to shape. Rather, a molecule’s vibrational frequency in the infrared range activates the olfactory neurons. It’s a “swipe card” model, versus the more widely accepted “lock and key” model.

ABOUT THIS COLUMN: This weekly feature, the Science of Smell, appears on Fridays and endeavors to answer basic questions about this ancestral sense.

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  1. Good stuff. Thanks, Scentinelle!

  2. Karen Carvallo says:

    Hey Gen, I saw a typo, is it Buck or Black?

    Loved the cat and creme brulee post. I can totally picture you sniffing Adam!


  3. livelybrowsers says:

    Thanks for good stuff

  4. Splendid article . Will definitely copy it to my blog,surely with link pointing towards you..Thanks.

  5. Inyour recent excellently written article in NYT on The Night My Liver Started Running My Life you mentioned that there is no test for Heatitis E. I beg to differ. Copied below is a link for HEV serology available from Canada:
    Pubmed reference: Use of Serological Assays for Diagnosis of Hepatitis E Virus Genotype 1 and 3 Infections in a Setting of Low Endemicity
    M. Herremans, J. Bakker, E. Duizer, H. Vennema, and M. P. G. Koopmans
    Clin Vaccine Immunol. 2007 May; 14(5): 562–568.

    • Unfortunately, I don’t live in Canada. Maybe my NYT essay will prompt clinical laboratories in the US to make a serology test widely available. But with incidence (i.e., demand) so low, I’m not sure we’ll ever have a test in the US other than the research-based PCR/Elisa methods. Thank you for sharing the information and for your comment!


  1. […] pathways usher smells to our olfactory neurons. The first is through the nose as described last week. The second is through the sensory cells that line the channel from the roof of the throat up to […]