Shakespeare’s often quoted line suggests “that which we call a rose; by any other name would smell as sweet.” The bigger question is how does it smell as sweet? Olfaction, the sense of smell, is still not well understood. There are multiple theories regarding how we actually are able to detect smells. One leading theory is that of a lock and key system. This theory suggests that if airborne molecules of a certain chemical can fit into the specialized sensory cells of the nasal cavity the nerve cell will respond and send a signal to the brain which is interpreted as a particular aroma. Another theory proposes that olfactory receptors respond not to the shape of the molecules but to their vibrations. This theory maintains that odours exist as a spectrum much as colour is detected as a spectrum of light by the sensors of the visual system. There is also a secondary system of olfactory receptors which may detect aromas without the brain being consciously aware of the smell. Some scientists suggest that this secondary system is highly tuned to pheromes and may be a basis for why we feel attracted to or warned away from certain situations. (Note added April 20th: Recent scholarly papers have concluded that this vomeronasal organ, although extremely active in most non-human animals, is non-functional in humans.# This is probably a good thing. The perfume and cosmetic industry would seek to exploit this function with irresistable sexual phermone fragrances. I can imagine the military creating phermone weapons which would cause the enemy to experience a sense of fear.)

As biological entities, we are always giving off fragments of ourselves and leaving a trail that can be detected by others. Think of a tracking dog that is able to follow the scent of one particular human based only on the specks of biology and their corresponding smells left behind as that individual walks through a forest containing thousands of other smells.

Dogs have a much greater capability of detecting odours than do humans. Some studies have found that dogs have as much as seventeen times more olfactory epithelium than humans and as much as one hundred times more nerves in that olfactory epithelium making it plausible that their sense of smell may be seventeen hundred times more sensitive than ours.* What a strange world this must be for a dog. To the dog, the air is alive with many more smells. Every human and animal in its environment is continually raining down a stream of biological bits with interesting aromas that the dog’s discerning nose can codify and quantify to learn helpful information about its environment. What a different perception of this world we would have if our sense of smell were as sensitive. Imagine the smell of that hockey locker room!

One day we may have a more complete understanding of olfaction. For now we revel in the mystery of smell.

*Bear, Connors and Paradiso, Mark, Barry and Michael (2007). Neuroscience: Exploring the Brain. USA: Lippincott Williams & Wilkins. pp. 265-275.
#Wysocki CJ, Preti G (November 2004). “Facts, fallacies, fears, and frustrations with human pheromones”. The Anatomical Record. Part a, Discoveries in Molecular, Cellular, and Evolutionary Biology 281 (1): 1201–11.
#Wyatt, Tristram D. (2003). Pheromones and Animal Behaviour: Communication by Smell and Taste. Cambridge: Cambridge University Press.

This blog entry was inspired by The Lives of a Cell: Notes of a Biology Watcher by Lewis Thomas.

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