How Does The Dog’s Nose Know?
This article is the first in a series to introduce the readers to the specifics of canine olfaction. A number of parts will be presented to cover various topics including: canine vs human olfactory systems, does breed or sex affect sense of smell, the accessory olfactory system, factors that affect the sense of smell and an introduction to the genetics of olfaction. If there are any other areas readers wish to be covered, please get in touch.
So to start off the series, What Is Olfaction?
Olfaction is also known as the sense of smell. The olfactory system comprises of the nose, which contains specially adapted cells designed for the detection of odours. These are then linked to a specific part of the brain, known as the olfactory bulb, via nerves. When odour molecules enter the nose, the cells send signals via the nerves to the brain where this information is interpreted.
Image from http://www.biologyjunction.com/olfaction.htm
There are two theories for the mechanism of olfaction; the “lock and key” theory and the vibration theory. These two theories will now be briefly described.
Lock and Key Theory
This is also known as “shape theory” was first proposed in 1949 and is based on the notion that for a smell to be detected, an odour molecule must fit into a receptor on a cell. Only a molecule of a specific size, shape and configuration will fit into a particular receptor to induce a signal to the brain. A modernised version of this theory based on findings from scientific research is known as the odotope or “weak shape” theory. However, similar shaped molecules can often give rise to very different smells which confuses this theory somewhat.
This was first proposed in 1928 but was abandoned in favour of the “lock and key” theory. However, a 1996 research paper resurrected the idea. This posits that a molecule must first fit into a receptor on a cell but will only trigger a signal if the molecule has the correct vibrational frequency. However, research testing this theory has so far proven inconclusive.
Image from http://www.sciencefocus.com/feature/health/how-we-smell-competing-theories
Neither of these theories completely explains the phenomenon of olfaction. And so the exact mechanism remains a mystery. The next part in the series will explore the differences between the canine and human olfactory systems in an attempt to explain why the dog’s sense of smell is superior to our own.
For more information on the theories of olfaction, please visit the following websites:
Turin L (1996). A spectroscopic mechanism for primary olfactory reception. Chemical senses. 21 (6): 773–91
Moncrieff, Robert Wighton (1949), What is odor? A new Theory. American Perfumer, 54: 453
Buck, L. and Axel, R. (1991) A novel multigene family may encode odorant receptors: a molecular basis for odor recognition. Cell, 65, 175-187.
k9WH are grateful to their ‘resident’ scientist for this article.