Lunch time thoughts inspired by stinky cheese: Smell part I

My family and I have recently returned from two weeks in France and I’m reminded daily of the trip by the smell of ripe cheeses that continue to waft from my fridge. Depending on my mood and state of hunger, the smells either start my stomach grumbling and mouth watering or evoke a flight response where I can’t get out of the house fast enough.

This olfactory overload has me thinking a great deal about the sense of smell of late. Perhaps, like me, you have been transported to distant memories by a smell. When I moved to Australia with my family I caught a first whiff of wattle flowers and it instantly took me back to the kitchen of a neighbour when I was a child – a place I had not thought about in my entire adult life, but no less profound. However, as I write this and I try to conjure up the smell of the wattle in my head, it seems an impossible task.

The great science essayist, Lewis Thomas, wrote about smell saying, “To be sure, I know that odor of cinnamon or juniper and can name such things with accuracy when they turn up in front of my nose, but I cannot imagine them into existence” [1].

How very true. I can bring up an image of a bicycle without issue. I can recall the sound of a chicken laying an egg with enough accuracy to replicate it (one of my party tricks). I can recall the taste of a passionfruit gelato I had in Australia well enough that my mouth waters at the very thought…but a smell? I can describe a smell reasonably accurately, but to recall it “into existence” as Lewis so accurately puts it, I don’t think so.

Yet, olfactory cues are rather critical to our existence. On the most basic level they convey information that helps us make decisions that affect our survival – such as whether a substance is toxic, choosing the right sexual partner, avoiding sickness and selecting food.  Beyond that, smells enrich our lives in ways that we likely can’t appreciate until the sense itself is removed.

The science of smell

Smells are made up of combinations of chemical odorants – molecules that evaporate and become airborne. The human olfactory system can detect and identify thousands of these odorants and due to our genetic makeup, cultural upbringing and personal experiences, two people can react very differently to the exact same odorants [2].

When we breathe in, we draw chemical odorants into our nasal passages. These passages are lined with a thin sheet of mucous-coated sensory tissue, known as the olfactory epithelium. The odorant molecules get trapped in the mucous and make contact with olfactory receptor cells, which are nerve cells with a direct connection to the brain (see image).  The human nose probably contains hundreds of different types of olfactory receptors whereas dogs may have over a thousand different types.

When the odorant molecule binds to one or more receptor cells, a biochemical chain reaction is triggered within the cell that causes a series of electrical pulses to be sent along the long nerve fibres – known as axons – to the brain. The millions of axons from all of the nasal receptor cells are bundled together to form the olfactory nerve (the nose equivalent of the eye’s optic nerve).

This electrical signal, carried along the olfactory nerve is sent to the olfactory bulb where the brain begins to process the information. The nerve endings of the receptor cells cluster together in regions –known as glomeruli – that are the switchboard of the olfactory system.  As the electrical signals come in from various receptors, this switchboard ‘lights up’ in unique patterns that are then processed by the olfactory bulb.

The olfactory bulb then passes this information on to the rest of the brain where it is processed further. The information goes to the limbic system – a part of the brain involved with emotion and memory – as well as the olfactory cortex and orbitofrontal cortex. It is this additional processing that is likely essential in forming lifelong memories, complete with emotion, which are connected with smells.

Next week…

Smell is a rather large topic and so next post I’ll investigate the olfactory equivalent of beer goggles, how olfactory cues can help detect illnesses and what happens when people lose their sense of smell.

Sources:

[1] On Smell is a short essay by Lewis Thomas, published in the book The Bedford reader (New York: St. Martin’s Press, 1985). It was made available online by Allisonian’s blog (http://allisonians.blogspot.co.uk/2011/10/on-smell-by-lewis-thomas.html)

[2] Monell Center’s Olfaction Primer http://www.monell.org/images/uploads/Monell_smell_primer.pdf