Naked Body: Our sense of smell

Our sense of smell can conjure vivid images from the past, but how does it work? Elisa Galliano is here with the answer...

Imagine being invited for dinner at your parents’ house on a cold and crispy November evening. You hug your mother, but then, all of a sudden, you become extremely confused. You are being transported back time and to a totally different place: there you are, so very happy, a young child making sand castles, eating ice-cream and swimming in the sea… and then you snap back to the present. You start paying attention to what you mum is saying: while cooking dinner, a splash of hot soup ended on her wrist, which she treated with some leftover after-sun lotion. Of course! You smelled the lotion, and the odour, so tightly linked with summer and the sea evoked that out-of-place memory!

In terrestrial vertebrates, such as mammals, odour molecules are inhaled through the nasal cavity. There, deep down the nose, where even the most probing fingers cannot reach, they encounter the first station of the smell system: the olfactory sensory neurons. These are specialised sensory brain cells which have sent out their long processes, called axons, into the back of the nasal cavity. Each olfactory sensory neuron has receptors on it with the incredible ability to recognise and bind to specific chemical molecules. For instance, if aloe is present, only the neurons equipped with the aloe-recognising receptors will switch on – all the others, including those with receptors for mint, pine, rose and so on, will stay silent.
Once the correct olfactory sensory neurons in the nose have been activated by the inhaled odour, they will start broadcasting their new status into the brain.

In the brain, this information will first arrive in a region called the olfactory bulb, and it is then passed on to the part of the cerebral cortex devoted to process smell, which is called the piriform cortex. While passing between the very many cells present in these two structures, the information coming from the nose will be processed and catalogued, and officially labelled as “this is the smell of aloe plant”.

But how do we go from knowing that aloe is in the nose, to feeling transported back to a summer beach trip ten years ago? How come odours are so good at bringing back memories and associations? The piriform cortex sits next door to the parts of the brains that make associations (parietal cortex), encode memories (hippocampus) and emotions (limbic system). So very rapidly we can go from “aloe!” to “this smells like after-sun lotion”,  and then on to “I remember that summer, when I played in the beach in Cornwall and got a bit sunburnt, and my mum put after-sun lotion on me”, and finally “how happy I was playing on the beach and enjoying the sun”. And so, as if by magic, in an instant your brain can not only decode the smell of sun cream, but also bring you back in time and make you feel

So next time a smell activates some long forgotten memory, just indulge, safe in the knowledge that you know why it’s happening.