Stop and smell the roses: how odour olfactory receptors work
Two mentions in medical news on breakthroughs in olfactory or smell related research in the month of October have stood out for
me. One was of a paralysed 38 year old Polish man who is learning to walk again after cells from his nose were transplanted into the damaged part of his completely severed spinal cord. The other was of a team of German biologists discovering that smell receptors that were thought to exist only in the nose are also found in human skin cells.
Of smell, scent, aroma and nostalgia
It’s funny how your nose picking up different smells can evoke the memories of moments in time long forgotten. I’m thinking of how seemingly random smells, scents and aromas can quickly remind you of friends and loved ones or even places you’ve been to even if it was decades ago. For example, the aroma of freshly baked bread as I pass through Queen Vic market tends to instantly transport me to my mother’s kitchen tens of thousands of miles away. So how does the child smelling oven fresh bread recognize the same aroma many years later and also recall memories associated to this smell?
A question whose answer was worthy of a Nobel Prize
Smell is one of the human faculties that we rely on each day. Imagine how hard it is for people who can’t smell to enjoy the taste of food. Despite its importance it was not until the early 1990s that biologists were first able to describe how the smell receptors actually do their job. Dr Richard Axel and Dr Linda Buck were in 2004 awarded a Nobel Prize in Physiology/ Medicine for their work in the late 80s and early 90s explaining how people can smell and recall over 10000 different odours.
The simplified version of how it works
Smell is a chemical sense detected by sensory cells or chemoreceptors. When you smell something, say a rose, you breathe in molecules that have evaporated from the rose and that are floating through the air to your nose.
At the top of your nasal passage inside your nose is a patch of neurons. These neurons come into contact with the air you breathe in and their hair like projections or cilia trap the rose molecules and lock them in. This action triggers the neuron that sends a message to the olfactory ‘bulb’ – the part of your brain that is associated with smell. The bulb in turn passes on this information to other parts of your brain which processes the information and causes you to perceive the smell and even recognise it as being the scent of a rose.
In order for molecules to be smelled, they have to be light and volatile (easy to evaporate). This explains why a piece of steel has virtually no smell – it’s a non-volatile solid and so nothing evaporates from it that your smell neurons can pick up.
Each smell receptor has different genes associated to it and if your DNA is missing a gene connected to a particular receptor, you won’t be able to pick up certain smells. This is why some people simply can’t smell certain substances over others, for example salt.
I think it’s pretty interesting that all that gene encoding for smell has now been discovered in the skin cells as well. The applications for this in future are filled with endless possibilities.