August 5, 2016

How do we smell scents

It is said that smell is the brother of breath. We cannot stop breathing, just as we cannot stop smelling scents. We can close our eyes not to see shapes and colours, we can cover our ears not to hear sounds, but scents accompany us at all times. Despite that, we still know too little about biological mechanisms of smelling scents. We are not able to objectively describe a scent in order to convey to another person its wealth and complexity. We cannot reliably compare two scents. We cannot even – apart from subjective feelings – determine the strength and intensity of a scent. Our own impressions and formulae of chemical compounds can assist us, but neither of these methods is sufficient. When we describe our impressions, for some the scent seems beautiful and inspiring, and for some – the same scent feels suffocating and tiring. In turn, chemical formula is only a symbol, deprived of the whole range of emotions and sensations that scents evoke; it does not influence our imagination in any way. Perhaps this is why the sense of smell for centuries has remained one of the most mysterious senses. Even in the middle of the 20th century, the world of science was divided into supporters of two theories of scent reception: odour and molecular vibration by Robert H. Wright and stereo-chemical theory for olfaction by John E. Amoore. Wright believed that fragrance particles have their vibrational frequencies and by vibrating they stimulate certain receptors in olfactory ephitelium. And Amoore differentiated seven scents he considered primary – each of them stimulated its own receptor in olfactory ephitelium. As it turns out, neither of these researchers was right.

Discovery worth its weight in Nobel… Prize

The processes of smelling and recognising scents by human have also been studied for years by Richard Axel and Linda B. Buck. They managed to establish that the only proper organ of smell is the olfactory ephitelium that is a part of nasal cavity lining. Ephitelium consists of three types of cells, the largest of which are olfactory neurons. It is at their endings that olfactory receptors are placed and they are responsible for detection of odour molecules from the air. Neurons transform this information into nerve impulse and send it directly to the brain. Neurons are supported by sustentacular cells which are for them a kind of scaffolding and isolation. The third type of ephitelium cells are Glands of Bowman, which accumulate odour molecules around the receptors. In the brain, it is axons that are responsible for stimulus reception. They are gathered in olfactory bulb in rhinencephalon, and in trigeminal nerve. However, the breakthrough in the studies on the sense of smell was the discovery that olfactory receptors are proteins, which are not found in the same form in other cells. These receptor proteins are encoded by over 1,000 different genes, and each olfactory neuron contains just one type of olfactory receptor. This proves how huge – and still unappreciated – the significance of the sense of smell is. For their studies on olfactory receptors, Alex and Buck received the Nobel Prize in Physiology and Medicine in 2004.

Nose or brain?

Although perfume creators have always been called “noses”, they themselves protest against this term. We tend to think that – since they are able to smell and differentiate so many fragrance notes and above all combine them harmoniously – they have their sense of smell more developed than an average person has. However, in fact it is not the nose that is the most important. It is the brain that can remember all the scents and then correctly recognise them. Composing perfume in that case is a complex process not only in the sensory-artistic way, but above all – in the analytical one, in which olfactory memory plays the main role. The same olfactory memory and individual sensitivity to smells are responsible for perceiving the ready fragrance composition. Not only does the perfume creator, but also perfume wearer form the actual scent only in their head. However, it is not known exactly how the brain builds its emotional response to olfactory stimuli and what really decides if a scent is received as beautiful and safe, or unpleasant and irritant. Another mystery is how scents influence our organism: they stimulate, calm it down, or even cure. Even now it is not known why and how exactly scents influence memory and memories so intensely.

Olfactory fatigue

Many times it happens that after several hours, or even minutes, we stop smelling our perfumes. It is a natural phenomenon and we should not worry about it. Paradoxically, not smelling our own perfumes means that the scent is perfectly chosen for us. It is not the fragrance that lost its strength, but our brain switched off its defence mechanisms and started ignoring the scent that it perceives as its “own”, “natural”, but above all – “safe”. Scientists called this phenomenon “olfactory fatigue”. This mechanism’s task is to selectively switch off perception and sensitivity to smells. If a new scent appears around us, then our sense of smell will let us know about it without any problem. If we experience the olfactory fatigue, we should not increase the amount of perfume we apply, as people in our surroundings can still smell it, and if we spray on a higher amount – the perfume may become irritating to others. People who are not entirely self-confident, when they do not smell the mist of a beautiful scent around them, are recommended to use more than one perfume in turns.