How Our Sense of Smell Works

Our sense of smell is our most powerful sense even though, generally, we often neglect it and use it the least of all our other senses preferring to use our senses of sight and hearing.

I can remember as a very small child visiting my grandmother. I must have been very small as, in my memory, while I was toddling around her garden I flopped onto my bottom on numerous occasions as I couldn’t walk very well. I can remember being given numerous flowers and leaves to smell, feel and look at, some even to taste. I can remember being told about the fairies that lived in that garden so a strong sense of magic and mystery pervaded the experience for me. My grandmother moved shortly after this visit and her next garden was mainly of roses. The memory of exploring the first garden must have faded soon after that experience as it wasn’t until I was about 24 years old that I again smelt the flower of a morning, noon and night bush, the scent of that flower bringing back that memory of my grandmother’s magical garden.

Even today smelling a particular type of apple in summer conjures up memories of my early childhood, sitting in my high chair and being given an apple to eat by my baby sitter. The smell of the first drops of rain after an extremely hot day take me back to my school days and many other smells evoke memories of past times. Although certain pieces of music and some visual experiences can evoke memories, smell is by far the strongest in provoking memories from the past to enter into our awareness in immense detail.

How we smell is a very complex process involving a part of our nose and various parts of our brain. We have the ability to recognize up to 10,000 separate odors but each thing we smell needs to be volatile (capable of entering a gaseous state) for our senses to be able to detect and identify it.

Noses come in all shapes and sizes externally but the external features are only for the passage of air as we breathe. Internally, olfactory epithelium (mucous membranes) located on the roof and upper part of both walls of the nasal cavity are the part of the nose that detects odors. This epithelium "contains some 5 million olfactory neurons, plus their support cells and stem cells…each topped by at least 10 hair-like cilia". During normal breathing the inhaled air slightly touches the cilia so that odors can be detected from the environment. Sniffing, which is deliberate, strong inhalation of air through our nose, pulls the air deeply into the cilia to increase the amount and strength of the odor that has been detected by increasing the amount of the gaseous chemicals that come into contact with the cilia.

For detection to occur the cilia are covered with a thin film of mucous that is necessary to dissolve the chemicals in the gaseous substance that carries the odor. It is estimated that each cilia cell has between 500 and 1,000 receptor proteins that are genetic in nature and determine which odors our brains can detect and identify according to which receptors are activated by the odor. Each neuron is connected, via a synapse to the olfactory bulb, which is part of the brain, by a long fiber called an axon. The axon passes through the bone above the neurons, called the cribiform plate. It is in the olfactory bulb that the first impressions are made. "Olfaction is…. the most direct interface between the brain and the outside world".

Once the odors have triggered the receptorsites, the olfactory bulb sends nerve impulses, via the olfactory bundles, to the thalamus (which then transmits the impulses to the neocortex) and to the hypothalamus. Its in the neocortex that olfactory discrimination, perception and memories occur allowing us to correctly identify what it is we are smelling.

The hypothalamus forms part of the limbic system (the oldest part of the brain) which activates smell-related emotions and "the olfactory relationships with memory, eating, survival, sex and other emotional behaviour". The limbic system is also involved with visceral functioning, via the sympathetic and parasympathetic nervous systems, "including the regulation of heart rate, blood pressure, respiration, digestive activity and levels of various hormones". In this way the hypothalamus and limbic system play a key role in emotional conditions such as stress and depression. Aromatherapy works by using various odors to achieve specific therapeutic results when utilising the person’s sense of smell to manipulate their limbic system. The odors of the essential oils trigger limbic responses that affect the person physically, mentally and emotionally with each oil having a variety of effects upon the person depending on the oil’s chemical constituents and the person’s memory association of that odour. Although it is not known exactly how essential oils absorbed through the skin and into the blood stream and lymphatic system affect the brain, skin absorption through massage has a therapeutic action on the brain functions controlled by the limbic system and on internal organ and tissue systems.

Although the use of aromas for therapeutic use has been around for centuries it is only recently that research has been done in earnest to determine how our brains work to determine smells and how these odours can affect us. One particular research is being conducted to determine the relationship between smell and the success of ventures such as attaining a job or selling a house. This research is aimed at rationalising what we already instinctively know and detailing how it actually occurs. Other research has determined the relationship of aromas and the productivity of workers in Japan using various scents and measuring the changes in productivity with each scent.

Lavender, for example, has been tested in various settings to determine the effect it has on animals and people. "By the seventeenth century essential oils had become a small but regular part of the herbalist’s repertoire of remedies" with Culpepper, a herbalist, noting that lavender was especially useful for symptoms of headaches from various causes, uterine problems and other physical ailments. Today, lavender is used therapeutically for a wide variety of problems and has the recognised properties of antidepressant, antiseptic, bactericidal, decongestant, hypotensive, insect repellent and sedative actions on the body with its main characteristics described as "calming, soothing and, above all, balancing". . The antidepressant, decongestant, hypotesive and sedative actions work through the olfactory system to enact changes within the limbic system. There is a large range of essential oils, each with its own individual properties and actions on the body with more essential oils being added to the list each year.

As more research is done on how the aromas of essential oils affect us perhaps, in the future, we will once again be using our sense of smell to its fullest capacity to increase our appreciation and awareness of our environment and the effect is has on our emotional, physical, mental and spiritual wellbeing.

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The Perfect Potion (Aust) Pty Ltd, Noosa, 1995, pp100-102

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Addison-Wesley Educational Publishers Inc, New York, 1993, pp160

Pines, M. "Seeing, Hearing and Smelling the World – new findings help scientists make sense of our senses",
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The Olfactory Research Fund, "Power Scents – the path to success may be through your nose",
undated, http://www.fragrance.org/oa-power.html

Worwood, V.A., "The Fragrant Mind – Aromatherapy for Personality, Mind, Mood and Emotion",
Bantam Books, London, 1995, pp23-38