Our Chemical Senses:  We possess at least 5 senses that respond to chemicals in our environment:

1) Olfaction – the sense of smell

Introduction: Olfaction is the sense of Joy: smell of a baby, a rose, the aroma of fresh brewed coffee… These are the things that add color and zest to our lives and are the reason why the sense of smell is called the “evocative” sense as it readily evokes moods and memories.  We all know that the sense of smell can provide healing, relaxation and pleasure – the reason why we love our essential oils!  But the main function of the oldest of our senses is to monitor our environment as we move through it and to provide information not available via our other senses…

Studies by Dr. Sobel of U.C. Berkeley have demonstrated that the nostrils are attuned to different odor chemistries just like our ears and eyes will perceive somewhat different spatial information.  This odor processing is necessary for localization and orientation toward important stimuli and done for the most part subconsciously, thus with every breath we take we sniff the air.  The sense of smell rests as we sleep (which is why people rarely wake up when there is a gas leak or smell of fire but they will awaken when there is a loud noise, for example).  As long as the odors are benign, our conscious mind hardly pays attention.  If the fragrance is really nice (like a blossoming rose bush) we may stop and smell the roses, but after a couple of whiffs we will move on as the process of “adaptation” (diminished response) has set in (see later this chapter).  Our nose will alert us when there is a foul odor around, such as animal excrement or a dead animal, or a chemical leak which is sign to keep a distance and stay away.  When the offensive odor is hazardous, another sense may kick in: the Trigeminal sense.  The trigeminal sense will alert us we are coming too close and will do so by inducing a prickly irritation sensation in our nose, sneezing or even tears.  Thus an important function of our nose is to ensure purity/cleanliness for protection:  How do we determine if the milk in the bottle is still fresh?  Mainly with our nose.  How do we determine if our bodies are clean or if our clothes need to be washed?  Again, with our nose.  We delight in fresh scents because they register in the brain as “healthy” and “pure” and we have learned to mask body odor with plant fragrances that we consider more appealing.  The fragrance industry applies this knowledge with a multitude of fragrant personal care products.

It is important to know that olfactory stimulation via the olfactory nerves is electro-chemical and that the nerves literally “fire” an electrical signal into the brain when the sense of smell is stimulated.  Thus the fragrance does not travel into the brain!  There are millions of olfactory nerves lining the upper part of the nose within a small sensory area called the olfactory “epithelium” or “mucosa”  (see chart).  All these nerves are extensions of the First (out of 12) Cranial (head) Nerves.  This attests to its importance and to the fact that it’s the oldest sense out of all our senses.  The “olfactory mucosa” has a yellowish appearance in Man and much darker brown color in more chemosensory animals such as dogs.  The mucosa is composed of olfactory nerve cells at different stages of maturity that secrete the mucus necessary for olfactory stimulation (when the air is dry the sense of smell is disrupted).  The olfactory nerves, probably because of the constant assault by chemicals from the environment have a very short life span (weeks) thus the olfactory tissue itself is at a constant flux of regeneration from stem cells.  The olfactory nerves, unlike most nerves in the body, are unique in their ability to rapidly regenerate and are therefore intensively studied.

Olfactory receptors:

A mature olfactory nerve has many thin hair-like extensions at its tip called “Cillia” which bathe in the olfactory mucus of the nose.  The cillia are dotted with olfactory “receptors” that recognize specific aspects of the scent chemicals.  All the receptors on a particular neuron are of one type and there are about 1000 different types of receptors.  Each receptor is optimally sensitive to only several odor molecules, and in total we recognize about 10,000 volatile odor molecules.  Research by Turin from Britain demonstrated that olfactory receptors distinguish volatiles based on their vibration.  Binding of odor molecules by the receptors initiates an electrical signal in the neuron which travels to the olfactory bulb. The amount of genetic material devoted to the sense of smell is very extensive and only surpassed by the immune system which attests to the importance of the sense of smell.  Thus there are at least 1000 “receptor genes” while for color vision there are only 3 genes!

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The Limbic System:

From the Olfactory bulbs it takes only a couple of synapses (nerve connections) for a scent to activate the ancient part of the brain known as the Limbic System – (also called the ‘smell brain’).   The limbic system is the center of emotions and memories: everything in the limbic system translates into either "like or dislike."  It is only after processing in this area that the scent information is processed in the higher cerebral hemispheric centers (neocortex).  In the neocortex cognitive processing takes place such as odor identification and labeling.  Thus the first immediate gut-response to an odor is the intuitive like or dislike, emotions and memories.  This initial response is triggered even at very low concentrations.  The secondary higher brain response that involves evaluating the odor at the cerebral cortex usually requires stronger odor intensity for processing.  Thus you need just a brief whiff to enjoy a smell of an essential oil, but in order to identify which oil it is and describe it, a higher odor concentration is necessary.