The Sense of Smell
The olfactory function (our ability to smell) allows for a direct connection to the brain. Sensory cells which are present in the mucous membrane lining of our nasal cavity are stimulated by the introduction of the presence of chemical particles into this lining. Fibres of the olfactory nerve run upward from smell receptors in that nasal mucosa high in the roof of the nose, through tiny holes in the skull where they enter the olfactory bulb of the brain. The signals are carried then to the rhinencephalon (which is a part of the limbic system of the brain concerned with the sense of smell) and from there the scent is analysed.
What is perceived by the brain as an aroma is chemical particles. The limbic system which is a centre for our emotions is connected to the hypothalamus (responsible for the control or our hormonal system through the pituitary gland) and this is how scent can trigger memory and emotions. Essential oils are simply made up of the volatile compounds of a plant, chemical particles if you will, which are extracted through a method of steam distillation. Being only a tiny part of the plant, it often takes kilos of plant matter to produce one milliliter of essential oil.
Because scent has such a direct interaction with the brain, it can have profound effects on the health and wellbeing of our nervous system.
It is worth noting at this point also that this is how synthetic fragrances can do a lot of harm to our hormones and bodies. The nose does note register that the fragrance is synthetic, but the brain and the body certainly do and these particles do not interact with our biology in a friendly way. Studies have now shown that artificial fragrances can be responsible for a myriad of health issues from headaches to hormonal issues with lead to sleep issues and reproductive problems as well.