Smell can be defined as the perception or detection of an odour or scent, olfaction is the medical term used for perception of smell. It is believed that humans can smell and distinguish between over 10,000 different aromas. In the animal world, a sense of smell provides the ability to find food, avoid danger and search for a mate, so it is vital for an animal’s survival to have an intact sense of smell. While humans are less dependent on olfaction than animals it is still used to assess safety, nutrition, pleasure and general wellbeing. Olfaction is a complex process that is still relatively new science, between thousands of airborne chemicals as it detects and differentiates betwwen them, and it also adds an emotional attribute which may influence mood and thought processes in humans, (Sharma et al 2019).
The nose is constantly monitoring the chemical composition of the environment surrounding us, . Odour molecules reach the nasal cavity either by direct inhalation from the front of the nose or via the throat. The aromatic molecules are inhaled up the nasal passage where they dissolve in the mucus lining, known as the olfactory epithelium. In the roof of the nasal septum, these volatile, inhaled molecules come into contact with the olfactory cilia which are microscopic hair-like endings of the olfactory receptor cells.
For a smell to be detected it must be volatile so in a gaseous state, and it will dissolve in the epithelium covering the cilia. As the odorant dissolves this stimulates the olfactory receptor cells and generates a nerve impulse which travels along the olfactory nerve and into the cranial nerve. In-between the brain and the olfactory epithelium there is a piece of bone known as the cribriform plate, which has several holes through which the nerve impulse is sent. The 1st stage of this impulse is to pass to the olfactory bulb where it divides into 2 pathways.
Processing of the nerve impulse produces the emotional response to the odour. A dangerous smell such as gas or smoke will trigger the sympathetic nervous system into the fight or flight response. An appetising smell such a chocolate will stimulate the digestive tract to produce saliva, and an unappetising smell may provoke sneezing or coughing as a protective mechanism.
Molecules will also travel directly into the respiratory system and the lungs when inhaled, where absorption through passive diffusion into the blood stream occurs. This can also have a physiological effect on symptoms,There may also be an immediate localised beneficial effect on breathing, due to direct action on the alveoli in the lungs,(Price and Price 2020).
Humans have an amazing ability to distinguish between 40,000 to 1 million different smells, although this exact number is controversial. Perception of a smell will evoke memories both positive and negative, (Price & Price 2009). There can also be a physiological response to a smell eg detecting the scent of freshly baked bread can stimulate production of saliva by the salivary glands. Emotional responses can also be stimulated by smells, which can also provoke a positive physiological effect. For example, smelling something, which brings a happy memory will cause the breathing and heart rate to relax, and the release of endorphins.
Ignacio Salazar, Pablo Sanchez-Quinteiro, Arthur W. Barrios, Manuel López Amado, José A. Vega, Anatomy of the olfactory mucosa, Smell and Taste, 10.1016/B978-0-444-63855-7.00004-6, (47-65), (2019).
Sharma A, Kumar R, Aier I, Semwal R, Tyagi P, Varadwaj P. Sense of Smell: Structural, Functional, Mechanistic Advancements and Challenges in Human Olfactory Research. Curr Neuropharmacol. 2019;17(9):891-911. doi:10.2174/1570159X17666181206095626
Price & Price, Aromatherapy for Health Professionals, 5th Edition, Publisher: Elsevier Health Sciences