It is so nice to wake up in the morning and smell that coffee aroma throughout the house; or, after a shower, to wear a perfume that translates our state of mind. Sometimes I have the impression that certain odors generate a magical feeling in us, as if they awaken something we don't even know what it is. A kind of vague memory, as if "there is something in the air." Mystery, perhaps that is the best word to define the sense of smell, because the truth is that despite the many existing theories, no one knows for sure how we perceive smells.
In the early study of smell, physiologists tried to discover if there were basic smells, which, when mixed in different proportions, would be responsible for generating the various types of odors, similar to what happens with primary colors. Based on this idea, in the 1950s, the first classification of odors emerged: 1. camphoraceous, 2. musky, 3. floral, 4. mentholated, 5. ethereal, 6. irritating, 7. putrid. However, this division served only to facilitate olfactory description, as the attempt to find basic smells was unsuccessful, and research continued.
The mucosa of the nasal cavity is covered by continuously secreted mucus, where the odor molecules from the air are dissolved and come into contact with bipolar neurons, which are the chemoreceptors involved in olfaction. The dendrites of these cells have cilia that extend throughout the nasal cavity, forming a dense network, while the axons are directed upwards and penetrate the cranial cavity. In the membrane of the cilia, there are olfactory receptors, which are produced by more than 1,000 genes that give rise to about 400 different olfactory receptors! The number of specific receptors is quite high compared to the other senses; however, humans can recognize a much larger number of odorants (from 10,000 to 400,000), and researchers are trying to discover how this occurs.
The axons of the bipolar neurons form nerve bundles that reach the olfactory bulb (in the brain), where they form synapses with other cells that project to cortical regions and, subsequently, to the limbic system, which explains the influence of smells on emotions and memories. Indeed, odors have a tremendous ability to make us remember, and these memories are often characterized by a strong emotional connotation and specificity, which allows the sense of smell to be of particular importance, for example, in studying autobiographical memory.
Just as smells can evoke emotions, these can also alter olfactory sensitivity. A study conducted in Germany with 32 individuals showed that olfactory sensitivity was significantly reduced in all subjects of the group after the presentation of a negative image and decreased only in men after the presentation of a positive image (the images used for emotional stimulation were selected from the IAPS – International Affective Picture System*). The pleasure and intensity ratings reported by participants depended on the content of the image: after the presentation of the negative image, the odor was rated as less pleasant and more intense, while the positive image led to an increase in reported pleasure. This example demonstrates the existing connection between smell and affectivity, which may help explain the benefits produced by smells on health, as we will see in the next post.
* IAPS: The International Affective Picture System was developed to provide a large number of standardized photographs capable of evoking emotions, internationally accessible, across various categories.
References
Hall, JE. Somatic sensations: The chemical senses – Taste and smell. In: Medical Physiology Treatise (Guyton, AC; Hall, JE). 2011; p. 683-90, Elsevier;
Lent R. The chemical senses. In: One Hundred Billion Neurons – Fundamental Concepts of Neuroscience (Lent, R). 2004; p. 340-65, Atheneu Publishing;
Pollatos O, Kopietz R, Linn J, Albrecht J, Sakar V, Anzinger A, Schandry R, Wiesmann M. Emotional Stimulation Alters Olfactory Sensitivity and Odor Judgment. Chem. Senses. 2007;32: 583–9;
Zucco GM, Aiello L, Turuani L, Koster E. Odor-Evoked Autobiographical Memories: Age and Gender Differences Along the Life Span. Chem. Senses. 2012;37: 179-89.
Image: http://commons.wikimedia.org/wiki/File:Tina_Liang_asian_model_smells_the_fennel.jpg
