| Zofia Hochtaubel
Have you ever imagined how your life would be without olfaction? It definitely wouldn't be the same without the smell of morning coffee or fresh buns from the oven.
We use our noses all the time but often underestimate their power. Until 2014, scientists believed that an average person could detect only about 10,000 different odors. However, now we know that the average person can discriminate between at least one trillion different odors with their nose. It only shows how the human olfactory system far outperforms the other senses regarding distinguishing between physical stimuli.
Olfaction is the oldest sensory system regarding evolution. Already about 2–3 billion years ago, single-celled bacteria called prokaryotes had a primordial sense of smell. Nevertheless, we still know little about the olfactory system compared to other senses.
The human species is a microsomatic species, meaning it has a poorly developed olfactory system. Compared to other mammals, we don't smell much. This is because our olfactory receptors are relatively less sensitive, and we have a smaller number of them compared to many other animals. However, our sense of smell still plays a crucial role in triggering certain memories and emotions. To understand how olfaction is connected to our memory, we have to understand the anatomy of the olfactory system.
Chemical stimuli called odorants interact with olfactory receptor neurons in the epithelial sheet lining the inside of the nose. On the surface of this epithelium are cilia coated with mucus, which is necessary for the sense of smell. For the receptor to be stimulated, the odorant must dissolve in the mucus. The receptor cells' axons project directly to neurons in the olfactory bulb, located on the orbital surface of the frontal lobe. Unlike other types of sensation, olfactory information takes shortcuts to the cerebral cortex without the mediation of the thalamus. This is a great solution, especially considering that the olfactory nerve cells are not covered with myelin, so the signals move much slower in them. The primary olfactory cortex includes the piriform cortex, the hook cortex, and the adjacent entorhinal cortex. The hook is a thickening located near the hippocampus and amygdala, which are entwined with memories and emotions. Thanks to this location, fragrances can evoke a memory that seems strong and very real because it is emotionally charged. What’s more, olfactory memory is surprisingly stable; if we know a certain scent, we will never forget it.
The olfactory system also shows remarkable plasticity in its response to odors, with rapid adjustments in sensitivity and acuity. These neural changes contribute to odor habituation, which allows us to focus on important sensory information rather than being constantly bombarded by familiar smells. This adaptive mechanism helps us navigate our environment more efficiently and enhances our overall sensory experience. Thanks to it, it takes from a few seconds to a couple of minutes for us to adapt to a smell. If it weren't for that, wearing perfume would be too overwhelming and distracting, as our brains would be overstimulated.
Nevertheless, the human sense of smell is greatly underappreciated since we often don't know how many things in our lives are connected to it. Have you ever noticed that, usually, when you catch a cold and your nose is blocked, your food does not taste so good? That's because odors are a component of flavor perception when eating. Our noses help us distinguish between different flavors and enhance our overall dining experience. Moreover, even before ingestion, they serve an important function. Olfactory food cues can alert us to food in our environment, shape our appetite, and appear to be able to provide nutritional information to humans. Studies have now shown that scents especially stimulate appetite for the cued product. This phenomenon is known as sensory-specific appetite, and this effect might apply to other foods with comparable properties, such as the taste category. Additionally, odors representing high- or low-energy-dense foods specifically induce appetite for food products high in or low in energy density. This suggests that humans, like other species, may use their sense of smell to navigate for high-calorie food.
Olfaction is also crucial for transferring information and fostering relationships among species. Sweat-based olfactory communication is simple and unaffected by constraints of space and time, making it applicable to people with cognitive and social impairments as well as those in different developmental stages. Sweat chemically encodes personal identity, kin and relatives, partner choice, friendship, health status, sexual availability, and emotional states. Smell loss significantly impacts human interactions. It may have an impact on emotional control, restrict the capacity to "read a room," and affect attachment. What's even more intriguing is that studies have shown that people are more ready to support strangers when they smell something pleasant, like freshly baked goods. It has to do with the way that pleasant smells bring back happy memories and lift our spirits.
Our noses are truly powerful. They can pick up on a variety of aromas, from the enticing aroma of freshly baked cookies to the foul stench of rotten eggs. They can impact our social lives and our dining experiences. We still have so much to learn about this sense, but every year we take another step toward discovering the full potential of olfaction.
Sources:
“Basic Clinical Neuroscience” by Ph.D. Paul A. Young, Paul H. Young, Ph.D. Daniel L. Tolbert.
“Your Superstar Brain” by Kaja Nordengen.
"Neurobiology of Sensation and Reward." by Jay A. Gottfried and Donald A. Wilson.
"Physiology, Olfactory" by Benjamin Branigan; Prasanna Tadi.
"Neuroscience. 2nd edition." by Purves D, Augustine GJ, Fitzpatrick D
"The importance of the olfactory system in human well-being, through nutrition and social behavior" by Sanne Boesveldt and Valentina Parma
"Humans can Discriminate more than one Trillion Olfactory Stimuli" by C. Bushdid, M.O. Magnasco, L.B. Vosshall, and A. Keller
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