We are now well into Spring, and walking outside is totally blissful. The temperature is comfortable, but more importantly, the scent in the air is somehow exhilarating. I then stop walking and take a moment to breathe in all the odours that surround me. I am fortunate enough not to be afflicted by any pollen allergies, so I go all out. I can smell flowers, freshly mown grass, dirt, laundry and more. Not only is the smell keeping me connected to the present, but it is also igniting an interior feeling of deep connection to my surroundings. This connection is bringing me its share of joy and tranquility. I know that as long as I’ll be standing right here, everything will remain perfect.
Our sense of smell, or olfaction, is (as far as we know) our oldest sense, right along with taste. It is such a remarkably developed sense that we are able to detect a tremendous number of odours. Despite what a study from 1927 suggested, humans have the potential to smell much more than 10,000 distinct odorants. Over one billion of them, to be exact, as reported by a 2014 study. Still, this incredible figure is considerably meagre compared to the ability displayed by dogs. Our domesticated canines are surpassing our smelling power by as much as 44 fold (44x), with 220 million olfactory receptor cells in their nasal cavity compared with 5–6 million for us, mere humans.
In comparison to the senses presented in two earlier posts (see Bring Up Hearing, Bring Up Sight), smell possesses a rather simple organ, the nose. This cavity has receptors spread over all of its epithelium. These receptors are cranial neurons that are astonishingly capable of renewing themselves every 30–60 days. Neurons are typically incapable of such feats. The nasal cavity encompasses a wide diversity of cells. It includes no less than six morphologically and biochemically distinct cell types. One especially relevant cell type for smell is the ciliated cells. Their sensory cilia are facing the inside of the nasal cavity. Their role is to catch the odorant molecules. Once activated, they send their signal to the olfactory cortex in the main olfactory bulb (there are two). The second olfactory bulb, called the vomeronasal organ (VNO), is not as important as the main. It serves to detect pheromones. In humans, we can find our VNO in the anteroinferior third of the nasal septum. The VNO also signals to the main olfactory bulb.
To be fair, all odours, scents, and pheromones are volatile compounds. However, pheromones have something that odours and scents do not have. Pheromones are chemical substances secreted by one individual and absorbed (or received) by another individual of the same species. The pheromones, once someone else picks them up, can act as hormones in their body. They have the ability to alter behaviour and physiology for survival and reproduction purposes. There are many kinds of pheromones, each carrying different signalling objectives. The signal emitted could be used to signal danger, food ingestion safety, food location and more, which can promote survival. As for reproduction, they can signal for potential and fertile mates and help with offspring recognition.
When it comes to choosing sexual partners, Smell seems to be a significant component for determining their attractiveness. Indeed, some studies have demonstrated that family members seem to have a scent perceived as “less attractive” compared with the scent of strangers. It is actually quite understandable as inbreeding correlates to an increased susceptibility to a wide range of disorders. Limiting inbreeding through regulating smell perception may seem like a reasonable solution to deter us from it. However, smell perception may not serve the unique purpose of limiting inbreeding. It also helps to recognize who’s family and who’s not. And we are able to do this very early on in life. Newborns are capable of identifying their mothers after being exposed for at least one hour.
The sense of smell is not as stable as the other senses; it varies greatly depending on conditions. In general, women have more robust, or more attuned, olfactory abilities compared to men. Although, this might not be true. Recent researchers have now started questioning this “common knowledge” and its validity. They have revealed that the difference noticed between the sexes is actually really small and possibly caused by anatomical differences. Now omitting the weakly possible sex differences, there are still other factors influencing olfactory variation. Circadian rhythm seems to be a strong influence; early morning appears to be accompanied by the weakest sensitivity to smell. Scientists are suggesting that its highest point is at night, right before going to bed. At last, an important factor seems to be the availability of moisture in our nasal cavity. The more, the merrier it seems. With this idea in mind, researchers are now advancing that smell is at its best during summer and spring. Workout sessions also seem to work wonders when it comes to increasing olfactory power.
It’s all well and good to know how our olfaction changes, but why should we care about it enough to protect it? The reason is utterly simple; it makes us feel great. Although, this is possible only if we perceive the scents as pleasant. From all the senses, olfaction is the one that shares the most links to memory and emotional brain regions. There is a belief in the activation of olfactory neurons to have the capacity to activate the brain reward system, which is a system also involved in addictive behaviours. Smell, as well as taste, is considered to have developed this strong connection as a way to avoid ingesting potentially harmful substances or running straight towards danger. By reinforcing survival behaviour through rewards, we are now not only able to safely feed ourselves, but we can also enjoy the process. Actually, if you didn’t already know, smell has a lot to do with taste as it provides 80% of the signalling necessary to perceive flavours. Another curious thing about smell resides in how our body responds to its signals. For most senses, when signals reach the brain, there will first be its identification, and only then would it be followed by a reaction. Olfaction works the other way around, reaction (emotional) first and then identification after.
When smell was introduced to you as having a lot to do with pleasure, some of you might have been quite skeptical. However, we did not produce that theory out of thin air and without anything to back it up. It turns out that people with olfactory dysfunctions tend to struggle more with malnutrition, apathy, depression and lower quality of life than the general population. This statement potentially indicates that a disrupted perception of scents leads to an overall decrease in hedonia (pleasure, enjoyment and comfort). These olfactory dysfunctions can be a reduced sense of smell (Hyposmia), a total lack of olfaction (Anosmia), a change in its perception (Parosmia, things that usually smell pleasant might now smell bad) or a perception of a smell that is actually not present (Phantosmia, olfactory hallucination). These dysfunctions could be present at birth or occur at any moment in our life. The general advice is to avoid excessive exposure to malodours (bad smells) as it can lead to the onset of a dysfunction.
At last, we are part of the mammalian family, and as such, we can infer that the ownership of a nose is not exclusive to only us but also to the rest of the family. If you also thought as much, then you wouldn’t be completely wrong. Most mammals have or had a nose, and still, not all of them use it as their primary olfactory organs. Toothed whales still have a nose but can’t smell anymore. Baleen whales, however, have kept the usage of their nose as an olfactory tool. Albeit we don’t know about the precise purpose of keeping them as they strongly rely on echolocation to find food. Other animals likewise can smell, but they might not smell the same way we do. Snakes, for example, smell mainly through their VNO located at the roof of their mouth, even though they also have a nose capable of sensing odours. They grab the molecules of scents with their tongue and insert them into the VNO for identification. As for fish, they do smell through an olfactory organ located in their nasal sac, where water, filled with chemical signals, is flowing in. As water comes in, it makes contact with the lamellae possessing olfactory receptor neurons which can provide the fish with the necessary information. Considerably more animals have olfaction than those that do not, which only encourages us to consider its importance towards our survival. Would you not agree?
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