Smell: an extraordinarily precise sense

Smell allows us to detect environmental dangers, identify the most appropriate foods or recognize individuals in a group. Scientists study the complexity and precision of this sense and why infections like Covid cancel it out.

Two seconds to inhale, three to exhale. In a resting situation, We breathe an average of 23.000 times each day and, during this constant succession of inspirations and exhalations, we soak up an endless amount of odorant molecules.. Smell is the sense with which we perceive the large amount of olfactory information that each of our inspirations contains.

Not all things have a smell. If we can smell an object, it is because microscopic volatile particles are released from it that travel in the air to our nose. On the contrary, many solids such as metals, stone, or glass do not give off an odor because at room temperature they do not release molecules.

In animals, the sense of smell has many functions; among them, the detection of dangers (such as fire or a predator), the recognition of other individuals in the group, mating or searching for food.

The perception of odors

The aromatic particles that penetrate our nostrils through the inhaled air come into contact with the olfactory epithelium. In humans, this tissue occupies an area of ​​about four square centimeters on the roof of the nasal cavity and is made up of six different types of cells. Some of them, the olfactory receptor neurons, are responsible for the identification of odors. In their membrane they have microscopic antennas called cilia where the olfactory receptors are located: the proteins that detect odorant particles dissolved in the mucus that covers the epithelium. In order to be detected, in addition to being volatile, odoriferous particles must be present in a sufficient concentration, have a suitable size and the ability to be moistened.

In 2004, Linda B. Buck and Richard Axel received the Nobel Prize in Physiology or Medicine for his pioneering discoveries about the functioning of the olfactory system. Buck and Axel discovered the genes that encode the different types of olfactory receptors; one of the most numerous gene groups in the genome, made up of approximately 1.000 genes in mammals.

Each olfactory neuron contains a single type of receptor, which can recognize several odorants., and although it is still largely unknown which receptor binds to which odorants, related receptors (within the same subfamily) tend to recognize molecules of the same type or with similar characteristics.

Olfactory information is therefore encoded using a combinatorial method. The simultaneous stimulation of some receptors and not others is what allows us to perceive different odors.

In humans, between ten and twenty million neurons send nerve impulses to the olfactory bulb, located in the most frontal part of the brain, where smells are processed and encoded in different microregions. Each odor produces a different spatial activation map in the bulb. From there, the information is sent to the cerebral cortex, responsible for processes such as consciousness, thinking, emotions, reasoning, language and memory, and to the limbic system, involved in behavioral and emotional responses.

The number of intact olfactory receptor genes varies between species such as rodents or canids, which have a keen sense of smell, and those that, like humans and other primates, do not have it. In the former it is close to a thousand or more, while in the latter we have just over 400 functional genes. Genetic variations in the same receptor, which occur naturally among people, have been related with changes in the perception of odors.

Smell is also closely related to the perception of taste. Apart from the five basic flavors, the rest of the nuances are provided by the smells. Therefore, when we want to taste a food, we tend to exhale so that the air enters our nostrils from the back of the mouth (retronasal route) and with it the volatile molecules of the food that stimulate our olfactory receptors.

Covid and anosmia

The loss of smell (or anosmia) caused by a virus is not exclusive to the recent covid pandemic. Specialists already used the term 'postviral olfactory dysfunction' to refer to alterations in smell caused by rhinovirus, other coronaviruses, parainfluenza viruses, or Epstein-Barr.

Loss of smell has been linked to damage to the olfactory epithelium. Generally, it recovers when the acute infection disappears, but in some people it persists for several months or even years. Olfactory training or a combination of olfactory training and corticosteroid drugs is used to treat anosmia. administered topically or orally.

In covid, the alteration of smell affects between 30 and 75% of infected people. Until now had identified a region on chromosome 4 related to anosmia. In it are located the UGT2A1 and UGT2A2 genes, both active in the olfactory epithelium, where they play a role in the metabolism of aromatic compounds, but The process by which the coronavirus causes loss of smell was not known. A recent study, published in February in the magazine 'Cell', proposes for the first time a biological mechanism to explain it.

Scientists have seen that, in the olfactory epithelium, the virus does not infect the olfactory neurons and, instead, accumulates specifically in the supporting or sustainacular cells which, unlike neurons, do express a large amount of ACE2 and TMPRSS2 proteins, the virus' entry point into cells. Under normal conditions, supporting cells maintain the structure of the tissue so that neurons can play their role in odor perception.

The study analyzed three groups of hamsters – infected with covid, with flu, and not infected. who were deprived of food for ten hours. Afterwards, they were introduced into cages where Choco Krispies, which hamsters adore, had been hidden mixed in the bedding. And during the 14 days following infection, the time it took to find them was recorded.

During the first two days, the hamsters with coronavirus were unable to find the cereal, unlike hamsters with the flu or uninfected ones. Clearly, they had lost their sense of smell. By the 15th, practically everyone had recovered it and was once again finding the hidden food. In hamsters with covid, scientists detected that more than half of the supporting cells died in the first two days of infection. As a result of this, An inflammation process begins and the olfactory neurons, which under normal conditions dedicate most of their activity to functions related to smell, are forced to reduce the production of receptors. and other components necessary for smell and, for this reason, anosmia occurs.

Olfactory neurons don't die, they just try to cope with inflammation. This is why for a short period of time, between three and five days after the start of infection, many people lose their sense of smell. This returns when inflammation subsides and the population of supporting cells can regenerate from progenitor cells. Even so, there are cases in which olfactory dysfunction persists for more than six months.

The direct connection between smell and memory

Have you ever walked into a room and experienced an intense emotion because a perfume reminded you of a familiar person? As Proust's Madeleine, A smell can be overwhelmingly nostalgic and trigger emotions and thoughts before we even have time to process them.

The first processing of the signals that we perceive as smells is carried out in the olfactory bulb and, from there, the neurons transport the information directly to two structures of the limbic system: the amygdala (in charge of processing emotions) and the hippocampus (related to memory and learning). Smell is the only sense that has a direct route to the areas of the brain involved in memory and emotions.. All other sensory information, including sight, hearing, taste, touch and balance, first passes through the thalamus, which acts as a switchboard, before reaching the cerebral cortex. But the sense of smell skips it and in one or two synapses it hits the amygdala and the hippocampus.

For neuroscientists, This particular brain architecture would explain why memories triggered by smells are more evocative and more emotional than those related to other stimuli.. Normally when we smell something that connects us to the past, we first experience an intense emotion and then remember; but there are cases in which we are unable to recover the memory. This probably happens because the context is different from what it was when our brain first established the relationship with that smell.

Finally, although the strong connection of smell with memory causes people to perceive these types of memories as very accurate, They are as inaccurate and susceptible to being rewritten as the rest.

Metallic smell is just body odor

Although we swear that we perceive a characteristic 'metallic smell' when we touch steel cutlery, keys or coins, this fragrance is not due to metal. Solid bodies, like these, have no smell. So why is it?

American and German researchers they discovered in 2006 that It is nothing more than the smell that our body produces when we touch a metallic object. The sweat of our skin, slightly acidic, corrodes iron, forming ferrous ions (Fe2+) that oxidize in seconds to ferric ions (Fe3+), while simultaneously reducing and breaking down the fats present in our skin (lipid peroxides), forming volatile molecules. – ketones and odorous aldehydes – which are what we perceive as a metallic odor. They also demonstrated it with other metals such as copper.

"We are the first to demonstrate that When humans describe the 'metallic' smell of iron, there are no iron atoms in the smells. "Odors are actually byproducts of metals reacting with the skin.", in the words of Andrea Dietrich, author of the study.

The smell of some ferruginous waters or blood would also be due to the same, and the ability to detect it would have represented an evolutionary advantage in humans for tracking injured prey. This same phenomenon would also explain why some people recommend removing bad odors from your hands by rubbing them against a metal object.