S2C8: Olfaction & Taste

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a) External signalling through pheromones

In Chapter 7 on Vision we saw this sense allows an organism to harvest light whilst capturing information about the angle of origin of the light source, its brightness and its wavelength. This data is then processed by our brain to infer position and identification, which in turn allows animals to orientate themselves and derive an understanding of the events unfolding around them so as to optimize their behaviour and actions. This requires a lot of neural processing and has taken a relatively long time to evolve.

In comparison, the sense of smell, also known as olfaction, is a lot more direct in its processing with much less interpretation baked into the system. For that reason, the fact that it was the first sense to have developed should not be unexpected. I am not suggesting olfaction is simple since, as we will see, it still relies on a chemical cascade to activate dedicated receptors and transduce the information in an electrochemical signal travelling through nerves. Fundamentally, the sense of smell is about chemical compounds containing molecules that bind to proteins, very much like neurotransmitters and hormones do. This information is then leveraged to identify the probable source of these compounds called odours and this signal triggers certain behaviours which are often unconscious and reflexive, a result of evolutionary adaptation imprinted in the networks of our nervous system.

The comparison to hormones isn’t coincidental because they have an analogous type of molecules called pheromones which can be detected by olfaction and plays a central role in the behaviours of many animal species. If hormones are about carrying information to another part of the organism itself, pheromones are excreted out of the organism’s body and detected by other organisms. They are thus inter-organism signalling molecules whereas hormones are intra-organism signalling molecules.

As always, we should fully expect our senses to be useful for the ultimate purpose of enhancing the odds of an organisms’ survival and replication, and therefore the replication of its genetic material. In the case of pheromones, this is very much on point. For example, in several species such as bees, the release by one individual of a specific pheromone when facing a predator will trigger a predetermined aggressive behaviour throughout the entire hive. This is called a releaser effect and in other species the reflex could be a mass escape away from the predator.

Pheromones are also used as social signals such as denoting a territory or the location of food by literally creating a scent trail. Respective examples would include your pet dog urinating on lampposts, or sniffing them, and ants marking their way back to the colony if they have chanced across some food. Female in various animal species also emits pheromones when they become sexually available, not because they are single since pair bonding is more the exception than the rule, but rather around the time they are fertile.

And finally, just like hormones, pheromones can initiate a development process, this is called the primer effect. For example, the queen bee regulates the various functions within the colony mostly through primer pheromones, these have consequences such as the suppression of worker bee reproduction and of the rearing of other queens. In fact, the very emergence of a queen out of a standard bee larva is the result of a protein contained within the royal jelly which it is fed, a primer pheromone of sorts.

b) Smelling danger

As mentioned, pheromones are a signalling tools within species, nonetheless our sense of smell is not restricted to these molecules and can theoretically detect any type of odour, as long as the relevant receptors have evolved. Going by evolution’s standard operating procedure, there is no prize for guessing the function of the receptors that first developed would have to do with providing an edge in terms of survival, either by detecting likely danger or items that are useful to the organism, in particular food since it is the main source of energy and nutrients, indispensable inputs for the maintenance of a proper metabolism and homeostasis – and therefore not die.

When a gas, liquid or solid has a bad smell, the default behaviour is to stay away from it and, in the case of food, avoid ingesting it. This makes complete sense and it is no coincidence harmful substances have a nauseating smell. What we need to remember is that just like there is no colour in the real world, there is no odour either. The experience of smell is a type of qualia, an internal phenomenon we explored in S2 Section 7.e that occurs within our brain, and generations after generations, this type of mental representation would have been fine-tuned through natural selection to result in an alignment between harmful substances and bad smell, and conversely between useful substances and enjoyable odours.

Hence, noxious gases will have a pungent repulsive smell, gazelles will be highly tuned to chemical compounds emanating from their predators and lions will be very sensitive to the odour of their preys. This is no exaggeration and, once again, our own sense of smell doesn’t afford us a proper grasp of how powerful this sense can be. Humans are quite good at discerning a wide range of odours but we are not very sensitive when it comes to concentration, hence we can’t pick a scent trail that is several days old nor can we smell food located underground like bears can. The difference between humans, dogs and bears when it comes to olfaction isn’t negligible, it is several orders of magnitudes.

Finally, just as we have two eyes, we also have two nasal cavities, which suggests some degree of ability to detect the provenance of odours. This most likely relies on detecting gradients of molecule concentrations between the two sides of our nose and inferring direction from these (though probably not distance), a skill called stereo-olfaction.

c) The olfactory system

Now that we have looked at function, it is time to dive into the mechanics and chemical processes of the olfactory system. In humans, it consists of various elements within the nose including receptors, the olfactory nerves and the brain. From a process perspective, the steps are similar to those involved in vision: gathering data via receptors, transducing this data and conveying it through nerves, processing it in the brain and creating a mental representation in the form of our experience of smell.

The olfactory receptors are located on sensory neurons in the epithelium, a tissue within our nasal cavities that is in contact with the outside air through the nose or the throat – this is a pathway we use to smell food in our mouth and which partially informs our sense of taste. Crucially these protein-based receptors come in various shapes to which different types of molecules can bind with more or less affinities, otherwise proper identification would be impossible or at the very least would be too generic and therefore quite ineffective. There are therefore two dimensions to the data being captured: the type of receptor a molecule binds to and the strength of this bonding.

The binding triggers the beginning of the chemical cascade of the transduction process and starts with the release of enzymes and other molecules enabling the increase in concentration of calcium and sodium ions (positively charged) and the reduction in the amount of chloride ions (negatively charged) thereby depolarizing the neuron and resulting in an action potential that will travel across its membrane. Interestingly, there is a built-in negative feedback mechanism that over time attenuates the sensitivity of activated receptors. We call this desensitization and it explains why we eventually stop noticing some smells, which serves the purpose of enabling us to detect other smells. Pretty smart, right?

As the signals travel through the neurons, an intermediary processing occurs within the part of the brain called the olfactory bulb. There, the olfactory receptor neurons corresponding to each type of receptor converge towards dedicated structures called glomerulus which aggregate the signals, thus working out odour concentration, before transmitting data onward to the olfactory cortex situated in the lower part of the temporal lobe.

Most of the processing within the olfactory system is distributed across various areas of the olfactory cortex with the main objective of identifying the exact nature of the odours perceived. You may wonder why identification is required since there is already a large variety of receptors to provide information relating to the type of odour. The answer is simply because the information provided isn’t granular or comprehensive enough and it is through the combination of information from various types of receptors, by way of the glomeruli, that proper discrimination is carried out and that several odours can be recognized at the same time. This should be reminiscent of the existence of three types of cone receptors within the retina which encode information relating to the mix of wavelengths present in light and allows us to match to known concepts in our brain and derive colours.

Unlike in vision however, there seems to be less need for interpretation requiring a complex neural processing and the role played by context-related information in computing probabilities is probably very secondary. To be sure, I am not talking here about conscious reasoning based on external clues that may help identify the true nature of an odour. The point is that most specific odour-related signal patterns probably have one-on-one behavioural or developmental associations hardwired in our central nervous system.

d) Taste as gatekeeper

The sense of taste is made possible by the gustatory system and is often associated with the sense of smell since together they inform our perception of flavours. This suggests some degree of overlap, or perhaps complementary is a better word, between both senses. And indeed, taste provides an additional layer of data to olfaction mostly when it comes to the decision of whether to ingest food inside the organism, or not.

In theory, the concept of odours binding to receptors in our nose could do the trick if it were not for the fact that food tends to come in the form of complex macromolecules so identification is much trickier compared to gaseous molecules. Nevertheless, it is not that taste receptors can identify macromolecules outright, rather our gustatory system has evolved a way to break down the food into simpler constituting units so it can perform this task. This is the job of the enzymes in our saliva.

So, what purpose does a better identification of the macromolecules present within food serve? The intuitive answer is to say: eat what you like and spit out what we don’t. Except we know we would have it in reverse in a way and that the phenomenology, the qualia, associated with the tasting of food would have evolved so that it provides us with an evolutionary advantage, namely to dislike what we should avoid and to like what we need to eat to maintain proper metabolism, which covers the energetic aspect as well as the maintenance of a proper balance of nutrients and minerals within our body. Indeed, I think everybody will have noticed how our body somehow seems to yearn for certain types of foods and drinks, not because those are our favourites but because we need to top up our level of iron, energy or electrolytes. This can ever alter our sense of taste, for example some sports drinks with a pronounced salty taste will not be very pleasing in the mouth when rested but during or shortly after an intense effort they will feel right, just what we needed.

To achieve this feat, our organism has evolved different kinds of chemoreceptors to distinguish four or five types of taste, according to our current classification: sweet, sour, salt, bitter, and savoury (also called umami), with classification of the latter being debated.

The chemoreceptors triggering the sensation of sweetness are those binding to sugars and some alcohol compounds such as glycerol. The commonality of this type of molecules is they form the main source of energy brought into our organism through the digestive process, thus it is not surprising we would enjoy the taste of energy-rich food. These molecules bind to chemical receptors belonging to the group of G protein-coupled receptors (or “GPCRs”) – the same group as olfactory receptors – and sometimes other non-sugar molecules will also bind and activate those receptors to create a sweet taste in our mouth. This is the mechanism on which sweeteners rely and in the case of stevia for example, this plant product is perceived by humans as being dozens of times sweeter than sucrose. There is actually a double trick with stevia and a few other sweeteners: not only are they perceived as sweet but they cannot be metabolized in our upper gastrointestinal tract and therefore do not contribute blood glucose, which makes them eligible to be rated as having 0 calories from a nutritional perspective.
Sourness, which reveals acidity, is informed by a mix of chemoreceptors and ions with the penetration of protons (H⁺) leading to membrane depolarization. This helps us avoid highly acidic food and beverages that could damage our tissues as well as unripe fruits.
Saltiness also relies on ions channels and is produced by the presence of a few types of cations such as sodium (Na⁺) and potassium (K⁺). We like a bit of salt in our food because we need electrolytes to ensure the proper functioning of all the ion channels in our body, including in our central nervous system and muscle tissues.
Bitterness, like sweetness, relies on the activation of chemoreceptors (of the GPCR kind) but its evolutionary purpose is less obvious and many food and drink items we consume in our modern world, such as tea and coffee, do have a pronounced bitter taste. Of course we have not evolved consuming those specifically, however the point I am making is that there is a wide range of healthy food tasting bitter and this sensitivity to bitterness tends to soften as we age. The reason the range of F&B with a bitter taste is wide is because the range of organic compounds activating the receptors is also wide and includes classes such as alkaloids (containing nitrogen) and amino acids. Clearly, we would need to go back to our specific environment when the taste evolved to pinpoint exactly what substances our gustatory sense tried to keep us away from. Perhaps it is simply a matter of default stance, a precautionary measure. Some plants taste bitter and are healthy and others are poisonous, why take the chance if you can’t readily make the difference?
As for savouriness it also relies on GPCRs and is activated by compounds involved in the synthesis of proteins such as nucleotides and glutamic acid. Accordingly, umami is triggered by meaty foods and carnivorous species have a very enhanced taste for these. In fact, they sometimes lack one or several other types of taste receptors.

e) The gustatory system

As mentioned in the previous section, the sense of taste starts with the dissolution of food and liquids thanks to enzymes located in the saliva. Most of the chemoreceptors are located on the surface of our tongue, forming clusters called taste buds. These are aggregated into rough structures called papillae that maximize the surface area coming into contact with food. Humans have a few thousands taste buds on the tongue and each of those has a up to 100 receptors. And no, there is no dedicated regions of the tongue for each of the four or five tastes although the back of the tongue has a relatively higher concentration of receptors for bitterness.

Whether it is through ion channels or chemoreceptors acting as switches, the taste information is transduced through a series of chemical cascades similar to those already explained previously, including in section c) above on the olfactory system. Once transduced, the data travels through three cranial nerves, depending on where the signal originated, with a fourth cranial nerve carrying additional information such as food texture and pungency – in that respect you may find it interesting to know that spiciness is not a taste, it is a pain signal. A lot of the information is aggregated in the solitary nucleus in the brainstem, not without inputs from various other areas of the brain including the hypothalamus, the amygdala and the prefrontal cortex. As with olfaction, the full range and intensities of tastes only arise when all those signals have been integrated and interpreted.

Whether it evolved on purpose or it is a leftover of a more primordial version of those ancient senses, both olfaction and taste are often associated with memories. It may have been a way not to better remember experiences but rather to remember the tastes and smells associated with positive and negative experiences. There is even a case to say that memories were formed to record exactly this type of information. Arguably the best-known literary illustration of the taste-memory pairing is the sense of nostalgia arising within the narrator when eating a madeleine in Proust’s In Search of Lost Time (the episode occurs in the first volume called Swann’s Way).

f) Trivia – Wine tasting and acquired taste

I have elected to briefly discuss wine tasting because it serves to illustrate two important aspects of the senses of taste and smell: their complementarity and the fact that we identify chemical compounds, not specific food and drink items.

Of course, some of the vocabulary used by both amateurs and professionals makes one want to roll her eyes, as when a wine is called “intellectual” but the part on tasting different fruits, herbs and minerals is grounded in the chemistry of the gustatory and olfactory receptors. These receptors are molecule-specific and the molecules present in fruits, herbs and minerals can be present in wines.

In order to best enjoy wine, it needs to be exposed to air to release a type of organic compound named aromatics we can detect via our sense of smell. The mix of aromatic sensations creates what is called the wine’s bouquet and it can be further enhanced by the swirling of the liquid in the mouth as well as by the temperature at which the wine is served because the release of aromatics increases with temperature.

Temperature also impacts other variables such as perceived acidity and tannins increasing as temperature decreases. Thus serving white wines a little chilled will showcase the acidity and provide some backbone to the flavours whereas serving red wine closer to room temperature (warmer than white wine) will de-emphasize the presence of tannins that we perceive as bitter and astringent on the tongue.

Wine is not the only drink or food humans learn to appreciate and whereas sometimes the enjoyment is there on day one it is regularly the case our first exposure isn’t a truly pleasurable one. In such instances, the over-riding and ultimately the absence of the initial reflexive sensation bears the name of acquired taste. Good examples of this include the appreciation of cheeses with strong flavours, some types of bitter alcoholic beverages and the ability to discern high quality (and expensive) caviar, wine or honey.

As with pain and pleasure, whether a specific stimuli is enjoyable or not is ultimately a matter of interpretation by our central nervous system and of cumulative experience; therefore it can evolve over time.