The body’s olfactory system converts chemical signals in the form of odours into understandable and communicable perception. Yet compared to other senses, the study of smell is relatively new. That little piece of cartilage attached to your face has a lot going on under the hood and not all of it is fully understood. This article hopes to put you in the shoes of an odour molecule and give you a guided tour of your nasal workings – from acquisition, to understanding.
Breathe In
The first part of the process is collecting enough odour molecules to trigger a sensory response. The human body’s tool for this task are the nostrils, assisted by lungs acting as a pump so that air can flow through the nasal canal.
Most particulate contaminants are kept out of the respiratory system by nose hairs and mucus, leaving the odour molecules free to travel along the passage. However, at times, tissue in the nostril swells with blood, hindering the passage of air. You can test this easily by breathing normally and blocking one nostril at a time. You will likely find it easier to breathe through one than the other at any given moment. After time, the given obstructed nostril switches in something called the nasal cycle. The reason for the cycle is debated (Pendolino, Lund, Nardello, & Ottaviano, 2018), but this process allows a variation in the acquisition of odour molecules from each nostril as well as ensuring that neither one dries out.
Some odours are easily detected in fast moving air, while others are easily detected in slow moving air. Studies speculate that this combination of slow and fast acquisition help increase the olfactory range of the nose (Kahana-Zweig, et al., 2016).
But it’s not just the nose that can deliver odorants to the brain. Air can also travel along the nasal passage by entering through the throat. This is why olfaction (the act or process of smelling) is also heavily associated with the sense of taste. Flavours of the food we consume can stimulate the olfactory system just as much as the gustatory (tasting) system. Researchers tend to agree that an important part of the culinary experience comes from the olfactory system at work (Spence, 2015).
So, what does this system do with odour molecules when we breathe, eat or drink?
Passing on the Information
Regardless of how quickly they travel through the nose, odour molecules must reach something that can register them and send a message to the brain. Olfaction is again associated with the sense of taste here since both processes provide information to the brain about a substance’s chemical composition via the process of transduction.
The method begins with the odorant molecules being bound to the surface of a tissue called the olfactory cilia – a series of fine hair-like sensors that coat the inside of the upper nasal passage as part of the olfactory epithelium. The molecules bind to receptors on these cilia after they have dissolved in the mucus of the upper nose. This is a threshold response, where enough odour must reach the inside of the nose before detection can occur (Borgas, 2013).
Now, a lot of clever electrochemistry occurs with ion channels opening to generate receptor potential, (Purves et al. (2001) gives a detailed description for those interested). The process depolarizes the receptor and sends a current along it to activate the corresponding neuron. Each of the approximately 10,000 receptors look for a specific kind of odorant molecule and, since they all work together, there are an estimated one trillion different odours that humans are thought to be able to detect (Bushdid, Magnasco, Vosshall, & Keller, 2016). The combination of receptors that fire, helps code the chemical information of the odorant for the brain to interpret.
Studies show that these receptors are not only looking for odours. Like other animals, the human olfactory system has evidence of detecting chemosignals that trigger social or emotional responses in the same species. A commonly cited study (Groot, Smeets, Kaldewaij, Dujindam, & Semin, 2012) performed a test that took sweat samples from males who were instructed to watch a scary or disgusting film. Females were then exposed to these samples while performing a visual sensory test. Facial recognition and sniffing rates were measured throughout the process. The study found that when exposed to “fearful sweat”, the participants showed a fearful facial expression as well as an increase in their sensory intake – higher scanning of surroundings and increased sniff rate. Conversely, when exposed to “disgusted sweat”, the sensory intake decreased, and the facial expression again reflected disgust. This is only one example of chemosignals being used in humans, but it could explain phenomena such as emotion propagation in crowds as well as menstrual synchronisation. All these reactions are potential responses to the olfactory system detecting things that we don’t even know we are smelling.
There could also be an entire secondary olfactory system that exists inside of our nasal cavity. The vomeronasal duct and organ have been the topic of scientific debate for some time and they exist in many other animals – snakes and elephants to name two. This second olfactory system is used for non-volatile chemical sensing and even includes its own bulb and sensory cells independent to the rest of the olfactory system.
However, unlike other animals, the vomeronasal organ in humans seems to do nothing. Other animals must directly expose the tissue to stimulate it, and humans would have difficulty in doing so. Evidence suggests that in adult humans, this second nose is just a leftover from evolutionary processes.
Biological Circuitry
Once the receptors have fired their signals, the information travels along the olfactory nerve to a lower part of the forebrain known as the olfactory bulb. This is a collection of neurons that process and pass on the sensory information to the rest of the brain. Since it only has one input – the receptors – and one output, it is thought that this bulb acts as a filtering circuit rather than an associative one. Instead of making connections between lots of input and associating which output it should correspond to, the olfactory bulb takes in all detected stimuli and filters out the ones that are unimportant to the smeller.
The signals go all manner of places after they have been detected at the olfactory bulb. These include:
- The primary olfactory cortex – a collection of neural structures involved in processing the olfaction signal
- The thalamus – crucial for perceiving sensory information and relaying it to other areas of the brain
- The amygdala – where associations between odours and behavioural responses occur
- The orbitofrontal cortex – which assesses the odour’s relation to any rewards and their value (the smell of baked apples, to food, to nutrition and delicious flavour for example)
- The hippocampus – responsible for interpreting it in roles of emotion, memory and learning.
These brain structures are intimately interconnected, allowing understanding and reaction to occur from the stimulating odour.
Perception
When we perceive any sensory signal, it is an involved and sometimes even subconscious event. Olfaction is one of the less relied upon senses in the human repertoire, but that does not mean that it should be neglected entirely.
We primarily perceive smell through odour intensity, detection threshold and discrimination of character. And though it is true that many other animals have impressive senses of smell, surprisingly, human olfactory capabilities are often close to, or on par with, that of other mammals (Laska & Freyer, 1997). We can use the odorant ethyl mercaptan as an example. Often added to propane as a warning agent, it can be detected as low as 0.2 ppb (Whisman et al., 1978). In terms of scent discrimination, studies have also demonstrated that human’s rate of scent tracking is limited by their movement speed rather than their ability to lock on to a certain odour. And with practice, this ability can be honed – where specialists can train to familiarise themselves with particular scents and provide expert analysis on the odours in the environment.
The most useful social cue for olfaction is the ability to understand and describe the odours encountered. This is a relationship unique to humans, where we can encode smell into language. Old cabbage, chocolate, fruit, garbage, sweat, coffee – the fact that you can likely imagine and understand these odours is inherently impressive, even without these items physically present. It is one of the reasons that we can push forward into the scientific study of olfactory processes.
But what if you don’t speak the language? A study looked into this problem and explored different cultures’ ability to describe smells on a fundamental level (Majid & Burenhult, 2014). In English, the phrases used to describe perceived odours are based on experiences – think back to the key link between olfactory signals and the parts of the brain involving memory. Something smells like coffee, or like sweat. Or, words linked to taste are often used – sweet, bitter, smokey. But other cultures, such as the Jahai people of the Malay Peninsula, have words that can describe fundamental and independent components of odour character. This helped them name smells as easily as colours even when they had little prior experience with the smells tested. English speakers given the same task however, struggled with odour naming whereas naming colours came naturally. This communication is a wonderful tool and demonstrates the human ability to diversely relate to perceived stimulus.
Yet, this language diversity also makes studying and classifying odours difficult. Multiple people may provide different descriptions of the same odour’s character based solely on their differing personal experience. If one person has little experience with an odour, they may have difficulty describing it in English at all.
Likewise, the receptors in the nose are sensitive and diverse, and complex odours cannot be easily described in terms of their constituent parts. Studies of olfactory perception therefore try to work around or enhance these oral descriptions with an unspoken language of subconscious reactions. For example, sniff rate decreases if the brain has been instructed that the incoming odour is unpleasant, and the breaths become shorter (Weiss, Secundo, & Sobel, 2014). Reactions like this can help the field of olfactory classification and further our understanding of how we perceive the smells around us.
Breathe Out
So, the molecule has entered our system, been transduced into a nerve signal, and fired through our brains to allow us to understand, remember and communicate just what it was that we smelled.
The human olfactory system can detect tiny concentrations of complex odours, and then find a way to interpret this information to act accordingly, even if it is simply holding your breath as you take off your shoes.
But most impressively, remember that this has all happened at the speed of thought – in the space of one breath. Breathe in again and the process starts from the beginning. Perhaps new smells and new understanding awaits.
So, the next time you stop to smell the roses, pause for a second longer and just appreciate the intricate system that allows you to do so.
References and Further Reading
Borgas, M. (2013). Atmospheric Odour Response of Human Noses. CASANZ 2013 Conference – Sydney.
Bushdid, C., Magnasco, M., Vosshall, L. B., & Keller, A. (2016). Humans Can Discriminate More than 1 Trillion Olfactory Stimuli. Science, 1370-1372.
Groot, J. H., Smeets, M. A., Kaldewaij, A., Dujindam, M. J., & Semin, G. R. (2012). Chemosignals Communicate Human Emotions. Psychological Science, 1417-1424.
Kahana-Zweig, R., Geva-Sagiv, M., Weissbrod, A., Secundo, L., Soroker, N., & Sobel, N. (2016). Measuring and Characterizing the Human Nasal Cycle. PLoS One, 11(10), e0162918. doi:10.1371/journal.pone.0162918.
Laska, M., & Freyer, D. (1997). Olfactory discrimination ability for aliphatic esters in squirrel monkeys and humans. Chemical Senses, 457-465.
Majid, A., & Burenhult, N. (2014). Odors are expressible in language, as long as you speak the right language. Cognition, 266-270.
Pendolino, A., Lund, V., Nardello, E., & Ottaviano, G. (2018). The nasal cycle: a comprehensive review. Rhinology Online, 67-68.
Purves, D., Augustine, G., Fitzpatrick, D., Katz, L., LaMantia, A., McNamara, J., & SM, W. (2001). The Transduction of Olfactory Signals. In D. Purves, G. Augustine, D. Fitzpatrick, L. Katz, A. LaMantia, J. McNamara, & W. SM, Neuroscience. Sunderland: Sinauer Associates. Retrieved from https://www.ncbi.nlm.nih.gov/books/NBK11039/.
Spence, C. (2015). Just how much of what we taste derives from the sense of smell? Flavour, 4(30). doi:https://doi.org/10.1186/s13411-015-0040-2.
Weiss, T., Secundo, L., & Sobel, N. (2014). Human Olfaction: A Typical Yet Special Mammalian Olfactory System. In K. Mori, The Olfactory System: From Odor Molecules to Motivational Behaviors (pp. 177-203). Tokyo: Springer.
