Olfactory – The First Steps of Digestion
What is the Olfactory Mucosa?
Olfactory – not to be mistaken with ‘old factories’ although, if that helps you to remember its name, then hey, whatever works right!? Why are we looking deeper into the olfactory system we here you ask? Well. Because it’s pretty darn important! The first step to us perceiving the world comes in the first breath we take right, and what is processed in this breath is smells, taste and this then signals the cascade as to how we signal response to this. Finding familiarities and unfamiliarity is a pivotal role in the olfactory system.
Airborne odorant molecules are detected in what is called the Olfactory Mucosa of which is comprised of the Olfactory Epithelium. So, your first line of sensory defense and the protective lining that creates a great wall between you and your surrounding environment. Within this wall, the epithelium, we have the ‘Olfactory Sensory Neurones’ and these little gems are responsible for initiating the odorant signaling to the brain.
How does this impact Digestion?
This is cool and all, that we have the nose of a greyhound, but what does this have to do with my digestion? I can smell how good my food smells or my sheer distaste for Brussel sprouts? Genius ATP! No, hear us out there is more to it… Our Olfactory system does directly correlate with our metabolic capacity. The olfactory system is intimately linked in with our endocrine system that regulates as a result; our energy balance (metabolism). Yep, the first signaling for digestion and the associated energy balance hormones such as insulin, neuropeptide-y, ghrelin, etc. all happens at the moment in which you take a whiff of that meal and your tongue has started salivating.
Energy balance is regulated through co-ordinated signaling of the peripheral organs (these are parts of the body that respond to changes in the environment) and the brain. Our drive to something being delicious comes from sight, but also the smell of the food we are eyeing off. Interestingly, this is why fasting is believed to increase the sensitivity to the smell of food and feeling full lowers the sensitivity to smell .
Where does the Vagus Nerve come into this?
In our earlier blog on the vagus nerve specifically, we talked about how critical it is in the body’s regulation of all systems. But to recap, the Vagus Nerve is the longest cranial nerve in the body and is intimately linked in the regulation of our gut, cardiovascular system, respiratory and endocrine systems. In traditional medicine, it is called ‘the wandering nerve’ as it wanders in a vine-like fashion over our organs, muscle, intestinal tract, spine, and cranium. It's absolutely mesmerizing, and in the olden days, they used to sever the nerve in what was called a vagotomy to relieve gastric ulcers… however, found afterward that this communication highway, once cut, leads to a whole host of issues .
So, what does this nerve have to do with our sense of smell and taste? The hypothalamus and the vagus nerve communicate in a highway exchange fashion between the gut where the vagus nerve entangles its way through, and the brain to provide feedback. From here, the olfactory bulb at the epicenter of the olfactory nerve modulates receptor activity based on the information that is communicated by the vagus nerve to the hypothalamus. When the distension of the gastric wall changes during the intake of food, the sensitivity of olfactory neurons changes to induce an almost repulse to food odors in order to signify that we are satisfied and do not require more. Fascinating!! 
Is this an Ancient Survival System?
One of the greatest human systems for learning behaviors and imprinting memories and patterns is through odor. Odors have the ability to bring forward memory and experiences from when you smelt that particular thing initially and what came after. Be it a pleasurable or not so pleasurable experience . This allowed for a pinnacle survival adaption to experience to occur instantaneously happening both in the amygdala and hippocampus for memory and emotional recall. This phenomenon has been termed as the “Proust Phenomenon” which; put simply, is a form of autobiographical memory. This was termed when French writer Marcel Proust stated that eating a Madeleine transcended him far back into a memory of his childhood instantly .
A study to test the groundings of evidence to the ‘Proust Phenomenon” was conducted on 70 female volunteers who were subjected to videos of traumatic events, car crashes, and war crimes. During the visual intaking of these videos, a single scent was sprayed into the room which provided a unique odor as well as colored lights on a wall, and soft music in the background to test sensory width later. A week after the videos where shown, the team conducting the research exposed the volunteers to the same smell, colors, and music during the reciting of what they remembered from the videos – the volunteers said with the exposure of the light and the scent that the memory recall was extremely vivid [4,5].
Evidence to the ‘Proust Phenomenon’?
More research on the ‘Proust Phenomenon’ needs to be conducted to isolate smell as having a distinctive difference in memory imprinting and recall when exposed likewise in the future vs no stimuli. Another study conducted in 1993-1994 by Hinton and Henley sort to prove this autobiographical memory theory with success – comparing reactions to stimuli including visual, lexical, and olfactory modalities. The conclusion found that odors did in fact elicit far more affective reactions .
Continued studies in 1996,1997,1999 by Herz and Cahill attributed this foundation of autobiographical memory to the part of the brain named the amygdala which is involved intimately in the olfactory system, memory, and emotion – one of which is fear. Leading to the connection between olfactory sensory input imprinting memory more vividly especially with regards to emotional memory and survival requirements in the future [8,9].
What happens if I lose my sense of smell?
There are a few smell disorders and they are :
- Hypersomnia – a reduced ability to detect odors.
- Anosmia – complete inability to detect odors. This can also be a born congenital disorder too.
- Parasomnia – a distortion of familiar smells, often seen in pregnant women where pleasant smells can become foul or repulsive smelling.
- Phantosomia – the sensation of having and perceiving and an odor that is not actually there.
There are many things that can create smell/olfactory disorders  –
- The build-up of amyloid plaque – an early indication of Alzheimer's and Parkinson's.
- Growths in the brain affecting receptor sensitivity and perception.
- Nasal Polyps.
- Head injury.
- Exposure to solvents and strong chemicals.
- Antihistamines – long term exposure.
- Radiation involving the neck and head.
How do you improve the sensitivity of your olfactory system?
Olfactory neurons do regenerate and can grow new nerve fibers/axons to which can then adhere to the olfactory bulb where the communication axis lies, restoring the connection between the two.
There is a method called ‘smell training’ which originated in a study (2009) that found significant results in the communication of the olfactory neurons and the olfactory bulb with a flow-on effect to the brain and so on by sniffing aromas on four different categories, at 10 seconds for each, twice a day for 12 weeks. These 4 categories are deemed as the ‘odor prism’, similar in principle to how we have our primary colors, these are the foundational core elements of smell and you can retrain the brain to recognize these again and rebuild the strength to perceive aromas.
These four categories were flowery, fruity, aromatic, and resinous. Examples used in these categories where:
There were notable improvements in the study group vs the group who did not participate when asked to perceive smells thereafter .
Take home message – if you are concerned that you are losing your smell or that your old olfactory network perhaps isn’t up to scratch – get the all-clear from your doctor that there is nothing deeper at play before jumping into the smell training, if all clear, would love to hear how you go with the study method provided above!
Vagus nerve stimulation modifies the electrical activity of the olfactory bulb. David Erasmo Garcia-diaz, Hector Ulises A. R. G, division of life sciences university of Texas. Brain Research Bulletin. Vol 12 PP 529-537 1984.
Memory and Plasticity in the Olfactory System: From Infancy to Adulthood
Odour-evoked Autobiographical Memories: Psychological Investigations of Proustian Phenomena. Simon Chu, John J. Downes. Chemical Senses, Volume 25, Issue 1, February 2000, Pages 111–116, https://doi.org/10.1093/chemse/25.1.111
Hinton, P.B., Henley, T.B. Cognitive and affective components of stimuli presented in three modes. Psychon. Soc.31, 595–598 (1993). https://doi.org/10.3758/BF03337365
The Role of the Amygdala and Olfaction in Unconditioned Fear in Developing Rats. Sean W. C. Chen, Alexei Shemyakin and Christoph P. Wiedenmayer Journal of Neuroscience 4 January 2006, 26 (1) 233-240; DOI: https://doi.org/10.1523/JNEUROSCI.2890-05.2006
Hemispheric Lateralization in the Processing of Odor Pleasantness versus Odor Names. Rachel S. Herz,Catherine McCall, Larry Cahill Chemical Senses, Volume 24, Issue 6, December 1999, Pages 693 695, https://doi.org/10.1093/chemse/24.6.691
Cahill, L., Babinsky, R., Markowitsch, H. J., & McGaugh, J. L. (1995). The amygdala and emotional memory. Nature, 377(6547), 295–296. https://doi.org/10.1038/377295a0
Effects of olfactory training in patients with olfactory loss. 2009;119(3):496.