Egads, What's that Smell?!?!

We are branching out from our longer stories and starting a series of single posts.  Enjoy!

For a brief time, we lived in Hershey, PA.  Now, my experience with company towns is limited, but living down the street from an iconic chocolate factory seems preferable to say, living by a paper mill.  Hershey is a cool place, worth a visit, and not just for chocolate.  But mainly for that. When the factory is in production, the aroma of chocolate is almost inescapable.

Since we were in a new town, I figured I would get a new start to my work-out routine.  In my head I was going to jog the trails that crisscrossed the picturesque area.  That didn’t quite happen, because as it turned out, I am easily dissuaded.  When the factory was running, I could not.  I found that as soon as I stepped outside, the aroma of chocolate brought on an overwhelming craving for chocolate chip cookies.  Please don’t get me wrong, I could exist solely off chocolate chip (and related cookies, not oatmeal raisin though, no thank-you) but running with that sensation, was too much.

I had no more success running when the factory wasn’t making chocolate.  Hershey is in dairy country, which was one of the reasons it was a good spot to build a factory over 100 years ago in the first place. On warm summer days when the factory wasn’t in production, the area smelled, well, like a dairy farm.  More specifically, it smelled of acres upon acres of sunbaked cow patties. And while steaming cow patties and melted chocolate can look similar, the nose has some nuanced differences. Sucking in lungfuls of Eau de Cow Plop while jogging was nauseating, and I pretty much just hung up my running shoes for the year.

Fig 1. Lancelet. While not a vertebrate, they provide clues about early vertebrate evolution, and are thought to be one of the earliest vertebrate ancestors with a primitive olfactory system. By © Hans Hillewaert, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=5712836

 Pleasant or not so much, odors have the ability to instantly trigger reactions, before we are even consciously aware of the smell.  Our olfactory system is designed that way.  Unlike our other senses, olfaction (sense of smell) is hardwired directly into the limbic system, the part of the brain that is the gateway for memories, emotions, and motivation. It is that direct link to the limbic system that forms these strong reactions and conditioned responses to smells. And the response is usually formed with our earliest exposures, so cinnamon might bring memories of baking pumpkin pies grandma’s kitchen or chlorine bleach might bring up memories of summers at the pool.  If, instead of swimming at the local pool, someone’s earliest exposure to the smell of chlorine was scrubbing the bathroom, that reaction might be not as positive.  Some smells almost universally elicit revulsion.  Natural gas, which is otherwise odorless, has a noxious smelling additive- Mercaptan, which smells of sulfur (think rotten eggs or meat) to help people notice leaks.  My parents had a small natural gas leak in their house years ago, and the smell drove them crazy (looking back, I guess it could have been the gas leak) but it took a while to pin down the source of the smell. They called multiple exterminators to find what smelled like a dead animal in the walls before they figured it out.

Fig 2. Olfactory System. Several million olfactory neurons extend from the top of the nasal cavity up through a perforated area of the skull called the cribiform plate. They meet up with the olfactory bulb (which is commonly taught to be cranial nerve I, CN I. the Olfactory Nerve, though technically the millions of neurons transversing the cribiform plate are CN I). Because the fibers pass through the bony cribiform plate, they are subject to injury The remainder of the pathway is not pictured but from there, connections go to the opposite olfactory bulb, amygdala, entorhinal cortex, and on to the thalamus, orbitofrontal hippocampus, and brainstem. By Patrick J. Lynch, medical illustrator - Patrick J. Lynch, medical illustratorhttps://commons.wikimedia.org/wiki/File:Head_olfactory_nerve.jpg, CC BY 2.5, https://commons.wikimedia.org/w/index.php?curid=68370471

 Olfaction is primitive sense, wired directly into one of the more primitive parts of the brain.  It was in fact, the first of the special senses to evolve in human ancestors which allowed our underwater ancestors to sniff out sustenance, mates, and hazards in the primordial seas (fig 1). Our sense of smell and our limbic systems have evolved over time, we (theoretically) have more conscious control over our reactions, or I would have left Hershey weighing twice what I did when we got there.  Compared with senses that evolved later, especially vison, the neural pathways are relatively simple (fig 2). 

Flash forward 700 million years, our other senses have done a lot of catching up, and it that time we have become predominately visually based.  Though those olfactory connections to our limbic system which drove the behavior of our ancestors are still in place though driving our cravings and revulsions.  Has anyone tried to capitalize on those connections (besides the perfume and bath scent industries)?  Yeppers.  Scent based advertising is a very much a thing, but as NPR reported back in 2006, sometimes it doesn’t go as planned. The ‘Got Milk’ campaign in San Francisco decided to go multisensory with 5 of its bus stop posters and installed chocolate chip cookie scented strips behind the posters.  The thought was the smell of cookies would make people crave a glass of milk.  Within hours, complaints from anti-obesity and anti-allergy groups drove the transit authority to order the strips be taken down. 

Since we are not dependent on olfaction, loss of sense of smell can go unnoticed for years.  Just as a part of the natural aging process, we tend to start losing our sense of smell around age 30. Initially we lose our sense of smell for noxious smells, so maybe no big loss. Besides aging, there are a multitude of other reasons that someone could completely or partially lose their sense of smell (called anosmia and hyposmia respectively.)  It can happen early in some neurodegenerative diseases like Parkinson’s, sometimes years before any other symptom.  Because of the anatomy, CN I is (are) susceptible to trauma from head injury or traumatic brain injury, usually shear injury as the fibers pass through the cribiform plate (fig. 2).  Colds, sinus infections, smoking can all cause short term loss of smell.   While loss of smell might go consciously unnoticed, research has shown there are some associated health concerns like weight loss and depression.  As a neurologist, we don’t spend much time testing it, sometimes we have you smell some coffee beans, maybe a little something else.  But clinically, smell is almost vestigial, and often overlooked.

.  Dolphins lost their sense of smell when they evolved from land based back to water based mammals.  I read Douglas Adams, I know where we rank on the evolutionary spectrum.   In another 700 million years olfaction might be lost to human descendants completely, which would be a terrible shame.  Chocolate chip cookies smell really, really good, never mind what the San Francisco transit authority thinks. 

Kandel, E. R., Schwartz, J. H. 1., & Jessell, T. M. (2000). Principles of neural science (4th ed.). New York: McGraw-Hill, Health Professions Division.

Edited by Linda Wilson-Pauwels, Elizabeth J. Akesson, Patricia A. Stewart, and Sian D. Spacey. illustrated, (2002). Cranial nerves in health and disease, 2nd edition London: BC Decker.

Marketing Campaign Targets Noses at Bus Stops.  https://www.npr.org/templates/story/story.php?storyId=6577600

Niimura Y. Olfactory receptor multigene family in vertebrates: from the viewpoint of evolutionary genomics. Curr Genomics. 2012;13(2):103–114. doi:10.2174/138920212799860706

Hoover, K. C. (2010), Smell with inspiration: The evolutionary significance of olfaction. Am. J. Phys. Anthropol., 143: 63-74. doi:10.1002/ajpa.21441

How Smell Works.  https://health.howstuffworks.com/mental-health/human-nature/perception/smell3.htm

Thumbnail image-https://www.flickr.com/photos/internetarchivebookimages/14571900957/Source book page: https://archive.org/stream/practicaltreatis1904muss/practicaltreatis1904muss#page/n928/mode/1up, No restrictions, https://commons.wikimedia.org/w/index.php?curid=43329570