5.3 Smell

Smell Basics

Imagine this: You walk out of your psychology class, round the corner to the breezeway and begin walking toward the Commons. You’re suddenly greeted by the rancid smell of rotting food and turn left to discover that someone forgot to bring his lunch home (and, judging by the scent, that person has been missing his lunch for quite a while). How does this smelling process work? What “brain bits” are involved in recognizing a scent? How are you able to differentiate between the smell of this rotting food and the smell of roses?

In the nasal cavity, about seven centimeters past the nostrils and behind the eyes, lies the olfactory epithelium, a mucus-coated “sheet of neurons and supporting cells that lines approximately half of the nasal cavities” (Purves). Most notable of these cells are your (approximately) 40 million olfactory receptor neurons. On one end of these neurons lie cilia, tiny hairlike sensors, which are embedded in a thick layer of mucus. On the other end lies an axon, which transmits olfactory information to your brain (Chudler). These neurons are extremely unique: they’re the only nerve cells “in your body that are directly exposed to the environment”--and therefore often incur damage due to old age, “nerve injury, or...expos[ure] to environmental agents that enter the nasal cavity” (Costanzo). So, it would only make sense that these newly-unhelpful neurons are “regularly replaced about once a month” (Hanson). Basal cells (also known as stem cells) allow for this incredible regeneration of damaged olfactory receptor neurons, the “re-establish[ment] [of] connections with the [nervous system] and” the complete “restor[ation] [of] sensory function” (Costanzo). This remarkable process provides some insight into neurogenesis and could prove to be vital to the advancement of the field of neural regeneration.

Have you ever wondered why you’re unable to smell or taste well when you have the common cold? The inflammation of the membranes lining your nasal cavity not only inhibits the traveling of volatile molecules (which are really what you’re “smelling” when you’re greeted by the scent of waffles when walking into Mr. Fink’s after PATH) to your olfactory epithelium, but it also can cause damage to your receptor neurons. So if you don’t find the scientific field of neurogenesis fascinating, you can still revere the awesomeness of your stem cells for allowing you to smell once your immune system has combatted your sickness. This is why you regain your sense of smell--and are capable of recovering from the damage that the inflammation inflicts--after you’ve been sick: replacing the once-damaged receptor cells with new, well-functioning ones your basal cells have your sense of smell back to being as good as new in a remarkably small amount of time--roughly two to four weeks (Causes of Anosmia). diagram.jpg

Now for how you actually smell (and how you’re able to tell the difference between the scent of a rose and the scent of a Yorkie’s rotting lunch):

Receptor proteins are located on the aforementioned receptor neurons’ cilia. When the odor molecules from the aforementioned, decomposing food enter the olfactory epithelium, they bind to these receptor sites, which is why “the intensity [of smell] perceived...depend[s] on the number of molecules that reach” these olfactory receptor neurons (Espinosa). This explains why the smell of the rotting lunch was very faint when you first exited the classroom, and gradually strengthened as you walked closer to the Lost and Found--as your proximity to the lunch increased, the number of molecules entering your nose increased. Two prominent theories regarding the nature of smell detection have emerged over the past fifty or so years: the Stereochemical Theory and the Vibrational Theory (see this textbook’s Odor section). 

Once your olfactory neurons register the types of odor molecules which entered their receptor sites--in this case those released by the rotting food--they have to relay the information to your brain. These “messages” travel from the neurons’ unmyelinated axons to your two olfactory bulbs (which, as Herz describes, are extensions of your brain, each located in one nostril and possessing a size and shape comparable to those of a blueberry). An extremely delicate and slim bone, the cribriform plate, separates the “neurons in our nose from the olfactory bulbs in our brain” and is replete with thousands of miniscule holes “through which...the receptor neurons[‘]...axons pass through to get into the brain” (Herz). Therefore,

head trauma oftentimes (and easily) damages a person’s ability to smell by severing the olfactory nerves. Yes, you could lose your sense of smell from being hit in the head at a soccer game if that plate is knocked out of alignment (Gilbert). The olfactory information then proceeds to the rest of the limbic system, most notably stopping at the amygdala and hippocampus. Because of this, smell can have a profound effect on emotion and memory (as described in Smell and Memory and Smell and Emotion). Some olfactory information also goes to the thalamus, and from there the frontal cortex, otherwise we would not be able to identify smells at all (Jacob).

Watch this Must Watch video on smell (see right). It gives a great overview of the basics of your olfactory sense and will really help solidify the aforementioned information. If you’d like a more in-depth overview, feel free to watch the Crash Course video on smell.

Smell and Taste:

Though taste and smell are often thought of as two independent senses, they are extremely interconnected, most notably in the process of flavor perception. Our general idea of the meaning of taste, or “gustation” tends to “[refer] only to the senses that are associated with receptors on the tongue: sweet, sour, bitter, salty, and — some people argue — umami” (Fields). We tend to solely associate our sense of smell with orthonasal olfaction — the process of sniffing odor through our noses. In reality, however, two types of olfaction (or smell) exist: orthonasal and retronasal olfaction (refer to the photo on the left). Olfactory.jpg

The process of orthonasal stimulation is constantly occurring; every time you breathe in, you inhale “volatile molecules” which “activate the sensory cells in the olfactory epithelium” (Binder) (see Smell Basics for an in-depth description of this process and the anatomy involved). Retronasal olfaction, however, occurs when you’re eating food. As you chew, “volatile molecules [are] released from the food [and] are pumped...from the back of the oral cavity up through the nasopharynx to the olfactory epithelium” (Binder) (the same site that the molecules you inhale end up). This sensation is then activated when you exhale through the nose “between mastications or swallowings” (Binder). This explains why, for example, when you pinch your nose and begin chewing on a piece of candy, you can identify that it is sweet but you cannot tell what its flavor is until you release your nose and swallow, thus allowing air to flow from behind the palate and carry the candy’s aromatic molecules to the receptors in the olfactory epithelium. (Fields)


As mentioned in the Smell Basics section, two prominent theories regarding the nature of the olfactory molecules’ binding to your olfactory neurons’ receptor sites: the Stereochemical Theory and the Vibrational Theory. The former, also often referred to as the Molecular Shape Theory, posits that a molecule’s shape determines its specific smell. Based off of the fact that shape plays a tremendous role in so many other biological processes (for example, enzymes working to break down the food you ate for lunch), this “lock and key model” suggests that different types of odor molecules (theoretical “keys”) each have a specific shape therefore bind to receptors (our theoretical “lock”) depending on their varying shape and size. (Cigoj). So, if we go by this theory, the reason why the aforementioned rotting lunch smelled like, well, a rotting lunch rather than roses is because the shape of odor molecules between the two differ substantially.

Enantiomers.pngThe second theory, the Vibrational Theory, proposes that each volatile molecule vibrates a certain amount, which, in turn, activates receptors. If you’re interested in learning more about the Vibrational Theory, this TED talk, given by Lucas Turin, a world-renowned biophysicist who is a prominent proponent of this theory, is highly informative and definitely worth a watch. If we choose to go by this theory, the reason the rotting lunch doesn’t smell like roses is because the two types of odor molecules vibrate in different ways.

Humans are capable of smelling an estimated one trillion scents (Williams). This is why, historically, the classification of odors has been so difficult. Originating in the 20th century (very recent in the “scientific timeline”), the first empirical classifications of odors were based on very vague qualities which did little to aid in the objective categorization of this tremendous number of smells. However, as previously mentioned, our categorization of smells now is nowhere near perfect. Kathrin Kaeppler of Germany’s Leuphana University argues that odor classification still remains somewhat subjective; “odor percepts [still] [can]not be linked to a few measurable physicochemical features of odorous compounds or physiological characteristics of the olfactory system.” For example, enantiomers are “subtly different molecules which possess shapes that are near perfect mirror images of one another. The chemical carvone, for example, smells like caraway (Persian cumin) in one form, and in the near-mirror form, like spearmint” (Wilson). Odor qualities are therefore often evaluated by rather ambiguous “perception-based ratings” (Kaeppler). Hopefully, as the field of olfactory psychology develops, scientists will create a simple, universal method of classification of these seemingly-unclassifiable odors.

Olfactory Disorders

According to the National Institute of Deafness and other Communication Disorders, “one to 2 percent of the North American population below the age of 65 years experience smell loss to a significant degree” (Statistics on Taste and Smell). However, due to the fact that many people aren’t aware of their inadequate sense of smell (realizing a lessened ability to smell York’s lunch truck as you pass by, for example, is markedly more difficult than realizing you’ve gone blind and can’t see the truck itself or the hungry Yorkies standing in line), it is likely that a much higher proportion of our population experiences smell loss.

Multiple origins of anosmia, or the “loss of the sense of smell,” (Binder) exist; many people develop this condition congenitally while others acquire this loss of sensation later in life. According to Ilona Croy (et al) of the University of Dresden Medical School’s Department of Otorhinolaryngology’s Smell and Taste Clinic, congenital anosmia only accounts for 0-4% of the total population of those affected by loss of smell. This condition typically occurs due to “abnormalities of the nasal cavity, disruptions in the pathway that carries information from the nose to the brain, and/or malformations of the portion of the brain that processes sense of smell,” according to the U.S. Department of Health & Human Services’s Genetic and Rare Disease’s Information Center (Congenital Anosmia Overview).

As mentioned earlier, the inflammation which occurs inside your nose when you have a fungal, bacterial or viral infection) (ie when you’re sick), often leads to temporary smell loss by inhibiting the “passage of odor molecules to the smell receptors” (Causes of Anosmia). Sometimes, these infections can incur permanent anosmia by damaging your nose’s receptor neurons. Head injury is also a common cause of anosmia, as it can lead to the critical damage of receptor neurons if your head’s cribriform plate is displaced even the slightest bit. So, if you realize your ability to smell is lessened (or completely nonexistent) after a Stevenson kid knocks you in the head with a soccer ball during a game, you can definitely blame him or her for your anosmia.

Oftentimes, smell loss coincides with cognitive impairments, such as Down syndrome, Alzheimer’s and Parkinson’s (Fields). Lastly, aging is also an extremely common contributor to smell loss--so make sure to enjoy the unique smell of the Student Center while you can. Olfactory receptors, after decades of being exposed to the environment and its air-borne toxins, often become damaged and can sometimes lose their ability to regenerate. 

Smell and Memory/Emotion

Unlike the sensory input of your other senses (tactile, auditory, gustatory and visual [see 5 Sensation and Perception in the YAS Psych Textbook]), olfactory information is not processed through the thalamus. With a direct connection from your olfactory bulb to your hippocampus and amygdala, smell is extremely influential in the formation of autobiographical memories and the evocation of emotions (see 2.2b Limbic System) (Fields). Though you may think that memories formed from visual input are more ingrained in your head, numerous studies have shown that memories formed from olfactory input are actually much more vivid and easily conjured (which makes complete sense from an anatomical perspective--in some studies, including Arshamian’s, these memories evoked through olfactory input displayed higher levels of activity in subjects’ limbic system than those evoked through sensory stimuli). For example, seeing a human brain probably wouldn’t bring you back to the day Torg let each student hold a brain as much as the enchanting smell of formaldehyde would.

As detailed by Jordan Gaines Lewis, a neuroscience PhD student whose work has been featured in Scientific American, NBC, The Washington Post, and The Guardian, Smell can also evoke traumatic memories and volatile emotions in individuals suffering from PTSD. In a case study completed by Vermetten and Bremnert, a veteran endured horrible, vivid flashbacks to an accident in Vietnam whenever he smelled diesel. Overcome with guilt, anger, and nausea even when driving behind a truck, the man had to avoid circumstances involving the smell of diesel (yes, sitting in traffic must be awful for him) (Lewis). If you’re interested in the relationship between smell and anxiety disorders, read this fascinating article published by Rutgers University.

Functions of Smell (optional reading)

The human olfactory system is extremely influential in many aspects of our lives. Its main, most prominent functions can be classified into three major categories relating to: ingestion, detection of environmental hazards and social communication (Stevenson)

Both orthonasal and retronasal olfaction, as described in Smell and Taste, are involved in the process of eating and drinking to, respectively, identify a substance before ingesting and detect an expectancy violation. For example, before pouring the rotten milk that has been sitting in your fridge for over a month into your cereal bowl, you might be inclined to first smell the carton (using your orthonasal olfaction) to detect whether or not the milk is safe to ingest. However, if you fail to notice the carton’s expiration date and proceed to ingest the milk without first smelling it, your retronasal olfaction will step in and let you know that the milk tastes more sour than expected, indicating that it is indeed rotten and unsafe to continue to drink.

Numerous studies, including one in which one group of people who expected strawberry ice cream were instead given salmon-flavored mousse and another was knowingly provided with the mousse, indicate that humans tend to reject substances much more quickly when their flavor expectancy is violated. Though the following function of smell is more prominent among other mammals who don’t have the privilege of purchasing their food from a grocery store, it is important to note that olfaction is often used to detect potential food sources, for example, among small, root and seed-eating rodents. As you might guess, people affected by anosmia— “a temporary or permanent loss of the sense of smell, which may be selective to a small number of odorants or affect detection of all odorants” (Binder) — are much more likely to ingest unsafe (rotten, poisonous, etc.) food (Stevenson).

One’s ability to detect potential hazards in his or her environment can also be quite crucial to his or her survival. Non-microbial scents (smoke, gas, etc.) can alert us of potential dangers. We also often use dogs to sniff out explosives and drugs. Temmel, of the University of Vienna’s Department of Otorhinolaryngology, investigated the consequences of smell loss in 2002 and found that 30% of the subjects that exhibited anosmia had trouble identifying burning food. 

Perhaps the least obvious (and definitely the least studied) of the three major functions of our olfactory system is its potentially large role in our social lives. Though the field of smell in the context of our social environment is relatively uncharted in comparison to other aspects of our sense of smell, numerous studies have been conducted that point toward scent influencing our choice of reproductive partner as well as emotional contagion (Croy). As Rachel Herz of Brown University’s Department of Psychiatry and Human Behavior posits, the “cluster of genes” that “[code] our immune system[,]...major histocompatibility complex, or MHC” is unique in every individual on the face of Earth--other than identical twins, of course, who share indistinguishable genetic makeups. “Your unique string of MHC genes is the genotype,” the genetic makeup, of “your immune system, and your phenotype, the external manifestations of the genes for your immunes system, is your body odor.” This not only explains why canines are capable of locating a person by his or her distinct scent, but also how our olfactory scent can (or has, evolutionarily) quite possibly play(ed) a significant role in our selection of sexual partners. The biological advantage of choosing a mate with an immune system that is opposite, or complementary, of yours is fairly straightforward: “your children will have much greater disease protection and will be unlikely to inherit unpleasant recessive syndromes,” thereby increasing their chance of reproducing successfully (remember Darwin?). Female mice appear to follow this trend, and studies have indicated that humans do too, to an extent (Herz, 124-128). Claus Wedekind, a Swiss zoologist associated with the University of Bern, conducted what is commonly known as the “Sweaty T-Shirt Experiment” in 2000, which corroborate Herz’s assertion. Using two groups of about 50 individuals (separated by gender), Wedekind sought out to investigate women’s reactions to men’s scents in the context of the subjects’ MHC makeup. Instructing the male participants to wear a t-shirt for two nights, Wedekind then brought in his female subjects who were then told to determine which t-shirts, out of the seven provided, smelled the sexiest most pleasant. The results of this study indicate, interestingly enough, that a woman is more attracted to a man whose MHC highly differs from her own (Wedekind).