Sense of Smell

“Each sense receptor responds to different inputs (electromagnetic, mechanical, chemical), transmitting them as signals that travel along nerve cells to the brain. The signals are then processed in the brain, resulting in immediate behaviors or memories. Changes in the structure and functioning of many millions of interconnected nerve cells allow combined inputs to be stored as memories for long periods of time.” AZ science requirements for middle school

Smells bring back Ned’s Barf Day six years ago

Ned, twelves years old,  and his dog Honey were sitting on the back porch one Saturday morning.  The new neighbors next door were scrambling a feast of what smelled to rotten eggs to Ned.  Ned felt the urge to hurl.  He was taken back to a time when he was six years old.  Mom made Ned and his older brother Tom eat scrambled eggs for breakfast.    One morning Ned refused to eat the eggs.  Tom, ever the bully, grabbed little Ned by the shoulder, pried Ned’s mouth open, and forced a forkful of the scrambled eggs into Ned’s mouth.   This day was not only the repulsive egg smell but also the putrid smell of Tom’s left arm pit. Ned proceeded to projectile vomit all over Tom’s  t-shirt.  Some of the purple grape jelly from Ned’s barfed toast bled into the green lettering on Tom’s t-shirt.   Details of the drama that followed came flooding back into Ned’s head:  The divorce his parents were going through, Tom’s bullying, and poor little Ned getting ignored when things weren’t working out in first grade.   She had cleaned up the egg hurl mess on Barf Day when Mom didn’t know whether to go off on Tom or the barf master Ned.  After Barf Day, Ned had permission to eat his cereal on the deck when the family was eating scrambled eggs.

This morning was bad  The smell coming from his own arm pits was not so good.   Mixed with the smell of those eggs, Ned felt a terror come upon him.  He not only felt nausea, but also his heart racing and a cold sweat.  Honey looked at Ned with knowing eyes, laid her head on Ned’s lap,  and gently thumped her tail on the deck.  People claim that dogs can smell cancer or an epileptic attack coming on.  Could Honey smell the terror Ned was experiencing just from the smells from long ago?

  • Do dogs smell things we cannot?
  • Why do smells evoke such detailed memories?
  • Why do some smells cause some people but not others to barf?

Better Smelling by more receptors?

Is dog smelling better by the gross number of receptors, the number of chemicals that the receptors recognize and/or by the way dogs’ brains process the information they receive from their olfactory receptors?

 Color vision  is a blog about different combinations of just red, green, and blue opsins that absorb light.   Yellow light appears as such because it activates the red and green light absorbing opsins while not activating the blue light absorbing opsins.  We humans have about 400 different odor receptors in our noses.  Dogs have about 800 different odor receptor genes.  Rats have about 1200 different odor receptor genes. Yoshihito Niimura’s review might have too much chemistry for a middle school student but not for a well motivated high school student.

Like color vision, smell detection seems to work on a given olfactory epithelial cell only expressing one olfactory receptor of 800 possible genes. Dr Niimura’s review gives some examples of nose blindness, or specific anosmia. On the flip side, Dr Niimura gets into hypernosmia, a hypersensitivity to some odors.

  • One in ten humans are nose blind to hydrogen cyanide, that many of us perceive as having an almond like odor.
  • One in 1,000 of us cannot smell butyl mercaptan, the odor of skunks that is used to spike natural gas.
  • The pig hormone androstenone is a well studied example of human nose blindness.   Individuals may perceive this hormone as  offensive (sweaty, urinous), pleasant (sweet, floral), or odorless.   Scientists located the genetic code for this pig hormone olfactory receptor to a gene named  OR7D4.  Slight variations in this human gene determines how we respond to this  pig hormone.

Niimura Y. (2012) Olfactory receptor multigene family in vertebrates: from the viewpoint of evolutionary genomics. Curr Genomics. 13(2):103-14.  Free Paper

A few years ago  the dog’s sense of smell was featured on PBS NOVA.  This is a fun article to read, and middle school kid friendly.  In a nut shell, dogs have  40x the brain capacity for analyzing olfactory signals.    We see thousands of different shades in the visible spectrum with just three different varieties of cone opsins.  Your Dog Advisor has an even better piece on the wonders of the dog’s nose.  Keep these things in mind when you read about how complicated the sense of smell can be in we humans.  We humans cannot fully comprehend why our dogs must smell so many things on a walk.

Any given odor we can smell may bind to more than one of our olfactory receptors to one degree or another.  The same olfactory receptor may bind many different compounds.  The data from the lemon grass  compound citral is from a paper published by Dr Karine Audouze and coworkers in 2014.  The actual paper is far more complicated with an intended audience of those with MD and PhD degrees.

Olfactory receptors are found in the nasal cavity. Humans have about 400 different receptors. Olfactory receptor neurons connect to the olfactory bulb. Four or more of our 400 bind citral, an aromatic compound in lemon grass. These same receptors may bind other compounds with perhaps slightly different affinities.

This is only a glimpse of how complicated things are.  Yes, 40x the processing power is probably more responsible for their superior sense of smell than the 2x different types of olfactory receptors.

Audouze K, Tromelin A, Le Bon AM, Belloir C, Petersen RK, Kristiansen K, Brunak S, Taboureau O. (2014) Identification of odorant-receptor interactions by global mapping of the human odorome. PLoS One. 2014 Apr 2;9(4):e93037. Free Paper

Smells and memories

Anne-Marie Mouly and Regina Sullivan have addressed this subject in Chapter 15 Memory and Plasticity in the Olfactory System: From Infancy to Adulthood in a book called

The Neurobiology of Olfaction. Menini A, editor. Boca Raton (FL): CRC Press/Taylor & Francis; 2010.  Free Link

A simplified version of olfactory pathways adapted from Neurology of olfaction . Wikipedia was used for descriptions (green) of portions of the brain involved in odor related memory processing.

The middle school student is free to further research for deeper learning.

Smells and vomiting

Ned’s vomiting in response to a particular smell could fit the AZ Science  definition of an “immediate behavior.”  Dr. Charles Horn has a really nice review on why smell and taste induced vomiting can be an important defense food poisoning.  Women in their first trimester of pregnancy are more likely to experience nausea and vomiting.  Dr Horn asserts that this protects the fetus during its period of very rapid development.   Dr Horn’s paper is more at the level of medical students, not middle school students.

Horn CC. (2008) Why is the neurobiology of nausea and vomiting so important? Appetite. 50(2-3):430-4. Free Paper

Summarizing and getting philosophical

Philosophically, why to do smells make some people barf while others don’t even get the pre-barf nausea?  Dr Mainland and others compared the   sequences of olfactory receptor genes from over 1000 human volunteers.

A greatly simplified table from a scientific paper by Mainland and others published in 2014 . Some mutations resulting in single amino acid substitutions in the  0R4E2 gene and the OR2C1 genes

Some of the several olfactory receptors that Dr Joel Mainland and others studied had increased sensitivities to odorants.  Some mutations resulted in proteins with increased sensitivity ( 11%); other mutations led to decreased sensitivity (6.8%).

Fig 4 from Mainland (2014) a. pie chart of the effect of mutations in select human olfactory receptor genes on function. Pseudogenes cannot be used to make proteins. b.  snake diagram showing parts of the olfactory receptor protein that is sensitive to mutations.

By the way, we’ve just knocked off two more AZ science requirements:

“Changes (mutations) to genes can result in changes to proteins, which can affect the
structures and functions of the organism and thereby change traits”

These pesky pseudogenes that cannot make proteins because they lack regulatory elements. This will not be a requirement until high school.

“Not all DNA codes for a protein; some segments of DNA are involved in regulatory or structural functions, and some have no as-yet known function.”

Mainland JD, Keller A, Li YR, Zhou T, Trimmer C, Snyder LL, Moberly AH, Adipietro KA, Liu WL, Zhuang H, Zhan S, Lee SS, Lin A, Matsunami H. (2014)The missense of smell: functional variability in the human odorant receptor repertoire.Nat Neurosci. 2014 Jan;17(1):114-20. Free Paper

In summary

The process of smelling a very complicated.  Just the processing is complicated.  How smells get coded into memories and immediate actions is even more complicated.

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