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Olfaction and Gustation

Seeing and hearing are often called the major senses, while smelling, tasting, and touching are called the minor senses. However, they are not really minor.

People who lose a minor sense such as olfaction (smelling) or gustation (tasting) are quite dis­turbed by their handicap. However, it is true the minor senses have less space devoted to them, in the brain and most textbooks, compared to vision and hearing.

cross-section of the nasal passages
Molecules from the air lodge on the olfactory epithelium.

Olfaction is odor detection. The olfactory receptors are located in the olfactory epithelium, a dime-sized area at the top of each nasal cavity.

Where are the olfactory receptors?

An epithelium is a layer of cells, so the olfactory epithelium is a layer of cells devoted to the sense of smell. The receptors feed directly into a part of the brain called the olfactory bulb.

In order to trap small amounts of airborne particles on the mucous membranes of the olfactory receptors, we divert a small amount of air upward into the nose when taking a breath. E. Paulsen proved this near the turn of the century.

Paulsen sawed a cadaver head in half, put litmus paper (which reacts to ammonia by turning blue) into the throat and sinus cavities, then he put the head back together and pumped ammonia fumes up the nostrils.

Sure enough, the litmus paper in the sinus cavities turned blue. However, the litmus paper in the sinus cavities did not turn nearly as blue as the litmus paper in the throat, illustrating that only a small part of the air stream was diverted into the sinuses.

How does sniffing benefit olfaction?

Sniffing increases the amount of air going into the sinuses by disturbing the air stream. This is why we automatically sniff the air when we wish to smell something.

You may be familiar with brandy snifters–large, circular glasses that hold a small amount of brandy. People who like brandy sniff gently at the air drifting out of the glass and comment on the quality of the odor.

What do professional wine tasters do?

Professional wine tasters know the olfactory sense is largely responsible for their sense of taste. They swirl wine in a glass and gargle it to "thrust the bouquet upward" into the olfactory cavities so they can sample the aroma.

The sense of smell, like the other senses, is remarkably acute. Humans can detect typical odor-producing substances in concentrations of one particle of odorant (odor-causing substance) per 500 million particles of air.

How acute is the sense of smell? Can it be trained?

The sense of smell, like other senses, can be trained and improved in its discriminating abilities. Some people are more sensitive to odors than others. Professional "noses" command high prices for specialized services provided to perfume companies.

The Perfume Society explains on its web site that "being a 'nose' is a challenging job that takes up to seven years of study–and (mostly) qualifications in chemistry." ("The Nose", 2017)

Thirty million receptors in each nostril are capable of responding to a few mole­cules of an odor-causing substance. Dogs are even better than humans at detecting odors–a million times better, by some estimates.

Tracking dogs improve their abilities further by keeping their noses low to the ground, where odor molecules are most likely to accumulate.

The floppy ears of a bloodhound help the process even more. They are shaped perfectly to channel molecules of odor to the dog's nostrils.

What characteristics must a substance have, to be smelled? Why?

For a substance to be detected through the nose, it must be volatile, which means capable of evaporating. Gasoline is volatile. So is the oil of an onion. In general, if you can smell something, its molecules are capable of being airborne.

A detectable odor must also consist of fat-soluble molecules (able to dissolve in fat). The olfactory membranes, like cell membranes in general, are largely composed of lipids (fat molecules). The membrane must absorb a molecule to analyze it. Therefore only molecules that dissolve in fat can have an odor.

The Vomeronasal System

The forked tongue of a snake is a delivery system for odor detection. A pair of chemosensors, one in each branch of the fork, provides direct inputs to a part of the snake's brain called the vomeronasal or Jacobson's organs.

Why is the snake's tongue forked? This provides directional infor­mation . The two receptors are stimulated to different degrees by odors coming from different directions.

The ability to know the direction of an odor is important to snakes. Many snakes are nearly blind and rely on odor to detect prey. The directional sensitivity gives them the ability to hunt at night.

Why do snakes have forked tongues?

Humans also have a vomeronasal organ. For years researchers considered it vestigial (obsolete and unused).

However, in the late 1980s and early 1990s, a number of research teams suggested otherwise. They discov­ered that the human vomero­nasal system responded to chemicals entering tiny pits on both sides of the nose.

Researchers suggested the vomeronasal system might be involved in sexual attraction, acting as a detector of human pheromones (hormones often used for communication, including sexual attraction). People would not necessarily be aware of this; the output of the vomeronasal organ is not known to be consciously perceivable.

What has been discovered about the VNO in humans?

You can imagine the commercial impact of a substance that unconsciously triggered sexual attraction. It could produce huge profits for pharmaceutical or cosmetic industries as well as aiding therapy for sexual disorders.

These implications have not escaped researchers. As soon as vomeronasal sensitivity was discovered in humans, research was plunged into secrecy. If there is a way to exploit this system to enhance sexual attraction, publicity will not occur until the discovery is patented.

In the meantime, all sorts of unvalidated "sexual pheromones" are for sale on the internet. In all likelihood, they cause only placebo effects.

Olfactory Adaptation

Sensory adaptation, the reduction in response to a continuing stimulus, occurs in olfaction as in other senses. Olfactory adaptation is unusually rapid and complete. After a few minutes, odors seem 80% less powerful.

Because of olfactory adaptation, people who are wearing too much perfume or cologne seldom realize it. The odor is only one-fifth as powerful to them as it is to other people.

What is olfactory adaptation? How complete is it after a few minutes? What are some consequences?

In many situations, olfactory adaptation is a blessing. A person who spends lots of time working near a foul odor gets to the point where olfactory adaptation is 100% and he or she cannot smell it at all. A student wrote about his exper­ience with this:

Wednesday's discussion of adaptation...made me think of an example that I experienced growing up in Savannah. Savannah was unfortunate/fortunate (?) enough to have one of the world's largest paper mills, Union Camp.

For nineteen years I lived in the city and not once did I ever smell the awful fumes this factory used to put out. (They have cleaned it up a bit since then.)

I was working one summer with a man from North Carolina. He was constantly mentioning the fact that there was a "terrible smell" in the air.

Even after a month out of Savannah I could return and still not smell Union Camp. Yet those who do not live in the city catch the wafting smell almost instantly. [Author's files]

The olfactory sense plays a major role in our perception of flavor in food. When the nose gets stuffed up, food is tasteless.

A simple experiment reveals the role of olfaction in taste. Take an apple and an onion, hold your nose, and sample each.

You will be unable to tell them apart. Or take a swig of a cola drink, while holding your nose. It will taste like carbonated water.

Does Odor Alter Mood?

Some psychologists believe that odors can alter mood. Small vials of pleasant odor-causing substances are sold to people who want to increase their alertness or improve their moods.

Odor-causing substances such as lavender and camphor have been used in medical treatments for thousands of years. The word aromatherapy first appeared in 1937.

What is "aroma therapy"?

Edwards (1995) interviewed an expert on olfaction, Susan Kasko, PhD, at the time employed as a research psychologist with Monell Chemical Senses Center. She expressed skepticism about beneficial effects of aromas.

Kasko said she is aware of 11 published studies involving the connection between odor and performance, and five of those showed no influence at all.

The other six were evenly divided between positive and negative effects. Her own studies have never shown any connection between odor and performance....

"There is no direct evidence that odors trigger more emotional responses than other sensory systems." (Edwards, 1995)

Aromatherapy is a large industry. There are web sites claiming evidence-based treatments using odors. Skeptical sites are not convinced, citing problems with the evidence cited by aromatherapists.

What happened when people were exposed to water but told it contained a pleasant fragrance?

Kasko herself suspected placebo effects were at work:

She cited one study in which parti­cipants were exposed to water vapor but were told that it contained a pleasant fragrance. Many of the participants reported an elevated mood when exposed to the vapor (Edwards, 1995)

Gustation

Gustation is usually called the sense of taste, despite the fact that olfaction dominates our ability to taste foods and liquids. Taste cells are gathered together in taste buds on the tongue, and taste buds are hidden in bumps on the tongue called papillae.

Which sense dominates the taste of foods?

taste bud
There are many taste cells in each bud, and each bud is inside a pore between the papillae

Look closely at your tongue. You will see many small bumps. These are not taste buds; they are papillae (papilla is the singular).

Taste buds are hidden in pores in the papillae. There are about 10,000 taste buds on the tongue, each with 10 to 20 taste cells that look like the sections of an orange.

Taste cells are the actual receptors for taste. They are constantly rebuilt and replaced within the taste bud. Every seven days you get a new set of taste cells. If you burn your tongue on a hot drink, your gustatory ability will return in a week or less.

What are taste cells, buds, and papillae?

Substances enter the taste buds through small pores on the papillae. For a substance to reach the receptors, it must dissolve in water so it can wash into the pores.

In a classic chemistry class demon­stration, a teacher calls a hapless student to the front of the room, dries off the student's tongue, and deposits a small mound of salt on it.

The student can taste nothing. Only substances dissolved in water (or saliva, which is mostly water) can be tasted. Some substances such as glass and most metals do not dissolve in water at all, and they cannot be tasted.

Why must a substance be dissolved in water, to be tasted?

Taste cells are chemoreceptors. They respond to chemical substances. The four classical taste qualities are sour (e.g. vinegar), saline (e.g. salt water), sweet (e.g. sugar) and bitter (e.g. quinine).

The tastes have different functions. Sweetness usually means a food has high caloric value. Bitterness is commonly associated with plant poisons.

What are the four classic taste qualities?

Japanese researcher Kikunae Ikeda discovered a fifth basic taste: umami. The Smithsonian Magazine told the story (Geiling, 2013):

In 1908, over a bowl of seaweed soup, Japanese scientist Kikunae Ikeda asked a question that would change the food industry forever: what gave dashi, a ubiquitous Japanese soup base, its meaty flavor?...

Ikeda was able to isolate the main substance of dashi–the seaweed Laminaria japonica. ...After days of evaporating and treating the seaweed, he saw the development of a crystalline form. When he tasted the crystals, he recognized the distinct savory taste that dashi lent to other foods, a taste that he deemed umami, from the Japanese umai (delicious.)

He determined the molecular formula of the crystals: C5H9NO4, the same as glutamic acid, an amino acid... In the body, glutamic acid is often found as glutamate, a different compound that has one less hydrogen atom. Glutamate is one of the most abun­dant excitatory neurotransmitters in brain, playing a crucial role in memory and learning.

In 1909, Ikeda began mass-producing Ajinomoto (meaning "essence of taste")... It was touted as a nutritional wonder, helping bland but nutritious food become delicious. A growing number of Japanese housewives used the product...

The sodium salt of glutamic acid remains prevalent today–anyone who has eaten KFC or Doritos has ingested it; it's just known by a different name: monosodium glutamate, or MSG. (Geiling, 2013)

In 1968, scary stories linked MSG to the "Chinese Restaurant Syndrome." Symptoms ranging from cold sweats to dizziness.

You might recall from Chapter 1 that both positive and negative effects can result from expectations and placebo effects. Double-blind research must be carried out to explore cause-effect claims involving foods and medicines.

A well controlled double-blind study of MSG effects in humans was carried out in 1971. The results were clear: "no subject" reported the so-called Chinese Restaurant Syndrome (Rosenblum, Bradley, and Coulston, 1971).

What is the fifth basic taste? What neurotransmitter is involved? What did a double-blind study show about MSG and the Chinese Restaurant Syndrome?

Scares based on bad publicity can persist long after the dangers are disproved. Many people are still wary of MSG.

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References:

Edwards, R. (1995, March) Pleasant aromas chase away those bitter moods. APA Monitor, p.20.

Geiling, N. (2013, November 8) It's the umami, stupid. Why the truth about MSG is so easy to swallow. Retrieved from: http://www.smithsonianmag.com/­arts-culture/its-the-umami-stupid-why-the-truth-about-msg-is-so-easy-to-swallow-180947626/

The Noses. (2017) PerfumeSociety.org. Retrieved from: http://perfumesociety.org/discover-perfume/an-introduction/the-noses/

Rosenblum, I., Bradley, J. D., & Coulston, F. (1971) Single and double blind studies with oral monosodium glutamate in man. Toxicology and Applied Pharmacology, 18, 367-373. http://dx.doi.org/10.1016/0041-008X(71)90129-3


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