Of the five senses that keep us in touch with the world, one that we tend to take for granted is the sense of smell. Compared to the others, this sense may not seem very complicated or amazing. Nevertheless a little research reveals that our sense of smell is not only exquisitely designed, but it is also poorly understood by biologists. Of all our senses, that of smell seems to be the most complicated.

Eye and ear vs. nose

When we consider the other senses, we discover that with our sight, color involves only three kinds of receptor: specifically for green, red and blue light. All visual images come from messages to the brain sent from these three color receptors as well as from a receptor for light itself.

The ear, on the other hand, could be thought the most sensitive human organ. The hair cells in the inner ear are designed to detect bass tones (low frequency sound waves) or treble tones (high frequency sound waves) or anything in between. Besides that they are able to detect extremely soft, low energy sound, and louder tones up to billions of times more energetic. However, all the receptors are much alike, whether they detect low or high pitched sounds.

But the sense of smell is quite a different proposition. Imagine a sense which involves 350 entirely different kinds of receptor. It is evident that smell is more interesting than we might have expected.

The nose is huge!

Biologists expect that the number of odors which an organism can detect, is proportional to the number of relevant genes. In people, about 350 different genes code for 350 different receptors. The reason that we need so many receptors is because of the great chemical diversity in odor-causing molecules in the air. The receptor molecules in the nose are located on tiny projections emerging from nerve cells. These projections are situated in the mucous membranes high up in the nose. When an odor molecule collides with an appropriate receptor, the two fit together like lock and key. The receptor protein then initiates a chain of chemical reactions in the nerve cell’s membrane so that the electrical condition in the nerve cell changes. As a result, the nerve cell sends an electrical impulse toward the brain.

The stimulation of different combinations of the 350 different kinds of receptor in the nose, results in the perception of at least 10,000 different odors. Each receptor responds to just one part of a molecule’s structure. Thus, if there are several reactive sites on the surface of one molecule, several different receptors may be stimulated at the same time by this one type of molecule. The blending in the brain of the different messages, leads to the sensation of a specific odor. Some smells are mixtures of large numbers of aromatic molecules. Wines, for example, may consist of as many as 200 different kinds of molecule, and that lovely aroma of coffee contains about 500 different kinds of molecule.

Although we understand these basics, the chemistry of our sense of smell is nevertheless far from clear. Some molecules with very different compositions nevertheless smell much the same. Moreover, some molecules that are extremely alike, nevertheless elicit entirely different sensations of smell. Mirror images of an organic molecule called carvone, for example, smell either like cumin or peppermint, depending upon which arrangement the component atoms assume.

Fearfully and wonderfully made

We really don’t appreciate the wonder that are noses are, and how important the sense of smell is…at least, not until our noses are clogged.

In each nostril, an area about two square centimeters in diameter lies high up in the nasal cavity, just below the brain. This area is packed with tiny thread like extensions from the myriad nerve cells. Each nerve cell deals only with one kind of chemical receptor. Thus all the cilia leading to one nerve cell, have only one kind of receptor on them. Many nerve cells with identical receptors are connected by “wiring” which passes through the skull into collector systems called glomeruli in the brain. The glomeruli are located in two small extensions of the brain which are called olfactory bulbs. These bulbs are about the size of small grapes and there is one above each nostril. The bulbs are lined by the glomeruli, small collection centers, each for the extensions from about 2000 identical nerve cells. Since there are about 350 kinds of receptor, this means there are also 350 kinds of nerve cells. Groups of identical nerve cells send messages to one collection centre or glomerulus. Thus all the messages going to one glomerulus come from stimulation of the same kind of receptor. From the glomeruli, the messages pass to other nerve cells which transmit further into the brain. How the stimulated parts of the brain make any sense of the incredible plethora of messages, is something scientists do not yet understand.

Better than a dog’s nose?

An article in the online journal Public Library of Science Biology (May 2004) was entitled “Unsolved Mystery – The Human Sense of Smell: Are We Better Than We Think?” The popular perception, so author Gordon Shepherd declares, is that the human sense of smell is vastly inferior to that of some other mammals such as dogs, cats and rodents. Well maybe we should think again! Although humans have only 350 functional olfactory receptor genes, compared to much higher numbers for other mammals, it turns out that humans perform extremely well in odor detection tests.

For example, when tested for the lowest amount of a chemical which they can detect, people performed better than dogs in some tests and much better than rats in others. Moreover, humans outperformed even the most sensitive machines (such as the gas chromatograph) designed to detect air-borne chemicals. Thus the author concludes “humans are not poor smellers …. But rather are relatively good, perhaps even excellent smellers.” The author ponders how it is that people have such excellent noses when they have so “few” detector molecules compared to other mammals. The popular evolutionary interpretation is that people lost their sense of smell as they gained in brain power and bipedal locomotion. Obviously the scientists need to reconsider.

A very brainy nose

We now know that people smell very well with far fewer kinds of receptor than animals require. The reason people are able to do this, apparently, lies in the much more sophisticated interpretive capability of the human brain. For any individual odor, the brain calculates how many different kinds of receptor are simulated and what is the relative proportion of these stimulated receptors.

Scientists have also recently discovered that smell perception involves many more areas of the brain than previously thought. The regions dedicated to odor interpretation include the olfactory cortex, olfactory tubercle, entorhinal cortex, parts of the amygdala, parts of the hypothalamus, the mediodorsal thalamus, the medial and lateral orbitofrontal cortex, and parts of the insula (“Unsolved Mystery…” p. 574). Dr. Shepherd points out that all these regions of the brain are involved in the immediate distinguishing of an odor. If memory is also involved, as is typical with smells, then the temporal and frontal lobes of the brain also become involved.

It is the view of Dr. Shepherd that people need such a sophisticated system for identifying smells. Not only do we need to identify natural smells, but we also create all sorts of artificial aromas such as those from cooking and manufacturing. The design of our olfactory system (for smell) thus involves not only the hardware such as nerve receptors and wiring in the brain, but also software design so that these inputs can be interpreted. It is evident that scientists who try to draw conclusions about organisms based on comparisons of their chemical components, may be in for a surprise. Dr. Shepherd therefore remarks: “The mystery being addressed here is a caution …. against any belief that behavior can be related directly to genomes, proteomes, or any other type of ‘-ome’” (p. 575). [The genome is the genetic information in the DNA, and proteome is the complete list of proteins in an organism.] None of these measures adequately characterizes an organism and its capabilities.

An experiment to try on your friends/victims

Now that we have established that the human sense of smell is extremely remarkable, we can turn our attention to the results of this gift. Most people understand, whether they are trained in biology or not, that our sense of smell is extremely important to our sense of taste. In this context, you might like to try a simple experiment on your friends or enemies. Separately puree some raw potato, apple and onion. Place each sample in an airtight container and provide each container with a medicine style dropper (or pipette). Now invite your friend (victim?) to undergo a taste test. Have the individual hold their nose and open their mouth. Drop a sample of puree on the tongue (apple first). As long as the nose is held, the person will not be able to identify the flavor except to say that it is sweet. Allow the individual to breathe through the nose in order to identify the sample. Repeat with the other samples with the onion administered last because after that the person will

a) refuse to cooperate further
b) chase you out of town
c) run for a glass of water or
d) all of the above.

Anyway, the experiment is lots of fun and it amply demonstrates the role of smell in flavor appreciation. Apparently the flavors of coffee, wine and chocolate are all largely controlled by our sense of smell as are those of many other foods. That is why food is tasteless when one is suffering from a cold.

In recent years, many people have become interested in the ways in which odors affect peoples’ moods. Obviously there is nothing like the aroma of freshly baked bread or of cinnamon buns to raise one’s spirits. It is said that the penetrating but pleasant fragrance of lily-of-the-valley or of peppermint enable some individuals to concentrate better on a given task. In some cultures the scents of lemon, jasmine or lavender may have the same effect. Other people have found that spiced apple scent or heliotropine (like vanilla and almond scents combined) are able to exert a relaxing effect. Not surprisingly, culture can affect our responses to certain stimuli. For example, a manufacturer tested three detergent samples which were identical except for scent. Test subjects in Toronto and Montreal were asked to compare the cleaning abilities of these three products. The people in Montreal (largely French speaking) preferred the sample which smelled the most like perfume. In Toronto (largely English speaking), on the other hand, the test subjects suspected that something this good smelling must not work very well. Thus they rated the perfumed product as least effective. The amusing thing is that all three samples were identical except for fragrance. There was no difference in their cleaning effectiveness.

Now that we know the nose…

Through the ages there have existed commercial interests which attempt to exploit the human sense of smell for commercial gain. Obviously the companies which market expensive perfumes and colognes top this list. There are other more subtle applications as well. The aroma of fresh baking can be purchased by store owners who keep their product protected in display cases. Furniture salesmen may spray an artificial scent of leather around their showrooms. Movie theaters may spray an artificial odor of fresh popcorn into the air. If there is a way to exploit people, we can be sure that someone will think of it. The use of scent has simply become another tool in that process.

For most people, smells that remind one of beautiful locations or happy events are the best scents of all. The scents of the sea shore, or of freshly mown grass, or of a roast beef dinner all conjure happy memories (or happy anticipation) in most of us. Now that we understand how complicated the design of our odor detection system really is, we will be doubly thankful for the wonderful gift of smell.

This was first published in the July/August 2004 issue. Dr. Margaret Helder is the author of “No Christian Silence on Science” which you can buy here.