This is the 2007 version. Click here for the 2017 chapter 04 table of contents.

Olfaction

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. It is true the minor senses have less space devoted to them in the brain and most textbooks, compared to vision and hearing. We will consider the remaining three of the traditional five senses proposed by Aristotle (taste, smell, and touch) and also two internal senses (limb position or kinesthesis and balance or equilibrium) before branching off to discuss some truly exotic types of perception in the final part of this chapter.

Where are the olfactory receptors?


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. 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.

Most of the air you draw into your nose goes directly down to your lungs, but a small amount is diverted up into the sinus cavities. E. Paulsen proved this near the turn of the century. He 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, Paulsen saw that 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 in order to "thrust the bouquet upward" into the olfactory cavities so they can sample the aroma. Ideally, to evaluate the taste of a wine fully, it should be swirled in the mouth and gargled. The process is noisy and looks comical, but professional tasters always do it.

How acute is the sense of smell?

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. Thirty million receptors in each nostril are capable of responding to one or a few molecules of 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.

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

For a substance to be detected by smell, it must be volatile, which means capable of evaporating. Gasoline is volatile. So is the oil of an onion. A substance must also be fat-soluble (capable of being dissolved in fat) to be smelled. The olfactory membranes, like cell membranes in general, are largely composed of lipids (fat molecules). The membrane must absorb a molecule in order to analyze it. Therefore only molecules that dissolve in fat can have an odor.


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