Smell Health Article

Definition

Smell is the ability of an organism to sense and identify a substance by detecting trace amounts of the substance that evaporate. Researchers have noted similarities in the sense of smell between widely differing species that reveal some of the details of how the chemical signal of an odor is detected and processed.

Description

The sense of smell has been a topic of debate from humankind's earliest days. The Greek philosopher Democritus of Abdera (460–360B.C.), speculated that humans smell "atoms" of different size and shape that come from objects. His countryman Aristotle (384–322B.C.), on the other hand, guessed that odors are detected when the "cold" sense of smell meets "hot" smoke or steam from the object being smelled. It was not until the late eighteenth century that most scientists and philosophers reached agreement that Democritus was basically right: the smell of an object is due to volatile, or easily evaporated, molecules that emanate from it.

In 1821, the French anatomist Hippolyte Cloquet (1787–1840) rightly noted the importance of smell for animal survival and reproduction; but his theorizing about the role of smell in human sex, as well as mental disorders, proved controversial. Many theories of the nineteenth century seem irrational or even malignant today. Many European scientists of that period fell into the trap of an essentially circular argument, that held that non-Europeans were more primitive, and therefore had a more developed sense of smell. The first half of the twentieth century saw progress in making the study of smell more rational. A Spanish neuroanatomist traced the architecture of the nerves leading from the nose to and through the brain. Other scientists carried out the first methodical investigations of how the nose detects scent molecules, the sensitivity of the human nose, and the differences between human and animal olfaction. But the most recent progress in studying the sense of smell and how it affects humans was made with the application of molecular science to the odor-sensitive cells of the nasal cavity.

The sense of smell is the most important sense for most organisms. A wide variety of species use their sense of smell to locate prey, navigate, recognize and perhaps communicate with kin, and mark territory. In a broad sense, the workings of smell in animals as different as mammals, reptiles, fish, and even insects are remarkably similar.

The sense of smell differs from most other senses in its directness; humans and other mammals actually smell microscopic bits of a substance that have evaporated and made their way to the olfactory epithelium, a section of the mucus membrane in the roof of the olfactory cavity. The olfactory epithelium contains the smell-sensitive ending of the olfactory nerve cells, also known as the

olfactory epithelial cells. These cells detect odors through receptor proteins on the cell surface that bind to odor-carrying molecules. A specific odorant docks with an olfactory receptor protein in much the same way as a key fits in a lock; this in turn excites the nerve cell, causing it to send a signal to the brain. This is known as the stereospecific theory of smell.

Recently, molecular scientists have cloned the genes for the human olfactory receptor proteins. Although there are perhaps tens of thousands or more of odor-carrying molecules in the world, there are only hundreds, or at most about 1,000, kinds of specific receptors in any species of animal, including humans. Because of this, scientists do not believe that each receptor recognizes a unique odorant; rather, similar odorants can all bind to the same receptor. It appears that a few loose-fitting odorant "keys" of broadly similar shape can turn the same receptor "lock." Researchers do not yet know how many specific receptor proteins each olfactory nerve cell carries, but recent work suggests that the cells specialize just as the receptors do, and any one olfactory nerve cell has only one or a few receptors rather than many.