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.
Function
It is the combined pattern of receptors that are tweaked by an odorant that allow the brain to identify it, much as yellow and red light together are interpreted by the brain as orange. (In fact, just as people can be color-blind to red or green, some can be "odor-blind" to certain simple molecules because they lack the receptor for that molecule.) In addition, real objects produce multiple odor-carrying molecules, so that the brain must analyze a complex mixture of odorants to recognize a smell.
Just as the sense of smell is direct in detecting fragments of the objects, it is also direct in the way the signals transmitted to the brain. In most senses, such as vision, this task is accomplished in several steps: a receptor cell detects light and passes the signal to a nerve cell, which passes it on to another nerve cell in the central nervous system, which then relays it to the visual center of the brain. But in olfaction, all these jobs are performed by the olfactory nerve cell. In a very real sense, the olfactory epithelium is a direct outgrowth of the brain.
Role in human health
In humans, the olfactory nerve cell takes the scent message directly to the nerve cells of the olfactory bulb of the brain. There multiple signals from different olfactory cells with different odor sensitivities are organized and processed. The signal then goes to the brain's olfactory cortex, where higher functions such as memory and emotion are coordinated with the sense of smell.
There is no doubt that many animals have a sense of smell far superior than humans. This is why, even today, humans use dogs to find lost persons, hidden drugs, and explosives although research on "artificial noses" than can detect scent even more reliably than dogs continues.
Because of their humble abilities of olfaction, humans are called microsmatic, rather than macrosmatic. Still, the human nose is capable of detecting over 10,000 different odors, some in the range of parts per trillion of air; and many researchers suspect that smell plays a greater role in human behavior and biology than has been previously thought. For instance, research has shown that human mothers can smell the difference between a vest worn by their baby and one worn by another baby only days after the child's birth.
Yet some olfactory abilities of animals are probably beyond humans. Most vertebrates have many more olfactory nerve cells in a proportionately larger olfactory epithelium than humans, which probably gives them much more sensitivity to odors. The olfactory bulb in these animals takes up a much larger portion of the brain than it does in humans, giving the animal more ability to process and analyze olfactory information. In addition, most land vertebrates have a specialized scent organ in the roof of the mouth called vomeronasal organ. This organ, believed to be vestigial in humans, is a pit lined by a layer of cells with a similar structure to the olfactory epithelium, which feeds into its own processing part of the brain, called accessory olfactory bulb, an area of the brain that is absent in humans.
Researchers have learned a lot about how the olfactory nerve cells detect odorants. However, they have not yet learned how this information is coded by the olfactory cell. Scientists are only beginning to understand the role that smell plays in animal and human behavior. The vomeronasal sense of animals is still largely not understood and some researchers have even suggested that the human vomeronasal organ might retain some function, and that humans may have pheromones that play a role in sexual attraction and mating. However, this hypothesis is still very controversial.
Detailed study of the biology of the olfactory system may yield gains in other fields. For instance, olfactory nerve cells are the only nerve cells that are derived from the central nervous system that can regenerate, possibly because the stress of their exposure to the outside world gives them a limited life span. Some researchers hope that studying regeneration in olfactory nerve cells or even transplanting them elsewhere in the body can lead to treatments for as yet irreversible damage to the spine and brain.
Common diseases and disorders
The most common complaint registered by patients is the loss of the sense of smell (anonosmia). Smell disorders usually develop after an illness or an injury. Loss of the sense of smell is commonly caused by upper respiratory illnesses or a head injury. It can result from polyps in the nose or nasal cavity, sinus infections, hormonal fluctuations, or dental problems.
KEY TERMS
Anosmia—A disorder in which one is able to detect no odors.
Olfactory bulb—The primitive part of the brain that first processes olfactory information.
Olfactory cortex—The cerebral cortex that makes use of information from the olfactory bulb.
Olfactory epithelium—The patch of mucus membrane at the top of the nasal cavity that is sensitive to odor.
Olfactory nerve cell—The cell in the olfactory epithelium that detects odor and transmits the information to the olfactory bulb of the brain.
Pheromones—Scent molecules made by the body that attract a mate and help initiate mating behaviors.
Receptor protein—A protein in a cell that sticks to a specific odorant or other signal molecule.
Stereospecific theory—The theory that the nose recognizes odorants when they bind to receptor proteins that recognize the odorants' molecular shape.
Volatile—Easily evaporated.
Vomeronasal—A pit on the roof of the mouth in most vertebrates that serves to detect odor molecules that are not as volatile as those detected by the nose.
BOOKS
Schiffman, Harvey. Sensation and Perception: An Integrated Approach. New York: Wiley and Sons, 2001.
Watson, Lyall. Jacobson's Organ: And the Remarkable Nature of Smell. W.W. Norton, 2000.
PERIODICALS
Dajer, Tony. "How the Nose Knows." Discover, Jan. 1992.
Farbman, Albert I. "The Cellular Basis of Olfaction." Endeavor, 18, no. 1 (1994).
