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.

The sense of smell has been a topic of debate from humankind's earliest days.The Greek philosopher Democritus of Abdera (460-360 B.C.) speculated that wesmell "atoms" of different size and shape that come from objects. His countryman Aristotle (384-322 B.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 18th century that most scientistsand 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.

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. The sense of smell differs from mostother senses in its directness: we actually smell microscopic bits of a substance that have evaporated and made their way to the olfactory epithelium, asection of the mucus membrane in the roof of the olfactory cavity. The olfactory epithelium contains the smell-sensitive endings 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 thesame 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.

In the past few years 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, orat most about 1,000, kinds of specific receptors in any species of animal. Because of this, scientists do not believe that each receptor recognizes a unique odorant; rather, similar odorants can all bind to the same receptor. In other words, a few loose-fitting odorant "keys" of broadly similar shape can turn the same receptor "lock." Researchers do not know how many specific receptor proteins each olfactory nerve cell carries, but recent work suggests thatthe cells specialize just as the receptors do, and any one olfactory nerve cell has only one or a few receptors rather than many.

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 intepreted by the brain as orange. (In fact, just as people can be color-blind tored or green, they can be "odor-blind" to certain simple molecules because they lack the receptor for that molecule.) In addition, real objects that we smell produce multiple odor-carrying molecules, so that the brain must analyzea 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 signal is transmitted to the brain. In most senses, such as vision, this task is accomplished in several steps: a receptorcell detects light and passes the signal to a nerve cell, which passes it onto another nerve cell in the central nervous system, which then relays it tothe 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.

The olfactory nerve cell takes the scent message directly to the nerve cellsof the olfactory bulb of the brain (or, in insects and other invertebrates that lack true brains, the olfactory ganglia), where multiple signals from different olfactory cells with different odor sensitivities are organized and processed. In higher species the signal then goes to the brain's olfactory cortex, where higher functions such as memory and emotion are coordinated with thesense of smell.

There is no doubt that many animals have a sense of smell far superior to that of humans. This is why, even today, humans use dogs to find lost persons, hidden drugs, and explosives, although research on "artificial noses" that candetect scent even more reliably than dogs continues. Humans are called microsmatic, rather than macrosmatic, because of their humble abilities of olfaction.

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 are beginning to wonder whether smell does not play a greater role in human behavior and biology than has been thought. For instance, research has shown that humanmothers 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 sensitivityto odors. The olfactory bulb in these animals takes up a much larger proportion of the brain than humans, giving them more ability to process and analyzeolfactory information.

In addition, most land vertebrates have a specialized scent organ in the roofof their mouth called the vomeronasal organ (also known as the Jacobson's organ or the accessory olfactory organ). This organ, believed to be vestigial in humans, is a pit lined by a layer of cells with a similar structure to theolfactory epithelium, which feeds into its own processing part of the brain,called the accessory olfactory bulb (an area of the brain absent in humans).

The vomeronasal sense appears to be sensitive to odor molecules with a less volatile, possibly more complex molecular structure than the odorants to whichhumans are sensitive. This sense is important in reproduction, allowing manyanimals to sense sexual attractant odors, or pheromones, thus governing mating behavior. It is also used by reptilian and mammalian predators in trackingprey.

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. Other topics of future research will be how olfactory cellsignals are processed in the olfactory bulb, and how this information relatesto higher brain functions and our awareness of smell.

Scientists are only beginning to understand the role that smell plays in animal and human behavior. The vomeronasal sense of animals is still largely notunderstood. 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--although this hypothesis is very controversial.

In addition, detailed study of the biology of the olfactory system might 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 lifespan. Some researchers hope that studying regeneration in olfactory nerve cells or even transplanting them elsewhere in the body can leadto treatments for as yet irreversible damage to the spine and brain.

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