How Smell Works

Smell begins when airborne molecules stimulate olfactory receptorcells。If a substance is somewhat volatile (that is, if it easily turns into a gas), it will give off molecules, or odorants. Nonvolatile materials likesteeldo not have a smell.

Temperature and humidity affect odor because they increase molecular volatility. This is why trash smells stronger in the heat andcarssmell musty after rain. A substance's solubility also affects its odor. Chemicals that dissolve inwaterorfatare usually intense odorants.

When an air current sweeps an odorant up through the nostrils, the molecules hit theolfactory epithelium-- the center of olfactory sensation. The epithelium occupies only about one square inch of the superior portion of the nasal cavity. Mucus secreted by the olfactory gland coats the epithelium's surface and helps dissolve odorants.

Olfactory receptor cellsare neurons with knob-shaped tips calleddendrites。Olfactory hairs that bind with odorants cover the dendrites. When an odorant stimulates a receptor cell, the cell sends an electrical impulse to theolfactory bulbthrough theaxonat its base.

Supporting cellsprovide structure to the olfactory epithelium and help insulate receptor cells. They also nourish the receptors and detoxify chemicals on the epithelium's surface.Basal stem cellscreate new olfactory receptors through cell division. Receptors regenerate monthly -- which is surprising because mature neurons usually aren't replaced.

While receptor cells respond to olfactory stimuli and result in the perception of smell,trigeminal nerve fibersin the olfactory epithelium respond topain。When you smell something caustic like ammonia, receptor cells pick up odorants while trigeminalnervefibers account for the sharp sting that makes you immediately recoil.

But how does odor actually become smell? In the next section, we'll learn more about olfactory receptors and odorant patterns.

How does thebrain认识、分类和记住的巨大variety of odors? In 1991, Richard Axel and Linda Buck published a groundbreaking paper that shed light on olfactory receptors and how the brain interprets smell. They won the 2004Nobel Prizein Physiology or Medicine for the paper and their independent research.

Axel and Buck discovered a largegenefamily -- 1,000 genes, or 3 percent of the human total -- that coded for olfactory receptor types. They found that every olfactory receptor cell has only one type of receptor. Each receptor type can detect a small number of related molecules and responds to some with greater intensity than others. Essentially, the researchers discovered that receptor cells are extremely specialized to particular odors.

Axel and Buck also found that each olfactory receptor type sends its electrical impulse to a particular microregion of the olfactory bulb. The microregion, orglomerulus, that receives the information then passes it on to other parts of the brain. The brain interprets the "odorant patterns" produced by activity in the different glomeruli as smell. There are 2,000 glomeruli in the olfactory bulb -- twice as many microregions as receptor cells -- allowing us to perceive a multitude of smells.

Another researcher, however, has challenged the idea that humans have a large number of receptor types that respond only to a limited number of molecules. Biophysicist Luca Turin developed the quantum vibration theory in 1996 and suggests that olfactory receptors actually sense the quantum vibrations of odorants'atoms。While molecular shape still comes into play, Turin purports that the vibrational frequency of odorants plays a more significant role. He estimates that humans could perceive an almost infinite number of odors with only about 10 receptors tuned to different frequencies.

Next, we'll learn about how smells trigger memory and find out how much cognition actually influences perception.

A smell can bring on a flood of memories, influence people's moods and even affect their work performance. Because the olfactory bulb is part of the brain'slimbic system, an area so closely associated with memory and feeling it's sometimes called the "emotional brain," smell can call up memories and powerful responses almost instantaneously.

The olfactory bulb has intimate access to theamygdala, which processes emotion, and thehippocampus, which is responsible for associative learning. Despite the tight wiring, however, smells would not trigger memories if it weren't forconditioned responses。When you first smell a new scent, you link it to an event, a person, a thing or even a moment. Your brain forges a link between the smell and a memory -- associating the smell ofchlorinewith summers at thepoolor lilies with a funeral. When you encounter the smell again, the link is already there, ready to elicit a memory or a mood. Chlorine might call up a specific pool-related memory or simply make you feel content. Lilies might agitate you without your knowing why. This is part of the reason why not everyone likes the same smells.

Because we encounter most new odors in our youth, smells often call up childhood memories. But we actually begin making associations between smell and emotion before we're even born. Infants who were exposed toalcohol, cigarette smoke orgarlicin the womb show a preference for the smells. To them, the smells that might upset other babies seem normal or even comforting.

在下一节中,我们将看看一些名人18新利最新登入e use smell's ability to trigger memory.

Advertisers are eager to cash in on the close link between smell, memory and mood. Real estate agents have long usedscent marketingas a way of putting clients at ease. Sellers set fresh pie or cookies on countertops to make a house seem comfy and livable. But because there's a limit to how many pies one agent can bake, companies that sell aroma-marketing systems are stepping up. Housing developments, hotels, stores and evencarmanufacturers are turning to customized scents to help set a mood and maybe even make an impression.

Scent marketing is the latest trick to stand out from the visual and auditory barrage that dominates advertising. These scents, however, are a far cry from the strong smells of incense and patchouli at the bead store. They're subtle and almost imperceptible to the unwitting sniffer. Developers use carefully tuned scents to lure customers into a sense of well-being. Stores that sell shoes or shirts, items ideally not associated with odor, formulate aromas of ivy or crisp linen. Some companies even strive to develop a "brand scent," something that customers will associate with the company as much as a logo.

To learn more about smell and the other senses, sniff out the links on the next page.