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Sense of smell comes
from brain, not nose

As those familiar holiday smells permeate the air and bring back memories, you might think that you're smelling with your nose. But the nose is just the catcher's mitt for certain kinds of odor molecules. The smelling is really done with the most primitive portions of the brain.

Smell is said to be the most mystical and emotional of the senses, yet we often it take for granted since it does not seem to be as necessary for survival as sight, touch and hearing. It was probably more necessary for our primitive ancestors, alerting them to predators and allowing them to locate and identify food.

Humans perceive the world largely through eyes and ears, neglecting the sense of smell and often suppressing awareness of what our nose tells us.

Although underrated, the olfactory sense adds richness to our lives, and we rely on it more than we realize, as it subtly affects memories and emotions and even our attraction to other people.

The sense of smell is important. It helps to stimulate the appetite when we catch the smell food coming from the kitchen, but also helps to alert us to danger, such as the smell of burning food, smoke from a fire or the presence of bacteria in food that warns us not to eat it.

It also soothes us when we smell a familiar fragrance such as a perfume, or newly cut grass that takes us back to a prior pleasant experience.

Smell plays an important role in taste because taste alone is not enough to identify most foods. The smell of foods help us to taste more fully.

Try a blind taste test of an orange or a piece of chocolate with your nose pinched if you don't believe it.

About 95 percent of the nasal cavity has nothing to do with smell. Most of it cleans the air of dust, bacteria and other chemicals that get trapped in the mucous lining. But up on the roof of the nasal cavity, behind the nose, is the olfactory membrane, or epithelium. This membrane contains special receptors that are sensitive to certain kinds of molecules; if a gaseous molecule in the air is the right shape and has a certain vibration frequency, it will fit into a receptor like a key in a lock.

In humans the olfactory epithelium is small, only about the size of a postage stamp, and contains around 10 million receptors of several hundred different kinds. The epithelia of dogs, with their keen sense of smell, have nearly 20 times the area and hundreds of times more receptors per square inch.

Any given odor contains many different kinds of molecules, and many different kinds of receptors are stimulated by a particular "smell." The brain interprets the combination of receptor signals and combines it into recognizable smells. A sniffer in good condition can distinguish 10,000 or so different smells but peaks around age 30 and diminishes from there.

The receptors are actually naked neurons, and they are the only cells in the nervous system that are replaced regularly, each one lasting four to eight weeks. Research into this regeneration could hold clues about how to replace other damaged nerve cells.

When a scent molecule docks with a matching receptor, it sets off a stimulus that then travels to the olfactory bulb, which lies underneath the front part of the brain just above the nasal cavity. There it combines with the signals from other receptors to converge on 2,000 clusters of neurons called glomeruli.

Following that single relay, the olfactory signals travel straight to the hypothalamus, the "primitive" region of the brain, then to the thalamus and neocortex where the information is interpreted and the smell is recognized before the signal is passed to the higher-level centers in the neocortex.

This explains why you might be able to identify a scent or associate it with a memory but not be able to name it.

As a result of this low-level processing, odors have an evocative nature; a smell can instantly recall an emotion, a particular place and time or a situation before we even recognize it, and often before we are even aware of it. Sight and hearing, by contrast, are processed at higher-level relay centers before reaching the primitive areas of the brain.

Some nerves in this region work backward, providing feedback such that the expectations of the smeller influences the way a scent is perceived.

For example, yellow might be associated with freshness, red with fruity sweetness. If the reality is different, the smell might be unpleasant, even if it is not intrinsically so.

The sense of smell is different from the other senses in another way. It is based on mass (number of molecules) rather than on energy (intensity of light, sound or pressure). After a while the sense of smell gets tired. At first you can smell dinner cooking, but soon the olfactory nerves get overtired and then you don't smell it anymore.

On the other hand, the brain might perceive fragrant molecules as pungent when highly concentrated because molecules are detected by the "wrong" receptors, leading to olfactory confusion.

The human nose is extraordinarily sensitive to certain small molecules, such as poisonous gases like hydrogen sulfide, hydrogen cyanide and ammonia.

Some people have a highly developed and trained sense of smell. A perfume maker can distinguish different scents and imagine their combinations the way a jazz pianist combines musical notes. A wine connoisseur has the same talent with wines. There are many other trained "noses" in various industries that are working to manipulate our emotions in one way or another.

Making sense of scents is what the nose knows, and it is a big interest in neuroscience these days. It not only tells us more about how scents affect our well-being, but it also gives us more clues about the mysteries of the human brain, which is the most complex and mysterious structure in the known universe.

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