Our sense of smell plays a large role in everyday life. It lets us enjoy pleasant scents, such as flowers and food, but it also helps us detect dangerous smells, such as gas or fire. Smell can also help us recall long-forgotten memories.  Not surprisingly, we often don’t realize how important it is to us until we lose it because of a disorder or a bad cold.

How does our sense of smell work?

Our smell is part of our chemosensory system, which also includes taste. High inside the nose, we have a group of specialized cells--called olfactory sensory neurons—and each expresses one odor receptor which connects directly to the brain. When microscopic molecules in our environment reach these receptors, they send a message to the brain which then identifies the smell.  Smells reach the olfactory sensory neurons in two ways--through the nostrils directly or through the roof of your throat that connects to the nose.

The common chemical sense also influences our smell. The common chemical senses are composed of thousands of nerve endings on the moist areas of the eyes, nose, mouth and throat help us sense irritating substances. For instance, when an onion makes us cry or peppermint tingles and cools our mouth, we are experiencing the common chemical sense.

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Why is it important?

A loss of smell can be indicative of a more serious health problem. 

Roughly 1 to 2 percent of people in North America say they have a smell disorder, and the likelihood increases with age. Men are more likely than women to experience a problem, and many people with a smell disorder also notice they have problems with taste. Smell disorders either cause people to lose their ability to smell or it changes the way they perceive an odor. Something that once smelled pleasant can be distorted to smell bad. Hyposmia is a reduced ability to detect odors, and anosmia is the inability to detect any odors at all.

What happens when we lose our smell due to a cold or allergies?

A recent study published in the journal Nature Neuroscience looked at how the brain responded to a short-term loss of smell, and found that brain activity rapidly changes in the olfactory regions to ensure that our sense of smell is just as sharp when our nose recovers.

For one week, researchers blocked the noses of 14 participants while they lived in a special low-odor hospital room. After the smell deprivation, researchers saw two regions of the brain that reacted – there was an increase of activity in the orbital frontal cortex and decreased activity in the piriform cortex. When the participants were allowed to smell again, they were immediately able to perceive odors due to the brain activity during the seven days of deprivation. A week later, the brain’s response to odor had returned to pre-experiment levels, showing that the olfactory system is agile in response to changes. The researchers pointed out that this is different from other senses, such as sight, which takes much longer to recover after deprivation. They hypothesized that the olfactory system has adapted because smell deprivation is a common occurrence because of allergies and viral infections.