symptoms

Your Brain on Odors: A HealthCentral Explainer

ATsai Editor August 21, 2012
  • 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.

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    Is smell connected to taste?

    Smell and taste are closely linked because they are both part of the chemosensory system, and many people who think they have a taste disorder actually have a smell disorder.  When your nose is stuffed due to a cold or allergies, food odors cannot reach the sensory cells and the flavor is lost. Without these olfactory sensory neurons, flavors such as chocolate or oranges are difficult to distinguish.

     

    Sometimes when smell is impaired people will change their eating habits to include more salt, which can be problematic for people with high blood pressure or kidney disease.

     

    Is smell linked to other health conditions?

    There are many health conditions that can affect the sense of smell, including sinus and upper respiratory infections, polyps in the nasal cavities, frontal head injuries, hormone issues, dental problems, exposure to certain chemicals, medications such as antihistamines and antibiotics, radiation for the treatment of neck cancers and aging. More serious conditions that affect the nervous system can also cause loss of smell, such as Alzheimer’s disease, Parkinson’s disease and multiple sclerosis. Plus, smell disorders can also be a sign of obesity, diabetes, hypertension and malnutrition. Smoking can likewise affect our sense of smell, and loss of smell can also lead to depression.

     

    Loss of smell is one of the first signs of Alzheimer’s disease. However, a study  published in the Journal of Neuroscience in 2011 found that it may be possible to restore. Researchers discovered that removing a certain plaque-forming protein, called amyloid beta, restored the sense of smell in mice. To remove the protein, the mice were given a liver x-receptor agonist. After two weeks on the drug, the mice were able to process smells normally.  Then after withdrawal from the drug for a week, the mice could not smell normally. This protein has been found to accumulate first in the parts of the brain associated with smell long before it builds up in areas associated with cognition and coordination.  Researchers believe that this could lead to earlier diagnosis and treatment of Alzheimer’s patients.

     

     

    Can it have an effect on emotional health?

    The loss of smell can have such an impact on quality of life that it can lead to depression. But it’s not the only emotional or psychological effect that can occur. One study published in March 2012 in Chemosensory Perception found that anxious people have a heightened sense of smell.  Researchers exposed 14 young adults to three types of smell: a pure neutral odor, a mixture of neutral odors and a negative odor mixture. They were asked to identify the presence or absence of these odors while in an MRI scanner. During this time, they also measured arousal level by the skin’s ability to conduct electricity, and measured breathing patterns. Participants were asked to rate their anxiety level after the odor test, while they were still in the scanner. When the researchers analyzed the brain images, they found that as anxiety levels rose, the ability to detect negative odors accurately also increased.

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    Another study, which was published in PLoS ONE in March 2012 found that people who were born without the sense of smell had higher social insecurity and an increased risk of depression.

     

    Sources:

    National Institute of Deafness and Other Communication Disorders. (2012, April 22). “Smell Disorders.” Retrieved from http://www.nidcd.nih.gov/health/smelltaste/pages/smell.aspx

     

    n.p. (2012, August 13). "Brain Changes After A Stuffed Nose Protect The Sense Of Smell." Medical News Today. Retrieved from http://www.medicalnewstoday.com/releases/248941.php

     

    n.p. (2011, December 1). "Loss Of Sense Of Smell, Early Sign Of Alzheimer's, Reversed In Lab." Medical News Today. Retrieved from http://www.medicalnewstoday.com/releases/238456.php

     

    Springer. (2012, March 26). "Sense Of Smell Boosted By Anxiety." Medical News Today. Retrieved from http://www.medicalnewstoday.com/releases/243302.php

     

    Public Library of Science. (2012, March 23). "Those Without A Sense Of Smell Suffer Enhanced Social Insecurity." Medical News Today. Retrieved from http://www.medicalnewstoday.com/releases/243216.php