Spotlight On: Smell

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Each time we stop to smell the roses, we should thank our sense of smell (olfaction) that allows us to detect airborne chemicals (odors) from the environment. Olfaction is one of the ancient senses used by animals to monitor the external environment.

Although olfactory systems vary in different animals—insects, for example, use antennae, while dogs have something called a vomeronasal organ—receptors involved with recognizing different smells are “conservative” across the animal kingdom. This means they are genetically and structurally similar in different animals. In 1991, Linda Buck, PhD, who worked as a postdoctoral researcher in the lab of molecular biologist Richard Axel, MD, discovered a pool of genes that encode olfactory receptors. This groundbreaking discovery won Buck and Axel the Nobel Prize in Physiology or Medicine in 2004 and helped outline the basic organization, genetics and molecular mechanisms of the olfactory system.

The human nose has 350 kinds of olfactory receptors—proteins that bind to odor molecules. Each scent-sensing nerve cell (olfactory neuron) embedded in the moist nasal mucus membranes has only one receptor type and can bind only one substance that gives off a scent (odorant). Olfactory neurons are constantly exposed to the external environment and only live for 30 to 60 days. Stem cells constantly replenish our pool of olfactory neurons, which is why people who temporarily lose their sense of smell due to COVID-19 regain olfaction back shortly upon recovery.

When the receptor for an olfactory neuron binds to an odorant molecule, it’s like a key fitting into a lock. This binding activates a signaling cascade that produces an electrical signal that passes through the roof of the nose to enter two olfactory bulbs, which are structures located beneath the frontal lobes of the brain.

Inside the olfactory bulbs are spherical structures called glomeruli. These glomeruli receive fibers from the olfactory neurons with a matching receptor type. Each receptor is dedicated to a specific odor, such as the smell of roses, freshly baked cookies or a skunk. Other areas of the brain process the signals from these odor-specific glomeruli to help us recognize the scents.

Our sense of smell greatly enhances our sense of taste. Between 75% and 95% of what we believe is taste is actually coming from smell. Thus, without the wonderful aroma of freshly brewed coffee, we will taste just a bitter fluid. Apples and onions will taste almost the same if we couldn’t smell them.

Frequent colds and air pollution can damage our olfactory system and may reduce our sense of smell over time. Exercise helps boost olfaction by increasing air flow to the nose and delivering more odorants to receptors.

Please take care of your nose by keeping it warm this winter. And stay active to preserve your sense of olfaction for years to come.

Natalya Zinkevich, PhD, is an assistant professor at the University of Illinois at Springfield. She teaches courses related to human anatomy and physiology, health and disease, and vertebrate zoology. Her research primarily focuses on the cardiovascular system.

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