- The Washington Times - Thursday, February 21, 2008

Olney resident Donna Saur can remember waking up one morning, hoping it would be the day she could smell the coffee brewing.

The virus that caused Ms. Saur’s two-week cold in October 2001 attacked the olfactory nerves in her nose, causing her temporarily to lose her sense of smell. The loss made it difficult for her to taste food, determine whether food was spoiled and identify potential dangers, such as a fire or gas leak.

“I felt claustrophobic. I could sniff and not smell anything,” Ms. Saur says. “It takes a lot of the pleasure out of eating. … It makes the texture of food a lot more important.”

At the tail end of her cold, Ms. Saur saw an otolaryngologist, who put her on a steroid medication. Her sense of smell returned gradually over the next year — first the bad odors, followed by savory and finally sweet odors, she says.

Sensory cells, also called receptor neurons, in the nose will regenerate, says Charles J. Wysocki, a member of the Monell Chemical Senses Center, a nonprofit scientific institute in Philadelphia that researches the senses of taste and smell and chemosensory irritation.

“Any one sensory cell lives for about a month in mammals,” says Mr. Wysocki, who holds a doctorate in neuroscience. “Since we have millions of them, to have them all turn over in a month, we need a lot replaced on a daily basis.”

The sense of smell is triggered by odorant molecules, most of which are volatile, or moving through the air, says Steven Munger, associate professor of anatomy and neurobiology at the University of Maryland School of Medicine in Baltimore. He holds a doctorate in neuroscience.

When a person breathes in or sniffs the air, the air flow carries the odorant molecules into the superior aspect, or upper region, of the nose, Mr. Munger says. The molecules come in contact with nerve cells within the nose called the olfactory epithelium, he says.

Mucus that overlays these nerve cells helps absorb the odorant molecules, says Dr. Philip Zapanta, assistant professor of surgery in the division of otolaryngology at George Washington University.

When the odorant molecules hit the mucus layer, certain proteins in the layer bind up the odorant molecules and carry them to the olfactory epithelium, Dr. Zapanta says.

“This causes a chain of cellular and neural events that stimulate the olfactory cortex,” says Dr. Zapanta, referring to the part of the brain responsible for odor processing and recognition. “It’s basically like a tour guide guiding one to the correct destination.”

Odors are made of particles so small they cannot be seen, though they can be detected through dissolving in the nasal cavity, says Dr. Stanley Chia, otolaryngologist at Washington Hospital Center and assistant professor of otolaryngology at Georgetown University.

The odors are interpreted in the olfactory bulbs on each side of the brain, Dr. Chia says. Odorant receptors in the sensory neurons do the interpreting, he says.

Humans have 300 to 400 odor receptors encoded by distinct genes that can be activated by tens of thousands of different odors, says Barry Davis, director of the Taste and Smell Program at the National Institute on Deafness and Other Communication Disorders, a branch of the National Institutes of Health in Bethesda.

“Different odors activate different receptors,” says Mr. Davis, who holds a doctorate in neuroscience. “In combination, you get a pattern of activity that would be the signature of that odor.”

Each receptor can recognize a small subset of odor chemicals, while an individual odor molecule usually is recognized by more than one receptor, Mr. Davis says.

The receptors signal a neurological response through the nerves in the criviform plate, an area of the skull with small holes so nerves can pass through between the nose and brain, Dr. Chia says.

The extension of the receptor cell called the axon goes through the plate into the olfactory bulb, Mr. Wysocki says.

“Different parts of the bulb are activated, and this is communicated to the brain,” he says.

The electrical pulses traveling down on the nerve’s axon converge on the dendrites, the extension of the next cell coming out of the olfactory bulb, Mr. Wysocki says.

“Those nerve impulses activate the next cell through a region called the synapses,” he says. “The olfactory axon is trying to make contact with the main output cell from the olfactory bulb.”

The receptors are stimulated to a greater or lesser degree by the different odorant molecules, Dr. Chia says.

“It’s the combination of the thousands of neurons being stimulated that allow us to interpret the smell,” he says.

However, whether odors result in the same smell is hard to determine, Dr. Chia says.

“People can consistently identify smells, but if they interpret it in the same way is unknown,” he says.

Smells cannot be determined by the substance, either, Mr. Wysocki says.

“You can’t predict what something’s going to smell like by looking at the chemistry of it,” he says. “Whether it has a smell or not is independent of the material. It’s what’s doing the smelling that’s important.”

In addition, the receptors that do the interpreting can be damaged or destroyed by a nasal viral infection or head trauma, Mr. Davis says.

Receptors “have the capacity to replace themselves, but if the damage is extensive, they can’t,” he says.

The most common way to lose some sense of smell is through aging, typically by age 60 or 70, Mr. Davis says.

“Complete loss of smell is a rare disease,” he says.

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