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Sniff longer to smell better
Neurons that replace themselves
Olfactory neurons are the heavyweights of turnover. Most neurons in the central nervous system come as one set for life, while these are replaced throughout the lifespan. How they turn over and tune to particular smells depends on their electrical activity, and sniffing odors is what generates it.
How to practice sniffing
Practice looks like laying out a few very different smells, such as a lemon and a rose, closing your eyes, and taking more sniffs than seems necessary, with long, deep inhales and full attention. The more times you sniff and the longer the inhales, the more those cells fire in the presence of the odor. Noxious smells are left out because a hardwired pathway detects them, and it persists unless the olfactory system is damaged.
Smell loss after injury
A respiratory illness or a head hit can shear off the olfactory axons where they pass through the cribriform plate, the Swiss-cheese-shaped bone at the back of the nose, and a person can lose smell for a period. Sniff training has been shown to help, especially right after the injury, and some evidence suggests smell can even improve over time.
Why do smell neurons keep regenerating?
Olfactory neurons sit in the lining of the nose, where they are exposed to air carrying dust, microbes, and chemicals. That exposure wears them out, so stem cells at the base of the lining keep producing replacements. The turnover takes roughly a month or two for each cell. Few other neurons in the adult human nervous system are replaced this way.
Why is smell so easy to injure?
Olfactory nerve fibers run from the nose to the brain through tiny holes in the cribriform plate, a thin bone between the nasal cavity and the skull. A blow to the head can make the brain shift and shear those fibers where they pass through. Viral infections can instead damage the supporting cells of the nasal lining. Head trauma is one of the best-known causes of a lost sense of smell.
How was smell training first tested?
Thomas Hummel and colleagues at the University of Dresden tested training with four odors, rose, eucalyptus, lemon, and cloves. Their 2009 study in The Laryngoscope compared people with smell loss who trained with a group who did not. The trained group improved more on standard smell tests.
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