Why do smell neurons get replaced but brain neurons do not?

Smell neurons sit exposed to dry air, dust, germs, and chemicals with every breath, so the body treats them as disposable. Most brain neurons are the ones you were born with; smell neurons are the exception, built to be thrown away and rebuilt.

Why do you still remember smells from years ago if the sensors are new?

Because the replacements plug into the same brain circuits as the neurons they replace. The sensors are new, but the wiring diagram they feed has not changed since childhood.

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Does Your Nose Really Replace Its Smell Sensors Every Month?

Q: Does your nose really replace its smell sensors every month?

Yes. The neurons that detect smell live only about 30 to 60 days before they die off, and stem cells in the nose lining grow brand-new ones to take their place, your entire life.

Yes. The neurons that detect smell only live about 30 to 60 days before they die off — and stem cells in your nose lining grow brand-new ones to take their place, your entire life.

Quick facts

Smell neuron lifespan: about 30 to 60 days
Replacement crew: stem cells at the base of the nose's smell lining
The replacements plug into the same brain wiring as the ones they replace
Smell neurons are the only neurons in your body that regularly grow back
Research range note: roughly 30 days in mice, closer to 60 in humans

A sensor with an expiration date

Your sense of smell runs on millions of tiny neurons sitting high inside your nose, each one topped with little antenna-like hairs that catch odor molecules drifting in with every breath. It's a rough neighborhood — every sniff exposes those cells to dry air, dust, germs, and chemicals — so your body treats them as disposable. Each smell neuron works for a few weeks, then dies.

Most neurons don't get that treatment. The neurons in your brain are mostly the ones you were born with. Your smell neurons are the exception: they are built to be thrown away and rebuilt, over and over, for as long as you live.

Who builds the replacements?

Underneath the smell lining sits a layer of stem cells whose whole job is manufacturing fresh smell neurons. When an old neuron dies, a new one grows in its place — and here's the remarkable part: the new neuron has to stretch a wire all the way to the same spot in the brain's smell center that the old one used. Wire it wrong, and one smell would register as a different one.

Why do you still remember smells from years ago?

Because the replacements plug into the same circuits, your smell memories survive the turnover. The sensors are new; the wiring diagram they're feeding hasn't changed since childhood. That's why a smell you haven't run into in twenty years can hit you like it happened yesterday — the hardware got swapped out dozens of times, but the connections it reports to stayed put.

Watch it

See the short version in the LOE PRESSURE "Things You Didn't Know" series — "Your Nose Replaces Its Smell Sensors Every Month."

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Keep learning

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Fascinating facts you didn't know. Come learn with me — no pressure.

Sources: Scientific American, "Experts: Olfactory Neuron Turnover" — https://www.scientificamerican.com/article/experts-olfactory-neuron-turnover/; ScienceDaily / UC Berkeley (2011) — https://www.sciencedaily.com/releases/2011/12/111207132649.htm.