Cranial nerve · one in each half of the brain
Olfactory nerve & bulb (I)
Also called Cranial nerve I, CN I, Olfactory bulb, Smell nerve
Your smell line: the only sense that skips the thalamus and heads straight to emotion.
Smell starts with millions of tiny sensor cells in the roof of your nose. Their thin fibers poke up through a bone full of little holes and plug into the olfactory bulb, a small bump under the front of the brain. From there, smell signals go almost straight to areas for emotion and memory. That is why one whiff of sunscreen or a certain perfume can bring back a memory in an instant.
Where it is
The sensor cells sit in a small patch of lining high up in the roof of your nose. The two bulbs rest on a thin plate of bone full of tiny holes, tucked under the front of the brain, behind and above the bridge of your nose.
What it does
Detecting smells
Each sensor cell carries one type of smell receptor. Humans have about 300 to 400 types, and every smell switches on its own mix of them, like a chord on a piano.
Sorting in the bulb
All the fibers carrying the same receptor type gather in the same small knot in the bulb, turning smells into a neat map.
Most of flavor
Aromas from food in your mouth drift up the back of the nose. That is why food tastes so bland when you have a stuffy cold.
Shortcut to feelings
Smell signals reach the amygdala and memory areas in very few steps. It is the only sense that does not first pass through the thalamus, the brain's main relay station.
Danger alarm
It warns you about smoke, gas leaks, and spoiled food, often before you can see any problem.
Think of the bulb as a mail sorting office: millions of scent messages arrive from the nose and get sorted into bins by smell type before being sent on.
Works with
- Superficial (cortical) amygdalaThis part of the amygdala gets smell signals directly from the bulb, which gives smells their instant emotional punch.
- Entorhinal cortexSmell signals reach this gateway to memory in just a couple of steps, helping smells trigger vivid memories.
- HippocampusThrough the entorhinal cortex, smells get tied to the places and moments where you first met them.
- Orbitofrontal cortexThis area is thought to be where smells become things you consciously notice and judge, like "delicious" or "gross", and blend with taste into flavor.
- Frontal lobeThe bulbs lie right under the frontal lobes, which is one reason a hard knock to the head can affect smell.
When it goes wrong
How doctors check it
With your eyes closed and one nostril gently pressed shut, you sniff something familiar like coffee or peppermint and name it. Then the other side is tested.
Losing smell
Colds, sinus problems, head injuries, and viruses like COVID-19 can take away smell. In a head injury, the delicate fibers can tear where they pass through the bone. Many people get some or all of it back over time.
An early clue
A fading sense of smell can show up years before the movement problems of Parkinson's disease, and is also common early in Alzheimer's disease.
For learning only, not medical advice. If you have a health concern, talk to a doctor.
Surprising facts
- Your smell sensor cells are among the very few nerve cells that keep being replaced throughout life. New ones grow from stem cells in the nose, roughly every few weeks to months.
- Smell is the only sense that does not first pass through the thalamus. It takes a shortcut to emotion and memory areas, which may be why smells can unlock such strong memories.
- Smell training, sniffing a few strong scents twice a day for months, is used to help some people recover their sense of smell after a virus.
By the numbers
Where the name comes from
From the Latin olfacere, "to smell" (from olere, "to give off a smell", and facere, "to make"). Bulb comes from the Latin bulbus, "onion", for its rounded shape.
Quick quiz
3 questions. Your answers are saved and come back in your daily review.
Which of these is a job of the olfactory nerve & bulb (I)?
Show the answer
Detecting smells. Each sensor cell carries one type of smell receptor. Humans have about 300 to 400 types, and every smell switches on its own mix of them, like a chord on a piano.
Detecting smells. Each sensor cell carries one type of smell receptor. Humans have about 300 to 400 types, and every smell switches on its own mix of them, like a chord on a piano.
The olfactory nerve & bulb (I) is part of which larger structure?
Show the answer
Cranial nerves. It belongs to the cranial nerves.
Cranial nerves. It belongs to the cranial nerves.
Which part fits this description? "Think of the bulb as a mail sorting office: millions of scent messages arrive from the nose and get sorted into bins by smell type before being sent on."
Show the answer
Olfactory nerve & bulb (I). It is the olfactory nerve & bulb (I). Your smell line: the only sense that skips the thalamus and heads straight to emotion.
Olfactory nerve & bulb (I). It is the olfactory nerve & bulb (I). Your smell line: the only sense that skips the thalamus and heads straight to emotion.
These parts now come back in your daily review, just before you would forget them.
Practice it and its neighbors in 3DSources
- 14.1 Sensory Perception, OpenStax, Anatomy and Physiology 2e
- 8.3 The Olfactory System, OpenStax, Introduction to Behavioral Neuroscience
- Smell Disorders, National Institute on Deafness and Other Communication Disorders (NIDCD)
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