How does the sense of smell work?
One-line takeaway
BLUF: Odor molecules drift into your nose, dissolve in mucus, and bind to specialized receptor proteins on nerve cells high in the nasal cavity. Those cells fire signals to the brain's olfactory bulb, which the brain reads as a distinct smell.
Smell shapes flavor, warns of danger, and triggers memory, and its sudden loss can be an early sign of illness.
How smell works
Smell, or olfaction, begins when volatile molecules from food, flowers, or smoke float through the air and enter your nose as you breathe. High in the nasal cavity sits a patch of tissue called the olfactory epithelium, packed with millions of sensory neurons. Each neuron sprays out tiny hair-like cilia coated in mucus. Odor molecules dissolve in that mucus and lock onto receptor proteins on the cilia, like keys fitting loosely into locks. When a molecule binds, the neuron fires an electrical signal up through the skull to the olfactory bulb, a relay station beneath the front of the brain. From there the pattern of activity spreads to regions that identify the odor and tie it to emotion and memory.
The combinatorial code
Humans carry only about 400 working types of odor receptor, yet we distinguish an enormous range of smells. The trick is combinatorial coding: most odor molecules activate several receptor types at once, and each receptor responds to several different molecules. A given scent therefore produces a unique pattern across many receptors, like a chord rather than a single note, and the brain learns to read these patterns. Linda Buck and Richard Axel mapped this receptor family in 1991, work that won the 2004 Nobel Prize in Physiology or Medicine. The genes encoding these receptors form the largest gene family in the mammalian genome, a sign of how much evolution invested in detecting chemicals in the air.
Smell in the real world
Smell drew wide public attention when COVID-19 caused sudden loss of it, or anosmia, in millions of people. Researchers found the virus mainly attacks support cells in the olfactory epithelium rather than the neurons themselves, which helps explain why most people recover. Loss or distortion of smell also tends to appear early in Parkinson's and Alzheimer's disease, making smell tests a promising screening tool. Meanwhile, much of what we call flavor actually comes from smell: aromas rise from the mouth to the nose through the back of the throat, a route called retronasal olfaction. That is why food tastes flat when a cold blocks your nose, and why perfumers and chefs train their noses as carefully as musicians train their ears.
Common misconceptions
Myth: humans have a feeble sense of smell compared with animals. Reality: studies show people can detect and discriminate a vast number of odors, rivaling many mammals. Myth: we can distinguish exactly 10,000 smells. Reality: that figure was a rough early-1900s estimate with no solid basis; the real number is far larger and hard to pin down. Myth: the tongue handles taste and the nose is a separate system. Reality: much of flavor is smell, working through the back of the throat. Myth: each odor lights up one dedicated spot, like a smell map. Reality: every odor activates a scattered combination of receptors. Myth: losing smell is trivial. Reality: it can signal disease and drains food, safety, and memory of vividness.