Smell is chemistry wired straight into memory circuits
Olfaction detects foods, hazards, and pheromones and shapes taste. In humans, odorants bind nasal receptors, signals pool in glomeruli, then hit the olfactory bulb before cortex. Damage, congestion, infection, injury, or neurodegeneration can dull or erase the sense.
Smelling starts when an odour molecule latches onto a receptor inside the nose. Clusters called glomeruli collect the resulting signals and pass them to the olfactory bulb, where they begin to engage brain regions handling identification, memory and emotion. The Roman poet-philosopher Lucretius guessed in the first century BC that odours differ because their atoms differ in shape and size. A modern version of that idea came when Linda Buck and Richard Axel cloned olfactory receptor proteins, work that won the 2004 Nobel Prize: each receptor responds to only one molecular feature or class of odorant, and mammals carry roughly a thousand receptor genes.
Exactly how the brain decodes a smell is still argued over. Shape theories hold that receptors read features of a molecule, or in the odotope version just small fragments that are combined. Luca Turin's rival vibration theory proposes that receptors sense a molecule's infrared vibrations through quantum tunnelling, but its behavioural predictions have not held up well. Genetics explains much of why people perceive odours differently; the receptor OR5A1, for instance, governs whether someone can detect beta-ionone.
Much of what we call taste is smell. While chewing, odorants travel up the back of the throat on the out-breath, and this retronasal route dominates flavour. The tongue separates only five basic tastes, whereas the nose can tell apart hundreds of substances in tiny amounts. Smell also carries social information: mothers can pick out their biological children by body odour but not stepchildren, and fish, mice and women can sense something of a partner's immune-system genes, generally favouring mates whose genes differ.
Ability varies hugely across animals. Scent hounds are estimated at one to ten million times more sensitive than people, bloodhounds more still, and grizzly bears reportedly outdo bloodhounds sevenfold, catching food smells up to eighteen miles off. Most birds smell poorly, apart from tubenoses such as albatrosses, some New World vultures and kiwis, and whales and dolphins lack the sense entirely. Salmon use smell to find their home streams. In people, congestion, infections, head injuries and neurodegenerative disease can all impair it.
Source: Sense of smell
