Sense of smell

• 1. Introduction

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  The sense of smell, known to scientists as olfaction, is quietly one of the most astonishing things the human body does. Research has suggested that the average person can distinguish over one trillion unique odors. That number was described by the researchers themselves as "conservative." The nose, it turns out, outperforms every other human sense in the sheer number of physically different stimuli it can discriminate. How does a cluster of nerve cells translate floating molecules into memory, appetite, and attraction? And why, after centuries of study, do scientists still lack a complete theory of how smell actually works?

• 2. A History of the Question

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  The Roman philosopher Lucretius, writing in the 1st century BC, proposed that different odors arise from different shapes and sizes of atoms. The idea sat dormant as speculation for nearly two thousand years. Eleanor Gamble's doctoral dissertation of 1898 took a more empirical turn, comparing smell to other senses and suggesting it had a lower intensity discrimination than they did. The decisive modern advance came when Linda B. Buck and Richard Axel cloned the olfactory receptor proteins that give the nose its detecting power. Their work paired specific odor molecules to specific receptor proteins, and in 2004 they were awarded the Nobel Prize. Mammals, the research revealed, carry about a thousand genes devoted to odor reception, though only a portion of those genes are functional. Humans have far fewer active odor receptor genes than other primates and other mammals.

• 3. How the Signal Travels

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  When an odor molecule drifts into the nasal cavity, it dissolves in the mucus lining the superior portion of the cavity. That mucus is replaced approximately every ten minutes, keeping the surface fresh. The molecule then binds to a receptor on a sensory neuron, operating like a key fitting a lock: only specific molecular features unlock specific nerve cells. This binding triggers a cascade involving a G protein called Golf, which stimulates the synthesis of cAMP, which opens an ion channel, which lets calcium and sodium into the cell. The resulting electrical signal travels down olfactory nerve fibers, which lack the myelin sheaths that speed up most other nerves. Those bare fibers pass through perforations in a bone called the cribriform plate and reach the olfactory bulb. In the bulb, roughly 25,000 axons converge on just 25 or so mitral cells within each tiny glomerulus, structures about 50 micrometers in diameter. From there the signal fans out to the anterior olfactory nucleus, the olfactory tubercle, the amygdala, the piriform cortex, and the entorhinal cortex. The entorhinal cortex projects to the hippocampus and is involved in motivation and memory. Crucially, the olfactory system is the only human sense that bypasses the thalamus entirely, connecting directly to the forebrain.

• 4. Smell and the Self

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  Mothers can identify their biological children by body odor alone, but not their stepchildren. Pre-adolescent children can detect their full siblings this way but not half-siblings or step-siblings. Functional imaging has traced this olfactory kin detection to the frontal-temporal junction, the insula, and the dorsomedial prefrontal cortex, not the primary olfactory cortices. The MHC genes, known as HLA in humans, govern much of the immune system, and offspring from parents with differing MHC genes tend to have stronger immunity. Fish, mice, and female humans can detect some aspect of a potential partner's MHC genes through smell, and most prefer partners whose MHC genes differ from their own. Some research suggests that hormonal contraception can shift that preference toward partners with more similar MHC genes. In the house mouse, a cluster called the major urinary protein (MUP) gene cluster provides a highly polymorphic scent signal that appears to underlie both kin recognition and inbreeding avoidance.

• 5. The Animal Spectrum

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  Dogs in general have an olfactory sense roughly ten thousand to one hundred thousand times more acute than a human's. Scent hounds as a group can smell one to ten million times more acutely than a human, while bloodhounds, the keenest-nosed dogs of all, have noses ten to one hundred million times more sensitive. They were bred specifically to track humans and can follow a scent trail several days old. The second most sensitive nose among dogs belongs to the Basset Hound, bred to track rabbits and small animals. Grizzly bears push even further: their sense of smell is seven times stronger than that of the bloodhound, and they can detect food from up to eighteen miles away. Humans, by contrast, have about 10 cm2 of olfactory epithelium. Some dogs have 170 cm2, and their epithelium is about a hundred times more densely innervated per square centimeter. At the other end of the scale, cetaceans have no functional sense of smell at all, compensating with a well-developed sense of taste. The kiwi, the tubenoses such as petrels and albatrosses, and certain new world vultures are notable exceptions among birds, which mostly lack a strong olfactory sense.

• 6. Flavor, Sound, and Coded Perception

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  Retronasal smell plays the biggest role in the sensation of flavor. During chewing, the tongue releases odorants that enter the nasal cavity on the exhale rather than the inhale, creating the sensation that the smell is coming from inside the mouth. The human tongue distinguishes only five distinct taste qualities, while the nose can distinguish among hundreds of substances even in minute quantities. Smell and sound information has been shown to converge in the olfactory tubercles of rodents, giving rise to a perception researchers call a "smound," a sensory hybrid analogous to the way smell and taste blend into flavor. The process by which the brain codes olfactory signals is still not fully understood. Each odor produces a particular spatial map of excitation in the olfactory bulb, but these maps shift over time, and temporal coding must also be factored in. Half the neurons in the orbitofrontal cortex, which mediates conscious smell perception, respond to only one odor, while the rest respond to only a few, suggesting a high degree of specialization at the top of the pathway. Genetic variation accounts for most of the differences in what individuals can smell: the receptor OR5A1 governs the ability to detect beta-ionone, a key aroma compound in foods and beverages, while OR6A2 has been linked to the perception of cilantro.

• 7. Disorders and Cultural Value

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  About 50% of patients infected with SARS-CoV-2 experience some form of smell disorder, including anosmia (complete inability to smell) and parosmia (distorted odor perception). SARS-CoV-1, MERS-CoV, and influenza can also disrupt olfaction. The vocabulary for smell disorders is unusually precise: presbyosmia names the natural decline of smell in old age, hyperosmia an abnormally acute sense, and phantosmia the hallucination of an odor when none is present. Desiderosmia, a compulsive craving for specific odors, has been associated with iron deficiency anemia. Western cultures have historically ranked smell below vision and hearing, treating it as a "chemical" sense tied to the body rather than the mind, partly because smells are so hard to describe without pointing to their source. Japan offers a contrasting tradition: Kodo, the art of appreciating incense, elevates smell to a refined cultural practice. Marcel Proust made the memory-triggering power of aroma central to In Search of Lost Time, a connection the source traces to the olfactory system's direct anatomical links to the limbic system and the hippocampus. The Zwaardemaker smell system, invented by Hendrik Zwaardemaker, and the Crocker-Henderson system, which rates smells on a 0-8 scale across four primary categories, are among the classification frameworks scientists have developed to bring order to this most elusive of senses.