[2] The sense of smell has many functions, including detecting desirable foods, hazards, and pheromones, and plays a role in taste.

[3] Glomeruli aggregate signals from these receptors and transmit them to the olfactory bulb, where the sensory input will start to interact with parts of the brain responsible for smell identification, memory, and emotion.

[13] According to a new study, researchers have found that a functional relationship exists between molecular volume of odorants and the olfactory neural response.

[19] The smell of food has the sensation of being in the mouth because of co-activation of the motor cortex and olfactory epithelium during mastication.

In the house mouse, the major urinary protein (MUP) gene cluster provides a highly polymorphic scent signal of genetic identity that appears to underlie kin recognition and inbreeding avoidance.

A number of scent-tracking strategies have been studied in different species, including gradient search or chemotaxis, anemotaxis, klinotaxis, and tropotaxis.

For instance, the odorant receptor OR5A1 and its genetic variants (alleles) are responsible for our ability (or failure) to smell β-ionone, a key aroma in foods and beverages.

Figures suggesting greater or lesser sensitivity in various species reflect experimental findings from the reactions of animals exposed to aromas in known extreme dilutions.

That is, the brain's smell-recognizing centers must react to the stimulus detected for the animal to be said to show a response to the smell in question.

The second-most-sensitive nose is possessed by the Basset Hound, which was bred to track and hunt rabbits and other small animals.

Grizzly bears have a sense of smell seven times stronger than that of the bloodhound, essential for locating food underground.

Using their elongated claws, bears dig deep trenches in search of burrowing animals and nests as well as roots, bulbs, and insects.

Bears can detect the scent of food from up to eighteen miles away; because of their immense size, they often scavenge new kills, driving away the predators (including packs of wolves and human hunters) in the process.

In many species, smell is highly tuned to pheromones; a male silkworm moth, for example, can sense a single molecule of bombykol.

Although conventional wisdom and lay literature, based on impressionistic findings in the 1920s, have long presented human smell as capable of distinguishing between roughly 10,000 unique odors, recent research has suggested that the average individual is capable of distinguishing over one trillion unique odors.

[42] Researchers in the most recent study, which tested the psychophysical responses to combinations of over 128 unique odor molecules with combinations composed of up to 30 different component molecules, noted that this estimate is "conservative" and that some subjects of their research might be capable of deciphering between a thousand trillion odorants, adding that their worst performer could probably still distinguish between 80 million scents.

The mucus overlying the epithelium contains mucopolysaccharides, salts, enzymes, and antibodies (these are highly important, as the olfactory neurons provide a direct passage for infection to pass to the brain).

cAMP, which is the second messenger here, opens a cyclic nucleotide-gated ion channel (CNG), producing an influx of cations (largely Ca2+ with some Na+) into the cell, slightly depolarising it.

This mechanism of transduction is somewhat unusual, in that cAMP works by directly binding to the ion channel rather than through activation of protein kinase A.

It is similar to the transduction mechanism for photoreceptors, in which the second messenger cGMP works by directly binding to ion channels, suggesting that maybe one of these receptors was evolutionarily adapted into the other.

In vertebrates, responses to an odor can be measured by an electro-olfactogram or through calcium imaging of receptor neuron terminals in the olfactory bulb.

Neuromodulators like acetylcholine, serotonin and norepinephrine all send axons to the olfactory bulb and have been implicated in gain modulation,[57] pattern separation,[58] and memory functions,[59] respectively.

[61] In insects, smells are sensed by sensilla located on the antenna and maxillary palp and first processed by the antennal lobe (analogous to the olfactory bulb), and next by the mushroom bodies and lateral horn.

The process by which olfactory information is coded in the brain to allow for proper perception is still being researched, and is not completely understood.

The signals from the glomeruli are transformed to a pattern of oscillations of neural activities[64] of the mitral cells, the output neurons from the olfactory bulb.

[71] While chemotopy remains a highly controversial concept,[72] evidence exists for perceptual information implemented in the spatial dimensions of olfactory networks.

The two organs insects primarily use for detecting odors are the antennae and specialized mouth parts called the maxillary palps.

Many air management districts in the US have numerical standards of acceptability for the intensity of odor that is allowed to cross into a residential property.

For example, the Bay Area Air Quality Management District has applied its standard in regulating numerous industries, landfills, and sewage treatment plants.

Systems of classifying odors include: Specific terms are used to describe disorders associated with smelling: Viruses can also infect the olfactory epithelium leading to a loss of the sense of olfaction.