Lably
Skip to content

Odor

Volatile chemical compounds perceived by the sense of smell

  • 25 min read
  • 6 chapters
Odor
Jan Brueghel the Elder / Peter Paul Rubens · Public domain

Related on Lably

Through American and British English spelling differences, Neuron and Organic compound

LabWord Ladder WorkshopLab · Class 11Neural Control and Coordination — Animated Chapter FilmLab · Class 10–11Neural ControlLab · Class 11One Day Inside the Human Body — Animated Chapter Film

Odor physiology

Sense of smell

The perception of odors, or sense of smell, is mediated by the olfactory nerve. The olfactory receptor (OR) cells are neurons present in the olfactory epithelium, which is a small patch of tissue at the back of the nasal cavity. There are millions of olfactory receptor neurons that act as sensory signaling cells. Each neuron has cilia in direct contact with the air. Odorous molecules bind to receptor proteins extending from cilia and act as a chemical stimulus, initiating electric signals that travel along the olfactory nerve's axons to the brain.

When an electrical signal reaches a threshold, the neuron fires, which sends a signal traveling along the axon to the olfactory bulb, a part of the limbic system of the brain. Interpretation of the smell begins there, relating the smell to past experiences and in relation to the substance(s) inhaled. The olfactory bulb acts as a relay station connecting the nose to the olfactory cortex in the brain. Olfactory information is further processed and forwarded to the central nervous system (CNS), which controls emotions and behavior as well as basic thought processes.

Odor sensation usually depends on the concentration (number of molecules) available to the olfactory receptors. A single odorant is usually recognized by many receptors. Different odorants are recognized by combinations of receptors. The patterns of neuron signals help to identify the smell. The olfactory system does not interpret a single compound, but instead the whole odorous mix. This does not correspond to the concentration or intensity of any single constituent.

Most odors consist of organic compounds, although some simple compounds not containing carbon, such as hydrogen sulfide and ammonia, are also odorants. The perception of an odor effect is a two-step process. First, there is the physiological part. This is the detection of stimuli by receptors in the nose. The stimuli are recognized by the region of the human brain which handles olfaction. Because of this, an objective and analytical measure of odor is impossible. While odor feelings are personal perceptions, individual reactions are usually related. They relate to things such as gender, age, state of health, and personal history.

Smell acuity by age and gender

The ability to identify odor varies among people and decreases with age. Studies claim that there are sex differences in odor discrimination, and that women usually outperform men. Conversely, there are some studies claiming a male advantage. A 2019 meta-analysis claimed that the differences in olfaction are extremely small, but confirmed a small advantage for women.

Pregnant women have increased smell sensitivity, sometimes resulting in abnormal taste and smell perceptions, leading to food cravings or aversions. The ability to taste also decreases with age as the sense of smell tends to dominate the sense of taste. Chronic smell problems are reported in small numbers for those in their mid-twenties, with numbers increasing steadily, with overall sensitivity beginning to decline in the second decade of life, and then deteriorating appreciably as age increases, especially once over 70 years of age.

Smell acuity compared to other animals

For most untrained individuals, the act of smelling acquires little information concerning the specific ingredients of an odor. Their smell perception primarily offers information that elicits an emotional response. Experienced individuals, however, such as flavorists and perfumers, can identify discrete chemicals in complex mixtures using only their sense of smell.

Odor perception is a primary evolutionary sense. The sense of smell can induce pleasure or subconsciously warn of danger, which may, for example, help to locate mates, find food, or detect predators. Humans have an unusually good sense of smell considering they have only 350 functional olfactory receptor genes compared to the 1,300 found in mice, for example. This is despite an apparent evolutionary decline in the sense of smell. The human sense of smell is comparable with many animals, able to distinguish between a diverse range of odors. Studies have reported that humans can distinguish in the region of one trillion unique aromas.

Habituation or adaptation

Odors that a person is used to, such as their own body odor, are less noticeable than uncommon odors. This is due to "habituation." After continuous odor exposure, the sense of smell is fatigued, but recovers if the stimulus is removed for a time. Odors can change due to environmental conditions: for example, odors tend to be more distinguishable in cool dry air.

Habituation affects the ability to distinguish odors after continuous exposure. The sensitivity and ability to discriminate odors diminishes with exposure, and the brain tends to ignore continuous stimulus and focus on differences and changes in a particular sensation. When odorants are mixed, a habitual odorant is blocked. This depends on the strength of the odorants in the mixture, which can change the perception and processing of an odor. This process helps classify similar odors as well as adjust sensitivity to differences in complex stimuli.

Genetic component

The primary gene sequences for thousands of olfactory receptors are known for the genomes of more than a dozen organisms. They are seven-helix-turn transmembrane proteins. But there are no known structures for any olfactory receptor. There is a conserved sequence in roughly three quarters of all ORs. This is a tripodal metal-ion binding site, and Suslick has proposed that the ORs are in fact metalloproteins (most likely with zinc, copper, and manganese ions) that serve as a Lewis Acid site for the binding of many odorant molecules. In 1978, Crabtree suggested that Cu(I) is "the most likely candidate for a metallo-receptor site in olfaction" of strong-smelling volatiles. These are also good metal-coordinating ligands, such as thiols. In 2012, Zhuang, Matsunami, and Block confirmed the Crabtree/Suslick proposal for the specific case of a mouse OR, MOR244-3, showing that copper is essential for detection of certain thiols and other sulfur-containing compounds. Thus, by using a chemical that binds to copper in the mouse nose, so that copper was not available to the receptors, the authors showed that the mice could not detect the thiols without the copper. However, these authors also found that MOR244-3 lacks the specific metal ion binding site suggested by Suslick, instead showing a different motif in the EC2 domain.

Evolutionary impact

Gordon Shepherd proposed that the retro-nasal route of olfaction (odorants introduced to the olfactory mucosa through the oral cavity often as food) was partially responsible for the development of human olfactory acuity. He suggested the evolutionary pressure of diversification of food sources and increased complexity of food preparation presented humans with a broader range of odorants, ultimately leading to a "richer repertoire of smells". Animals such as dogs show a greater sensitivity to odors than humans, especially in studies using short-chain compounds. Higher cognitive brain mechanisms and more olfactory brain regions enable humans to discriminate odors better than other mammals despite fewer olfactory receptor genes.

Sign in to keep reading

5 more chapters are free with a Lably account.

More in Anatomy and morphology

BrainOrgan central to the nervous systemNeuronPrimary cell of the nervous systemCentral nervous systemBrain and spinal cordLeafPhotosynthetic part of a vascular plantImmune systemBiological system protecting an organism against diseaseLiverVertebrate organ involved in metabolism
Source and licence. Adapted from the Wikipedia article “Odor” (this version, 2026-10-01) by its contributors, shared under CC BY-SA 4.0. Lablypedia has left out its tables, references and most pictures, and linked it to other Lablypedia articles.