Basics
The sense of smell gives rise to the perception of odors, mediated by the olfactory nerve. The olfactory receptor (OR) cells are neurons present in the olfactory epithelium, a small patch of tissue in 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 air. The olfactory nerve is considered the smell mediator, the axon connects the brain to the external air. Odorous molecules act as a chemical stimulus. Molecules bind to receptor proteins extended from cilia, initiating an electric signal.
The primary sequences of thousands of olfactory receptors are known from the genomes of more than a dozen organisms: they are seven-helix transmembrane proteins, but there are (as of July 2011) no known structures of any OR. There is a highly conserved sequence in roughly three quarters of all ORs that is a tripodal metal ion binding site, and Suslick has proposed that the ORs are in fact metalloproteins (most likely with zinc, copper and possibly manganese ions) that serve as a Lewis Acid site for binding of many odorant molecules. Crabtree, in 1978, had previously suggested that Cu(I) is "the most likely candidate for a metallo-receptor site in olfaction" for strong-smelling volatiles which are also good metal-coordinating ligands, such as thiols. Zhuang, Matsunami and Block, in 2012, 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 wasn’t available to the receptors, the authors showed that the mice couldn't detect the thiols. 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.
When the signal reaches a threshold, the neuron fires, sending a signal traveling along the axon to the olfactory bulb, part of the limbic system of the brain. Interpretation of the smell begins, relating the smell to past experiences and in relation to the substance(s) emitted. The olfactory bulb acts as a relay station connecting the nose to the olfactory cortex in the brain. Olfactory information is further processed and projected through a pathway to the central nervous system (CNS), which controls emotions and behavior as well as basic thought processes.
Odor sensation depends on the concentration (number of molecules) available to the olfactory receptors. A single odorant stimulus type is typically recognized by multiple receptors, and different odorants are recognized by combinations of receptors, the patterns of neuron signals helping to identify the smell. The olfactory system does not interpret a single compound, but instead the whole odorous mix, not necessarily corresponding to concentration or intensity of any single constituent.
The widest range of odors consists 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; the detection of stimuli by receptors in the nose. The stimuli are processed by the region of the human brain which is responsible for olfaction. Because of this, an objective and analytical measure of odor is impossible. While odor feelings are very personal perceptions, individual reactions are related to gender, age, state of health, and personal history.
Common odors that people are used to, such as their own body odor, are less noticeable to individuals than external or uncommon odors. This is due to habituation; after continuous odor exposure, the sense of smell fatigues quickly, but recovers rapidly after the stimulus is removed. 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, the conditioned odorant is blocked out because of habituation. This depends on the strength of the odorants in the mixture which can change perception and processing of an odor. This process helps classify similar odors as well as adjust sensitivity to differences in complex stimuli.
For most untrained people, the process of smelling gives little information concerning the specific ingredients of an odor. Their smell perception primarily offers information related to the emotional impact. Experienced people, however, such as flavorists and perfumers, can pick out individual chemicals in complex mixes through smell alone.
Odor perception is a primal sense. The sense of smell enables pleasure, can subconsciously warn of danger, help locate mates, find food, or detect predators. Humans have a surprisingly good sense of smell (even though they only have 350 functional olfactory receptor genes compared to the 1,300 found in mice) correlated to an evolutionary decline in sense of smell. Human's remarkable sense of smell is just as good as many animals, and can distinguish a diversity of odors- approximately 10,000 scents. This is because of the retro nasal route in humans to increase sensation. However, animals such as dogs show a greater sensitivity to odors than humans especially in studies using short-chained compounds. Higher cognitive brain mechanisms and more olfactory brain regions enable humans to discriminate odors better than other mammals despite fewer olfactory receptor genes.
Different categorizations of primary odors have been proposed, among others this, which relies on seven primary odors: (with examples)
- Musky- perfumes/aftershave
- Putrid- rotten eggs
- Pungent- vinegar
- Camphoraceous- mothballs
- Ethereal- dry cleaning fluid
- Floral- roses
- Pepperminty- mint gum
Although recently progress has been made, the idea of primary perceptions is disputed, and more so probably the concept of primary odors.
Read more about this topic: Odor