Chemicals dissolved in air chemically bind to upper-nose odor receptors {smell, sense}| {olfaction}. Smell qualities depend on molecule electrical and spatial-configuration properties, such as shape, acidity, and polarity. Smell is a synthetic sense, with some analysis. People can distinguish 20 to 30 primary odors and more than 10,000 different odors.
physical properties
Smellable molecules include many types of typically hydrophobic volatile substances with molecular weights between 30 to 350. Air-borne molecules vary in size, shape, chemical sites, and vibration states. Air-borne chemicals vary in concentration. Smellable chemicals chemically bind to upper-nasal-passage chemical receptors.
primary-odor receptors
Some people cannot smell camphorous, fishy, malty, minty, musky, spermous, sweaty, or urinous odors (primary odor). Camphorous molecules have multiple benzene rings. Fishy molecules are three-single-bond monoamines. Malty molecules are aldehydes. Minty molecules have a benzene ring and an oxygen-containing side group. Musky molecules have multiple rings. Spermous molecules are aromatic amines. Sweaty molecules are carboxylic acids. Urinous molecules are steroid ketones. Fruity molecules are organic alcohols.
types
Odors can be acidic, acrid or vinegary, alliaceous or garlicy, ambrosial or musky, aromatic, burnt or smoky, camphorous or resinous, ether-like, ethereal or peary, floral or flowery, foul or sulfurous, fragrant, fruity, goaty or hircine or caprylic, minty, nauseating, peppermint-like, pungent or spicy, putrid, spearmint-like, sweaty, and sweet.
Perhaps, the first smells were mating, food, or poison signs.
qualities
Smells can be sweet, acidic, or sweaty. For example, musk, ether, ester, lowery, fruity, and musky are dull, sweet, and smooth. Vinegar and acid are sharp, sour, and harsh.
Smells can be cool, like menthol, or hot, like heavy perfume. For example, menthol is cool, and perfume is hot.
Aromatic, camphorous, ether, minty, musky, and sweet are similar. Acidic and vinegary are similar. Acidic and fruity are similar. Goaty, nauseating, putrid, and sulphurous are similar. Smoky/burnt and spicy/pungent are similar. Camphor, resin, aromatic, musk, mint, pear, flower, fragrant, pungent, fruit, and sweets are similar. Putrid or nauseating, foul or sulfur, vinegar or acrid, smoke, garlic, and goat are similar. Vegetable smells are similar. Ethers are vegetable. Animal smells are similar. For example, caprylic acid and carboxylic acids are animal. Halogens are mineral
Acidic and sweet smells are opposites. Sweaty and sweet smells are opposites.
Smell always refers to object that makes smell, not to accidental or abstract property nor to concept about smell. In contrast, color always refers to object property.
Odors have same physical properties, and smell physiological processes are similar, so odor perceptions are similar, with same odors and odor relations, for people with undamaged smell systems. Smells relate in only one consistent and complete way. Smells do not have symmetric smell relations, so smells have unique relations. Smells cannot substitute or switch.
People can smell specific odors and not others. People can smell sweet as putrid and have other smell exchanges. People can always smell something.
mixing
Smells blend in concordances and discordances, like music harmonics. Pungent and sweet can mix. Pungent and sweaty can mix. Perhaps, smells can cancel other smells, not just mask them.
timing
Brain detects aldehyde smells first {top note, smell}. Brain detects floral smells second {middle note, smell}. Brain detects lingering smells, such as musk, civet, ambergris, vanilla, cedar, sandalwood, and vetiver, later {base note, smell}.
properties
Smell habituates quickly. Smell is in real time, with a half-second delay. Smell short-term memory is poor. Smell strength decreases with age. Fats absorb pungent food odors.
Butyrate and squalene odor patterns identify species members. In mammals, small pheromone amounts establish territories [Pantages and Dulac, 2000]. Humans have strong odors from hair-follicle apocrine glands. Perhaps, human odor warns predators away. Babies have small glands. Stress seems to cause odor. Menses smells like onions.
source location
Olfactory bulb preserves odor-receptor spatial relations. Smell cortex can detect smell location in space. Smell can detect several sources from one location. Smells from different sources can interfere.
diseases
Diabetes smells like sugar or acetone. Measles smells like feathers. Nephritis smells like ammonia. Plague smells like apples. Typhus smells like mice. Yellow fever smells like meat.
emotions
Smells can make people feel disgusted, intoxicated, sickened, delighted, revolted, excited, hypnotized, and pleasured. Smells can be surprising, because smells have many combinations.
evolution
Perhaps, the first smells were mating, food, or poison signs.
Butyrate and squalene odor patterns identify species members. Humans have strong odors from hair-follicle apocrine glands. Stress seems to cause odor. Perhaps, human odor warns predators away. (Babies have small glands.)
In mammals, small pheromone amounts establish territories [Pantages and Dulac, 2000].
development
In first few days, newborns can distinguish people by odor.
relations to other senses
Taste and retronasal-area smell can combine to make flavor. Taste has higher concentration than smell. Smell uses air as solvent, and taste uses water. Smell does not use molecule polarization, but taste does. Smell does not use molecule acidity, but taste does. Smells interfere with each other, but tastes are separate and independent. Taste does not use molecule vibrations, but perhaps smell uses vibrations. Taste and smell are both often silent. Taste and smell have early, middle, and late sensations. Smells and tastes have spatial source.
Smell is at body surface and so has touch. Touch can feel air near smell-receptor cells and react to noxious smells. Touch locates smell-receptor cells in upper nose. Trigeminal nerve carries signals from nose warmth-coolness, touch, and pain receptors.
Trigeminal nerve carries signals from nose warmth-coolness, touch, and pain receptors. Smell is at inner-nose surface and so has touch. Touch can feel air on inner nose and react to noxious odors. Touch locates olfactory receptors in upper nose.
Smell uses tactile three-dimensional space to locate smells in space.
Odor is painful at high concentrations.
People have upper-nostril skin areas, with molecule shape, size, and vibration receptors {smell, anatomy}. Smell uses more than 30 odor-receptor types, each with variations, making a thousand combinations. Smell-neuron axons go to older mammal-forebrain rhinencephalon, near frontal lobe, not to thalamus as other sense axons do. Invertebrates have skin odor receptors.
Odor receptors send to olfactory-bulb glomeruli, which send to cortical regions.
Behind eyebrow, where nose meets skull, is bone {cribriform plate} with many nerve-sized holes, through which olfactory-neuron axons go to olfactory bulb.
Olfactory epithelium has cells {basal cell} that can become olfactory neurons.
Olfactory-receptor cells send to neurons {mitral cell}, whose top dendrites go to horizontal cells to receive lateral inhibition and whose bottom branches are recurrent collateral axons to spread lateral inhibition. Mitral-cell axons go to anterior-olfactory-nucleus and prepyriform-cortex superficial and deep pyramidal neurons.
Olfactory-receptor cilia have molecules that bind odorants. Smell system has a thousand different protein receptors {olfactory receptor}, with seven to eleven major odor-receptor types, which each have a dozen minor types. People have ten million odor-receptors in each nostril. Dogs have 200 million. Odor-receptors die every month, and then new ones grow.
Of 1000 olfactory-receptor genes, 65% are not functional in humans. In Old World monkeys, 30% are not functional. In New World monkeys, 18% are not functional. In dogs, 20% are not functional. Odor-receptor chemical sites are for alcohols, aldehydes, amines, aryls, carboxylic acids, esters, ethers, halogens, ketones, cysteines, thiols, sulfides, or terpenes. Sites can be for small, medium, or large molecules [Firestein, 2001] [Laurent et al., 2001]:
Alcohols that are small, such as methanol and ethanol, smell alcoholy, biting, and hanging.
Alcohols that are medium-chain, such as butanol and octanol, smell sweet and fruity.
Alcohols that are cyclic, such as menthol, smell cool and minty.
Alcohols that are monoterpenoids, such as geraniol and linalool, smell flowery and fresh.
Alcohols that are monophenols, such as phenol and guaiacol, smell burnt and smoky.
Alcohols that are polyphenols, such as cresol, smell tarry and oily.
Aldehydes that are small, such as diacetyl aldehyde, smell buttery.
Aldehydes that are short-chain, such as isovaleraldehyde, smell malty.
Aldehydes that are alkene aldehydes, such as hexenal, smell grassy and herby.
Amines that are alkyl and aryl monoamines, such as trimethylamine and phenethylamine, smell fishy.
Amines that are alkyl multi-amines, such as putrescine, smell spermous.
Amines that are heterocyclic amines, such as pyrroline, smell spermous.
Amines that are heterocyclic aromatic, such as alkyl pyrazines, smell nutty, earthy, and green peppery.
Amines that are heterocyclic aromatic, such as 2-acetyl-tetrahydro-pyridine, smell roasted, fermented, and popcorny.
Aryls that are benzene alkyls, such as benzene, toluene, and xylenes, smell aromatic.
Aryls that are monophenols, such as phenol and guaiacol, smell burnt and smoky.
Aryls that are polyphenols, such as cresol, smell tarry and oily.
Aryls that are polycyclic aromatic hydrocarbons, such as anthracene and pyrene, smell burnt and smoky.
Aryls that are polycyclic in small concave sites, such as camphor, smell camphorous and resinous.
Aryls that are aryl monoamines, such as phenethylamine, smell fishy.
Carboxylic acids that are small, such as acetic acid, smell acrid, vinegary and pungent.
Carboxylic acids that are medium-short polar chains, such as butyric acid (butanoic acid), smell putrid, sweaty and rancid.
Carboxylic acids that are medium-length polar chains, such as caprylic acid (octanoic acid), smell goaty and hircine.
Carboxylic acids that are carboxylic-acid thiols, such as dithiolane-4-carboxylic acid, smell asparagusy and bitter.
Esters that are non-polar chains, such as methyl butyrate, smell sweet and fruity.
Ethers that are linear in concave and trough-shaped sites, such as ethyl methyl ether, smell fragrant, ethereal, floral and flowery.
Ethers that are cyclic, such as dioxacyclopentane, smell earthy, moldy and potatoey.
Halogens, such as fluorine, chlorine, and bromine, smell pharmaceutical, medicinal, pungent, and unpleasant.
Ketones that are heterocyclic, such as furanone and lactones, smell savory and spicy.
Ketones that are alkane ring ketones, such as steroid ketones, smell urinous.
Ketones that are macrocyclic in large concave sites, such as muscone (methylcyclopentade-canone), smell musky and ambrosial.
Ketones that are alkenes with one ring, such as ionones, damascones, and damascenones, smell tobaccoy.
Ketones that are cyclic alkene ketones in V-shaped sites, such as terpenoids and R-(-)-carvone (2-methyl-5-(1-methylethenyl)-2-cyclohexenone), smell minty, spearminty, and pepperminty.
Sulfur compounds that are cysteines, such as gamma-glutamylcysteines and cysteine sulfoxides, smell alliaceous and garlicy.
Sulfur compounds that are carboxylic-acid thiols, such as dithiolane-4-carboxylic acid, smell asparagusy and bitter.
Sulfur compounds that are small thiols, such as methyl mercaptan (methanethiol), smell foul, sulfurous, and rotten.
Sulfur compounds that are sulfides, such as methyl sulfides, smell cabbage-like and rotten at high concentrations.
Terpenes that are cyclic alkene ketones in V-shaped sites, such as terpenoids and R-(-)-carvone (2-methyl-5-(1-methylethenyl)-2-cyclohexenone), smell minty and pepperminty.
Terpenes that are monoterpenoid alcohols, such as geraniol and linalool, smell flowery and fresh.
Terpenes that are isoprenes and monoterpenes, such as isoterpene, smell rubbery.
Terpenes that are sesquiterpenes and triterpenes, such as humulene, smell woody.
Some sites are for both alcohol and terpene, alcohol and aryl, amine and aryl, carboxylic acid and thiol, or ketone and terpene.
Some sites are for carbon chains and rings: alkyls, alkenes, single rings, multiple rings, single heterocyclic rings, multiple heterocyclic rings, single aromatic rings, and multiple aromatic rings.
A limbic-system region {amygdala-hippocampal complex} measures smell associations and emotions.
Olfactory nerves, mitral cells, and tufted cells converge on olfactory-bulb spheres {glomerulus, smell} {glomeruli, smell}. Olfactory receptors send to one lateral glomerulus and one medial glomerulus. Glomeruli receive from one or more olfactory receptors and detect one odor or odor combination.
At nose tips, mammals have a ganglion {Grueneberg ganglion} that detects alarm pheromones (Hans Grueneberg) [1973].
Mammal nasal-cavity bases have smell neurons {vomeronasal system} {Jacobson's organ} {Jacobson organ} for sex-signal and other pheromones. Axons go to accessory olfactory bulb and then to amygdala [Holy et al., 2000] [Johnston, 1998] [Keverne, 1999] [Stowers et al., 2002] [Watson, 2001].
Odor receptors send output directly, left to left and right to right, to 2-mm-diameter brain region {olfactory bulb}| above and behind nose. Olfactory receptors send axons to mitral cells. Mitral-cell axons go to anterior-olfactory-nucleus and prepyriform-cortex superficial and deep pyramidal neurons. Pyramidal neurons send recurrent collateral axons to superficial pyramidal neurons and stellate cells. Pyramidal neurons have post-synaptic apical dendrites that receive from other pyramidal neurons. Tufted cells are local. Olfactory nerves, mitral cells, and tufted cells meet in olfactory-bulb glomeruli. Olfactory bulb preserves odor-receptor spatial relations. Olfactory bulb has fewer neurons than number of odor receptors.
Olfactory-bulb signals go to pyriform cortex, amygdala-hippocampal complex, and entorhinal complex {olfactory cortex}.
Nasal passages guide air onto olfactory epithelium {olfactory cleft} at nose back.
Upper-nose olfactory-cleft mucus cells {olfactory epithelium} {orthonasal olfactory system} are olfactory receptors, basal cells, and supporting cells. In mammals, odor receptors are at nose air-passage top or back. In humans, smell regions are four square-centimeters. Olfactory epithelium is mostly small sensory cells {olfactory sensory neuron} (OSN), with cilia that have odor receptors.
Olfactory region is light yellow in humans and dark yellow or brown in animals. Albinos have white regions and typically have poor smell ability.
Chewing and swallowing can send odorant up rear nasal tract {retronasal olfactory system}. People think sense qualities are in mouth. Orthonasal olfactory system is about outside environment, while retronasal olfactory system is about nutrients and poisons.
Inner-nose ridges {turbinate}| channel inhaled air to olfactory epithelium.
Objects can have smell {odor, smell}| to humans. Odorants mix to make odor.
Molecules can have smell {odorant} to humans. Odorants must be volatile. Airborne-molecule chemical-bond configurations (shapes) and vibration and rotation frequencies and intensities cause smell. Odorant molecules have molecular weight greater than 35 and less than 350, not too small nor too large for olfactory receptors. Odorants are typically hydrophobic.
Pungent odorants are compact non-polar aryl compounds. Sweet odorants are non-polar chain esters. Sweaty odorants are polar chain organic acids. Right-handed and left-handed chiral molecules, like spearmint and caraway, smell different.
primary
People can distinguish 30 primary odorants:
alliaceous and garlicy: cysteine sulfur compounds
aromatic: benzene alkyls
asparagusy, bitter: carboxylic-acid thiols
biting, hanging, alcoholy: small alcohols
burnt, smoky: monophenols and polycyclic aromatic hydrocarbons
buttery: small aldehydes
camphorous, resinous: polycyclic aryls
cool and minty: cyclic alcohols
earthy, moldy, potatoey: cyclic ethers
fishy: alkyl and aryl monoamines
flowery, fresh: monoterpenoid alcohols
foul, rotten, sulfurous: small thiol sulfur compounds
fragrant, floral, flowery, ethereal: linear ethers
fruity, sweet: medium-chain alcohols and non-polar chain esters
goaty, hircine: medium-length polar chain carboxylic acids
grassy, herby: alkene aldehydes
malty: short-chain aldehydes
minty, spearminty, pepperminty: cyclic alkene ketones
musky, ambrosial: macrocyclic ketones
nutty, earthy, green peppery: heterocyclic aromatic amines
pharmaceutical, medicinal, pungent, unpleasant: halogens
pungent, acrid, vinegary: small carboxylic acids
putrid, sweaty, rancid: medium-short polar chain carboxylic acids
roasted, fermented, popcorny: heterocyclic aromatic amines
rubber: monoterpenes (isoprenes)
cabbage-like, rotten: methyl sulfides
savory, spicy: heterocyclic ketones
spermous: alkyl multi-amines and heterocyclic amines
tarry, oily: polyphenols
tobacco: alkenes-with-one-ring ketones
urinous: steroid ketones
woody: triterpenes (sesquiterpenes)
Odorants mix to make odor, and people can distinguish 10,000 different odors.
categories
Smells can range through sweet/flowery/fruity, mild/vegetably, mild/animaly, mild/mineraly, strong/vegetably, strong/animaly, putrid/animaly, and sharp/mineraly.
The smell-category sequence correlates with molecule reactivity:
Ether -C-O-C-
Alcohol -CH2OH
Ester -COO-
Aryl =CHC=
Terpene =CC2
Ketone -COC-
Aldehyde -CHO
Acid -COOH
Amine -CH2NH2
Sulfhydryl -CH2SH
Halogens Br2
similarities based on chemical group
Similar chemical types make similar smells. Similar chemical origins make similar smells.
Alcohols are similar: biting, fruity, sweet.
Aldehydes are similar: malty, grassy (herby).
Amines are similar: spermous, fishy, nutty, roasted.
Aryls are similar: aromatic, burnt (smoky), camphorous (resinous), tarry (oily).
Carboxylic acids are similar: pungent (acrid, vinegary), putrid (sweaty, rancid), goaty (hircine).
Ethers are similar: fragrant, floral, fruity and sweet.
Ketones are similar: minty, spicy, savory, tobacco, musky (ambrosial), urinous.
Sulfur compounds are similar: asparagusy, cabbage-like, alliaceous (garlicy), foul, rotten.
Terpenes are similar: minty, flowery (fresh), rubbery, woody.
similarities based on similar chemical groups
Alcohols and aryl ketones are similar: biting, fruity, minty, musky.
Alcohols and esters are similar: fruity, sweet.
Aldehydes and alkene ketones are similar: malty, grassy, tobacco.
Aldehydes and ethers are similar: malty, grassy, earthy.
Aldehydes and terpenes are similar: malty, grassy, rubbery, woody.
Amines and steroid ketones are similar: spermous, fishy, nutty, roasted, urinous.
Amines and carboxylic acids are similar: spermous, fishy, nutty, roasted, pungent, putrid, goaty.
Polycyclic aryls and halogens are similar: camphorous, pharmaceutical.
Carboxylic acids and steroid ketones are similar: pungent, putrid, goaty, urinous.
Alkene ketones and terpenes are similar: tobacco, rubbery, woody.
Polycyclic aryl ketones and ethers are similar: minty, camphorous, musky, fragrant, flowery, fruity.
similarities based on organism type
Vegetable smells are similar: alcohols, aldehydes, ethers, aryl and alkene ketones, sulfur compounds, terpenes.
Animal smells are similar: carboxylic acids, amines, polycyclic aryl ketones, steroid ketones.
opposites
Carboxylic acids (sour, putrid, animal) and esters (sweet, fruity, vegetable) are opposites.
Carboxylic acids (sour, putrid, animal) and alcohols (sweet, fruity, vegetable) are opposites.
Amines (animal) and aldehydes (vegetable) are opposites.
Amines (animal) and terpenes (vegetable) are opposites.
Odors have pleasantness, familiarity, and intensity {odor hedonics}, which define how much people like them.
In mammals, chemicals {pheromone}| establish territories and find mates [Pantages and Dulac, 2000]. Sex-hormone-derived pheromones are in skin secretions [Savic et al., 2001] [Savic, 2002] [Sobel et al., 1999]. Baboons secrete female pheromones during sexual receptive period. Perhaps, pheromones synchronize ovulation [Gangestad et al., 2002] [McClintock, 1998] [Schank, 2001] [Stern and McClintock, 1998] [Weller et al., 1999].
Women living in close proximity menstruate at same time {McClintock effect}, perhaps from sweat pheromone.
Animals mark locations with scent {scent marking}. Cats and antelope use urine and face or cheek scent glands. Skunk and badger use anal glands.
Linnaeus said smells can be alliaceous like garlic, ambrosial like musk, aromatic, foul, fragrant, hircine like goat, and nauseating {primary odor}. Primary odors can be putrid, flowery, fruity, burnt, spicy, resinous or camphor, musk, floral, peppermint, ether, pungent, and putrid. Primary odors can be floral, minty, ethereal like pear, musky, resinous like camphor, foul or sulfurous, and acrid like vinegar. Primary odors can be acidic, burnt, caprylic like goat, and fragrant. Primary odors can be camphorous, fishy, malty, minty, musky, spermous, sweaty, or urinous odors.
Almond oil, honey, cinnamon, orange blossom, and henna {aegyptium} can mix.
Sperm-whale-stomach oil {ambergris, smell} can protect stomach lining.
Steroid molecules {androstenone} smell musky to 25% of people and urinous to 25% of people, and have no smell for 50% of people.
Orange-rind oils {bergamot, smell} can mix.
Violets can make drops {cacous}. Casca preciosa is sassafras.
d-carvone {carvone} is caraway, and l-carvone is spearmint.
Far-northern-beaver abdomen-gland oil {castoreum} marks territory.
Ethiopian-cat near-genitalia-gland honey-like compound {civet, smell} is a sex pheromone.
Violets make compounds {ionone} that can inhibit odors.
Rose, crocus, and violet oils {kyphi} can mix.
A genetic disease causes urine to smell like maple syrup {maple syrup urine}.
East-Asian deer-intestine red jelly {musk, smell} has steroids.
Oranges can make attar {neroli}.
Smell processes use molecule shape and electric-field differences to distinguish odorants {smell, physiology}. After seven or eight molecules bind to cilia odorant receptors, olfactory receptors signal once. People need 40 signals to perceive odor. Odorants affect several olfactory-receptor types, which send to smell neurons that excite and inhibit each other to form intensity ratios. Smell neurons work together to distinguish odors.
Odors are painful at high concentrations. Smell can detect very low concentrations. Odor intensity and sense qualities mix.
Smell can detect source location. Smell can detect many sources from one location.
Lower air pressure increases volatility and so smell intensity. Higher humidity increases volatility and so smell intensity. Light typically decreases smell, by breaking down chemicals.
After smelling an odor, smell is less sensitive to later odors {cross-adaptation}, probably because both odors share one or more odorant-receptor types. Different odor sequences result in different sensitivities.
People can be unable to name familiar odors {tip-of-the-nose phenomenon}. Unlike tip-of-the-tongue phenomena, there are no lexical cues.
Sinus problems or head blows can cause inability to smell anything {anosmia}. People can be unable to smell specific odors {specific anosmia}.
People can have heightened sucrose, urea, and hydrochloric-acid sensitivity {hyperosmia}.
People can have reduced smell sense {hyposmia}.
Air chemicals and odorant receptors have shapes. Perhaps, chemical shapes must be complementary to receptor shapes to detect odorants {shape-pattern theory}. Odorant-receptor firing pattern determines odor.
Perhaps, molecule geometry correlates with odor type {stereochemical theory}. Smell receptor sites are small concave for camphorous smell, large concave for musky smell, V-shaped for minty smell, trough-shaped for ethereal smell, and concave-and-trough-shaped for floral smell. Receptor sites can have electric charges that attract oppositely charged moelcules, with negative charge for pungent smell and positive charge for putrid smell [Amoore, 1964] [Moncrieff, 1949].
Perhaps, odorant molecules have vibration frequencies {vibration theory} (Luca Turin). Molecules with similar vibration frequency have similar smell.
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Date Modified: 2022.0225