Olfaction

Olfaction

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Olfaction is the sense
Sense
Senses are physiological capacities of organisms that provide inputs for perception. The senses and their operation, classification, and theory are overlapping topics studied by a variety of fields, most notably neuroscience, cognitive psychology , and philosophy of perception...

 of smell. This sense is mediated by specialized sensory cells of the nasal cavity of vertebrates, and, by analogy, sensory cells of the antennae of invertebrates. Many vertebrates, including most mammals and reptiles, have two distinct olfactory systems—the main olfactory system, and the accessory olfactory system (mainly used to detect pheromones). For air-breathing animals, the main olfactory system detects volatile chemicals, and the accessory olfactory system detects fluid-phase chemicals. For water-dwelling organisms, e.g., fish or crustaceans, the chemicals are present in the surrounding water. Olfaction, along with taste
Taste
Taste is one of the traditional five senses. It refers to the ability to detect the flavor of substances such as food, certain minerals, and poisons, etc....

, is a form of chemoreception. The chemicals themselves which activate the olfactory system, generally at very low concentrations, are called odorants. The chemical reactions that cause the sense of taste are not stored in our memory system as well as other senses are; such as the information received by our visual and auditory (echoic) systems.

Volatile small molecule odorants, non-volatile proteins and non-volatile hydrocarbons may all produce olfactory sensations. Some animal species are able to smell carbon dioxide in minute concentrations. Taste sensations are caused by small organic molecules and proteins.

History



As the Epicurean and atomistic Roman philosopher Lucretius
Lucretius
Titus Lucretius Carus was a Roman poet and philosopher. His only known work is an epic philosophical poem laying out the beliefs of Epicureanism, De rerum natura, translated into English as On the Nature of Things or "On the Nature of the Universe".Virtually no details have come down concerning...

 (1st Century BCE) speculated, different odors are attributed to different shapes and sizes of odor molecules that stimulate the olfactory organ. A modern demonstration of that theory was the cloning of olfactory receptor
Olfactory receptor
Olfactory receptors expressed in the cell membranes of olfactory receptor neurons are responsible for the detection of odor molecules. Activated olfactory receptors are the initial player in a signal transduction cascade which ultimately produces a nerve impulse which is transmitted to the brain...

 proteins by Linda B. Buck
Linda B. Buck
Linda Brown Buck is an American biologist best known for her work on the olfactory system. She was awarded the 2004 Nobel Prize in Physiology or Medicine, along with Richard Axel, for their work on olfactory receptors....

 and Richard Axel
Richard Axel
Richard Axel is an American neuroscientist whose work on the olfactory system won him and Linda B. Buck, a former post-doctoral scientist in his research group, the Nobel Prize in Physiology or Medicine in 2004....

 (who were awarded the Nobel Prize
Nobel Prize
The Nobel Prizes are annual international awards bestowed by Scandinavian committees in recognition of cultural and scientific advances. The will of the Swedish chemist Alfred Nobel, the inventor of dynamite, established the prizes in 1895...

 in 2004), and subsequent pairing of odor molecules to specific receptor proteins. Each odor receptor molecule recognizes only a particular molecular feature or class of odor molecules. Mammals have about a thousand gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...

s that code for odor reception. Of the genes that code for odor receptors, only a portion are functional. Humans have far fewer active odor receptor gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...

s than other primates and other mammals.

In mammals, each olfactory receptor neuron
Olfactory receptor neuron
An olfactory receptor neuron , also called an olfactory sensory neuron , is a transduction cell within the olfactory system. J. Rospars, Dendritic integration in olfactory sensory neurons: a steady-state analysis of how the neuron structure and neuron environment influence the coding of odor...

 expresses only one functional odor receptor. Odor receptor nerve cells function like a key-lock system: If the airborne molecules of a certain chemical can fit into the lock, the nerve cell will respond. There are, at present, a number of competing theories regarding the mechanism of odor coding and perception. According to the shape theory
Shape theory of olfaction
The Shape theory of smell states that a molecule's particular smell is due to a 'lock and key' mechanism by which a scent molecule fits into olfactory receptors in the nasal epithelium.-History:In 1949, R.W...

, each receptor detects a feature of the odor molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...

. Weak-shape theory, known as odotope theory
Odotope theory
Odotope theory, also known as weak shape theory, is a theory of how olfactory receptors bind to odor molecules. The theory proposes that a combination of shape factors determine the coupling. The word itself is an analogy to epitopes.- References :...

, suggests that different receptors detect only small pieces of molecules, and these minimal inputs are combined to form a larger olfactory perception (similar to the way visual perception is built up of smaller, information-poor sensations, combined and refined to create a detailed overall perception). An alternative theory, the vibration theory proposed by Luca Turin
Luca Turin
Luca Turin is a biophysicist with a long-standing interest in the sense of smell, the art of perfume, and the fragrance industry.-Vibration theory of olfaction:...

, posits that odor receptors detect the frequencies of vibrations of odor molecules in the infrared range by electron tunnelling. However, the behavioral predictions of this theory have been called into question. As of yet, there is no theory that explains olfactory perception completely.

Main olfactory system


The Olfactory Epithelium:
In vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...

s smells are sensed by olfactory sensory neurons in the olfactory epithelium
Olfactory epithelium
The olfactory epithelium is a specialized epithelial tissue inside the nasal cavity that is involved in smell. In humans, it measures about 1 square centimetre and lies on the roof of the nasal cavity about 7 cm above and behind the nostrils...

. The proportion of olfactory epithelium
Epithelium
Epithelium is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. Epithelial tissues line the cavities and surfaces of structures throughout the body, and also form many glands. Functions of epithelial cells include secretion, selective...

 compared to respiratory epithelium (not innervated) gives an indication of the animal's olfactory sensitivity. Humans have about 10 cm² (1.6 sq in) of olfactory epithelium, whereas some dogs have 170 cm² (26.4 sq in). A dog's olfactory epithelium is also considerably more densely innervated, with a hundred times more receptors per square centimetre.

Molecules of odorants passing through the superior nasal concha
Superior nasal concha
The back part of the medial surface of the labyrinth of ethmoid is subdivided by a narrow oblique fissure, the superior meatus of the nose, bounded above by a thin, curved plate, the superior nasal concha....

 of the nasal passages dissolve in the mucus
Mucus
In vertebrates, mucus is a slippery secretion produced by, and covering, mucous membranes. Mucous fluid is typically produced from mucous cells found in mucous glands. Mucous cells secrete products that are rich in glycoproteins and water. Mucous fluid may also originate from mixed glands, which...

 lining the superior portion of the cavity and are detected by olfactory receptor
Olfactory receptor
Olfactory receptors expressed in the cell membranes of olfactory receptor neurons are responsible for the detection of odor molecules. Activated olfactory receptors are the initial player in a signal transduction cascade which ultimately produces a nerve impulse which is transmitted to the brain...

s on the dendrites of the olfactory sensory neurons. This may occur by diffusion or by the binding of the odorant to odorant binding protein
Odorant binding protein
Odorant binding proteins are abundant small soluble proteins secreted in the nasal mucus of many animal species and in the sensillar lymph of chemosensory sensilla of insects....

s. 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
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...

).

In insect
Insect
Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...

s smells are sensed by olfactory sensory neurons in the chemosensory sensilla
Sensilla
A sensillum is an insect sensory organ protruding from the cuticle, or sometime lying within or beneath it. Sensilla are divided in chemical, mechanical , thermal and visual....

, which are present in insect antenna, palps and tarsa, but also on other parts of the insect body. Odorants penetrate into the cuticle pores of chemosensory sensilla and get in contact with insect Odorant binding protein
Odorant binding protein
Odorant binding proteins are abundant small soluble proteins secreted in the nasal mucus of many animal species and in the sensillar lymph of chemosensory sensilla of insects....

s (OBPs) or Chemosensory protein
Chemosensory protein
Chemosensory proteins are a class of small , soluble proteins characterised for the first time by Angeli et al. as secreted into the sensillar lymph of insect chemosensory sensilla...

s (CSPs), before activating the sensory neurons.

Receptor neuron


The binding of the ligand
Ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...

 (odor molecule or odorant) to the receptor leads to an action potential
Action potential
In physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and...

 in the receptor neuron, via a second messenger pathway, depending on the organism. In mammals the odorants stimulate adenylate cyclase
Adenylate cyclase
Adenylate cyclase is part of the G protein signalling cascade, which transmits chemical signals from outside the cell across the membrane to the inside of the cell ....

 to synthesize CAMP
Cyclic adenosine monophosphate
Cyclic adenosine monophosphate is a second messenger important in many biological processes...

 via a G protein
G protein
G proteins are a family of proteins involved in transmitting chemical signals outside the cell, and causing changes inside the cell. They communicate signals from many hormones, neurotransmitters, and other signaling factors. G protein-coupled receptors are transmembrane receptors...

 called Golf. CAMP, which is the second messenger here, opens a cyclic nucleotide-gated ion channel
Cyclic nucleotide-gated ion channel
Cyclic nucleotide-gated ion channels are ion channels that function in response to the binding of cyclic nucleotides. CNG channels are nonselective cation channels that are found in the membranes of various types of cells.- Discovery :...

 (CNG) producing an influx of cations (largely Ca
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...

2+ with some Na
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...

+) into the cell, slightly depolarising it. The Ca2+ in turn opens a Ca2+-activated chloride channel
Chloride channel
Chloride channels are a superfamily of poorly understood ion channels consisting of approximately 13 members.Chloride channels display a variety of important physiological and cellular roles that include regulation of pH, volume homeostasis, organic solute transport, cell migration, cell...

, leading to efflux of Cl
Chlorine
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...

-, further depolarising the cell and triggering an action potential
Action potential
In physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and...

. Ca2+ is then extruded through a sodium-calcium exchanger
Sodium-calcium exchanger
The sodium-calcium exchanger is an antiporter membrane protein that removes calcium from cells. It uses the energy that is stored in the electrochemical gradient of sodium by allowing Na+ to flow down its gradient across the plasma membrane in exchange for the countertransport of calcium ions...

. A calcium-calmodulin
Calmodulin
Calmodulin is a calcium-binding protein expressed in all eukaryotic cells...

 complex also acts to inhibit the binding of CAMP to the CAMP-dependent channel, thus contributing to olfactory adaptation.
This mechanism of transduction is somewhat unique, in that CAMP works by directly binding to the ion channel
Ion channel
Ion channels are pore-forming proteins that help establish and control the small voltage gradient across the plasma membrane of cells by allowing the flow of ions down their electrochemical gradient. They are present in the membranes that surround all biological cells...

 rather than through activation of protein kinase A. It is similar to the transduction mechanism for photoreceptors, in which the second messenger cGMP
Cyclic guanosine monophosphate
Cyclic guanosine monophosphate is a cyclic nucleotide derived from guanosine triphosphate . cGMP acts as a second messenger much like cyclic AMP...

 works by directly binding to ion channels, suggesting that maybe one of these receptors was evolutionarily adapted into the other. There are also considerable similarities in the immediate processing of stimuli by lateral inhibition
Lateral inhibition
In neurobiology, lateral inhibition is the capacity of an excited neuron to reduce the activity of its neighbors. Lateral inhibition sharpens the spatial profile of excitation in response to a localized stimulus.-Sensory inhibition:...

.
Averaged activity of the receptor neurons can be measured in several ways. 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. In insects, one can perform electroantenogram or also calcium imaging within the olfactory bulb.

Olfactory bulb projections



Olfactory sensory neurons project axon
Axon
An axon is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body or soma....

s to the brain within the olfactory nerve
Olfactory nerve
The olfactory nerve, or cranial nerve I, is the first of twelve cranial nerves. It is instrumental in the sense of smell. Derived from the embryonic nasal placode, the olfactory nerve is capable of regeneration.-Anatomy:...

, (cranial nerve I). These axons pass to the olfactory bulb
Olfactory bulb
The olfactory bulb is a structure of the vertebrate forebrain involved in olfaction, the perception of odors.-Anatomy:In most vertebrates, the olfactory bulb is the most rostral part of the brain. In humans, however, the olfactory bulb is on the inferior side of the brain...

 through the cribriform plate
Cribriform plate
The cribriform plate of the ethmoid bone is received into the ethmoidal notch of the frontal bone and roofs in the nasal cavities....

, which in turn projects olfactory information to the olfactory cortex and other areas. The axons from the olfactory receptor
Olfactory receptor
Olfactory receptors expressed in the cell membranes of olfactory receptor neurons are responsible for the detection of odor molecules. Activated olfactory receptors are the initial player in a signal transduction cascade which ultimately produces a nerve impulse which is transmitted to the brain...

s converge in the outer layer of the olfactory bulb
Olfactory bulb
The olfactory bulb is a structure of the vertebrate forebrain involved in olfaction, the perception of odors.-Anatomy:In most vertebrates, the olfactory bulb is the most rostral part of the brain. In humans, however, the olfactory bulb is on the inferior side of the brain...

 within small (~50 micrometers in diameter) structures called glomeruli
Glomerulus (olfaction)
The glomerulus is a spherical structure located in the olfactory bulb of the brain where synapses form between the terminals of the olfactory nerve and the dendrites of mitral, periglomerular and tufted cells...

. Mitral cell
Mitral cell
Mitral cells are neurons that are part of the olfactory system. They are located in the olfactory bulb in the mammalian central nervous system. They receive information from the axons of olfactory receptor neurons, forming synapses in neuropils called glomeruli...

s, located in the inner layer of the olfactory bulb, form synapses with the axons of the sensory neurons within glomeruli and send the information about the odor
Odor
An odor or odour is caused by one or more volatilized chemical compounds, generally at a very low concentration, that humans or other animals perceive by the sense of olfaction. Odors are also commonly called scents, which can refer to both pleasant and unpleasant odors...

 to other parts of the olfactory system, where multiple signals may be processed to form a synthesized olfactory perception. A large degree of convergence occurs, with twenty-five thousand axons synapsing on twenty five or so mitral cells, and with each of these mitral cells projecting to multiple glomeruli. Mitral cells also project to periglomerular cells and granular cell
Granule cell
In neuroscience, granule cells refer to tiny neurons that are around 10 micrometres in diameter. Granule cells are found within the granular layer of the cerebellum , the dentate gyrus of the...

s that inhibit the mitral cells surrounding it (lateral inhibition
Lateral inhibition
In neurobiology, lateral inhibition is the capacity of an excited neuron to reduce the activity of its neighbors. Lateral inhibition sharpens the spatial profile of excitation in response to a localized stimulus.-Sensory inhibition:...

). Granular cells also mediate inhibition and excitation of mitral cells through pathways from centrifugal fibers and the anterior olfactory nuclei.

The mitral cells leave the olfactory bulb in the lateral olfactory tract, which synapses on five major regions of the cerebrum: the anterior olfactory nucleus
Anterior olfactory nucleus
The anterior olfactory nucleus is a portion of the forebrain of vertebrates.It is involved in olfaction.-Location:...

, the olfactory tubercle
Olfactory tubercle
The olfactory tubercle is a structure involved in Olfaction.It is present in humans, but much smaller than it is in some other animals.It is a frequent subject of research.-External links:...

, the amygdala
Amygdala
The ' are almond-shaped groups of nuclei located deep within the medial temporal lobes of the brain in complex vertebrates, including humans. Shown in research to perform a primary role in the processing and memory of emotional reactions, the amygdalae are considered part of the limbic system.-...

, the piriform cortex
Piriform cortex
In anatomy of animals, the piriform cortex, or pyriform cortex is a region in the brain.-Anatomy and function:The piriform cortex is part of the rhinencephalon situated in the telencephalon....

, and the entorhinal cortex
Entorhinal cortex
The entorhinal cortex is located in the medial temporal lobe and functions as a hub in a widespread network for memory and navigation. The EC is the main interface between the hippocampus and neocortex...

. The anterior olfactory nucleus projects, via the anterior commissure
Anterior commissure
The anterior commissure is a bundle of nerve fibers , connecting the two cerebral hemispheres across the midline, and placed in front of the columns of the fornix...

, to the contralateral olfactory bulb, inhibiting it. The piriform cortex projects to the medial dorsal nucleus
Medial dorsal nucleus
The medial dorsal nucleus is a large nucleus in the thalamus.It is believed to play a role in memory.-Anatomy:...

 of the thalamus, which then projects to the orbitofrontal cortex. The orbitofrontal cortex mediates conscious perception of the odor. The 3-layered piriform cortex projects to a number of thalamic and hypothalamic nuclei, the hippocampus and amygdala and the orbitofrontal cortex but its function is largely unknown. The entorhinal cortex projects to the amygdala
Amygdala
The ' are almond-shaped groups of nuclei located deep within the medial temporal lobes of the brain in complex vertebrates, including humans. Shown in research to perform a primary role in the processing and memory of emotional reactions, the amygdalae are considered part of the limbic system.-...

 and is involved in emotional and autonomic responses to odor. It also projects to the hippocampus and is involved in motivation and memory. Odor information is stored in long-term memory
Long-term memory
Long-term memory is memory in which associations among items are stored, as part of the theory of a dual-store memory model. According to the theory, long term memory differs structurally and functionally from working memory or short-term memory, which ostensibly stores items for only around 20–30...

 and has strong connections to emotional memory. This is possibly due to the olfactory system's close anatomical ties to the limbic system
Limbic system
The limbic system is a set of brain structures including the hippocampus, amygdala, anterior thalamic nuclei, septum, limbic cortex and fornix, which seemingly support a variety of functions including emotion, behavior, long term memory, and olfaction. The term "limbic" comes from the Latin...

 and hippocampus
Hippocampus
The hippocampus is a major component of the brains of humans and other vertebrates. It belongs to the limbic system and plays important roles in the consolidation of information from short-term memory to long-term memory and spatial navigation. Humans and other mammals have two hippocampi, one in...

, areas of the brain that have long been known to be involved in emotion and place memory, respectively.

Since any one receptor is responsive to various odorants, and there is a great deal of convergence at the level of the olfactory bulb, it seems strange that human beings are able to distinguish so many different odors. It seems that there must be a highly-complex form of processing occurring; however, as it can be shown that, while many neurons in the olfactory bulb (and even the pyriform cortex and amygdala) are responsive to many different odors, half the neurons in the orbitofrontal cortex are responsive only to one odor, and the rest to only a few. It has been shown through microelectrode studies that each individual odor gives a particular specific spatial map of excitation in the olfactory bulb. It is possible that, through spatial encoding, the brain is able to distinguish specific odors. However, temporal coding must be taken into account. Over time, the spatial maps change, even for one particular odor, and the brain must be able to process these details as well.

Inputs from the two nostril
Nostril
A nostril is one of the two channels of the nose, from the point where they bifurcate to the external opening. In birds and mammals, they contain branched bones or cartilages called turbinates, whose function is to warm air on inhalation and remove moisture on exhalation...

s have separate inputs to the brain with the result that it is possible for humans to experience perceptual rivalry in the olfactory sense akin to that of binocular rivalry
Binocular rivalry
Binocular rivalry is a phenomenon of visual perception in which perception alternates between different images presented to each eye.When one image is presented to one eye and a very different image is presented to the other, instead of the two images being seen superimposed, one image is seen for...

 when there are two different inputs into the two nostrils.

In insect
Insect
Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...

s smells are sensed by sensilla located on the antenna and maxillary palp and first processed by the antennal lobe
Antennal lobe
The antennal lobe is the deutocerebral neuropil of insect which receives the input from the olfactory sensory neurons on the antenna. Functionally, it shares some similarities with the olfactory bulb in vertebrates....

 (analogous to the olfactory bulb
Olfactory bulb
The olfactory bulb is a structure of the vertebrate forebrain involved in olfaction, the perception of odors.-Anatomy:In most vertebrates, the olfactory bulb is the most rostral part of the brain. In humans, however, the olfactory bulb is on the inferior side of the brain...

), and next by the mushroom bodies and lateral horn.

Accessory olfactory system


Many animals, including most mammals and reptiles, but not humans, have two distinct and segregated olfactory systems: a main olfactory system, which detects volatile stimuli, and an accessory olfactory system, which detects fluid-phase stimuli. Behavioral evidence suggests that these fluid-phase stimuli often function as pheromone
Pheromone
A pheromone is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting outside the body of the secreting individual to impact the behavior of the receiving individual...

s, although pheromones can also be detected by the main olfactory system. In the accessory olfactory system, stimuli are detected by the vomeronasal organ
Vomeronasal organ
The vomeronasal organ , or Jacobson's organ, is an auxiliary olfactory sense organ that is found in many animals. It was discovered by Frederik Ruysch and later by Ludwig Jacobson in 1813....

, located in the vomer, between the nose
Nose
Anatomically, a nose is a protuberance in vertebrates that houses the nostrils, or nares, which admit and expel air for respiration in conjunction with the mouth. Behind the nose are the olfactory mucosa and the sinuses. Behind the nasal cavity, air next passes through the pharynx, shared with the...

 and the mouth
Mouth
The mouth is the first portion of the alimentary canal that receives food andsaliva. The oral mucosa is the mucous membrane epithelium lining the inside of the mouth....

. Snakes use it to smell prey, sticking their tongue out and touching it to the organ. Some mammals make a face called flehmen to direct air to this organ.

The sensory receptors of the accessory olfactory system are located in the vomeronasal organ
Vomeronasal organ
The vomeronasal organ , or Jacobson's organ, is an auxiliary olfactory sense organ that is found in many animals. It was discovered by Frederik Ruysch and later by Ludwig Jacobson in 1813....

. As in the main olfactory system, the axons of these sensory neurons project from the vomeronasal organ to the accessory olfactory bulb, located on the dorsal-posterior portion of the main olfactory bulb
Olfactory bulb
The olfactory bulb is a structure of the vertebrate forebrain involved in olfaction, the perception of odors.-Anatomy:In most vertebrates, the olfactory bulb is the most rostral part of the brain. In humans, however, the olfactory bulb is on the inferior side of the brain...

. Unlike in the main olfactory system, the axons that leave the accessory olfactory bulb do not project to the brain's cortex but rather to targets in the amygdala
Amygdala
The ' are almond-shaped groups of nuclei located deep within the medial temporal lobes of the brain in complex vertebrates, including humans. Shown in research to perform a primary role in the processing and memory of emotional reactions, the amygdalae are considered part of the limbic system.-...

 and hypothalamus
Hypothalamus
The Hypothalamus is a portion of the brain that contains a number of small nuclei with a variety of functions...

 where they may influence aggressive and mating behavior.

Human olfactory system


In women, the sense of olfaction is strongest around the time of ovulation
Ovulation
Ovulation is the process in a female's menstrual cycle by which a mature ovarian follicle ruptures and discharges an ovum . Ovulation also occurs in the estrous cycle of other female mammals, which differs in many fundamental ways from the menstrual cycle...

, significantly stronger than during other phases of the menstrual cycle
Menstrual cycle
The menstrual cycle is the scientific term for the physiological changes that can occur in fertile women for the purpose of sexual reproduction. This article focuses on the human menstrual cycle....

 and also stronger than the sense in males.

The MHC
Major histocompatibility complex
Major histocompatibility complex is a cell surface molecule encoded by a large gene family in all vertebrates. MHC molecules mediate interactions of leukocytes, also called white blood cells , which are immune cells, with other leukocytes or body cells...

 genes (known as HLA
Human leukocyte antigen
The human leukocyte antigen system is the name of the major histocompatibility complex in humans. The super locus contains a large number of genes related to immune system function in humans. This group of genes resides on chromosome 6, and encodes cell-surface antigen-presenting proteins and...

 in humans) are a group of genes present in many animals and important for the immune system
Immune system
An immune system is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumor cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own...

; in general, offspring from parents with differing MHC genes have a stronger immune system. Fish, mice and female humans are able to smell some aspect of the MHC genes of potential sex partners and prefer partners with MHC genes different from their own.

Humans can detect individuals that are blood related kin (mothers and children but not husbands and wives) from olfaction. Mothers can identify by body odor their biological children but not their stepchildren. Preadolescent children can olfactorily detect their full siblings but not half-siblings or step siblings and this might explain incest avoidance and the Westermarck effect
Westermarck effect
The Westermarck effect, or reverse sexual imprinting, is a hypothetical psychological effect through which people who live in close domestic proximity during the first few years of their lives become desensitized to later sexual attraction. This phenomenon was first hypothesized by Finnish...

. Functional imaging shows that this olfactory kinship detection process involves the frontal-temporal junction, the insula
Insular cortex
In each hemisphere of the mammalian brain the insular cortex is a portion of the cerebral cortex folded deep within the lateral sulcus between the temporal lobe and the frontal lobe. The cortical area overlying it towards the lateral surface of the brain is the operculum...

, and the dorsomedial prefrontal cortex
Prefrontal cortex
The prefrontal cortex is the anterior part of the frontal lobes of the brain, lying in front of the motor and premotor areas.This brain region has been implicated in planning complex cognitive behaviors, personality expression, decision making and moderating correct social behavior...

 but not the primary or secondary olfactory cortices, or the related piriform cortex
Piriform cortex
In anatomy of animals, the piriform cortex, or pyriform cortex is a region in the brain.-Anatomy and function:The piriform cortex is part of the rhinencephalon situated in the telencephalon....

 or orbitofrontal cortex
Orbitofrontal cortex
The orbitofrontal cortex is a prefrontal cortex region in the frontal lobes in the brain which is involved in the cognitive processing of decision-making...

.

Olfactory coding and perception


How olfactory information is coded in the brain to allow for proper perception is still being researched and the process is not completely understood. However, what is known is that the chemical nature of the odorant is particularly important as there may be a chemotopic map in the brain; this map would show specific activation patterns for specific odorants. When an odorant is detected by receptors, the receptors in a sense break the odorant down and then the brain puts the odorant back together for identification and perception. The odorant binds to receptors which only recognize a specific functional group, or feature, of the odorant, which is why the chemical nature of the odorant is important.

After binding the odorant, the receptor is activated and will send a signal to the glomeruli. Each glomerulus receive signals from multiple receptors that detect similar odorant features. Because multiple receptor types are activated due to the different chemical features of the odorant, multiple glomeruli will be activated as well. All of the signals from the glomeruli will then be sent to the brain, where the combination of glomeruli activation will encode the different chemical features of the odorant. The brain will then essentially put the pieces of the activation pattern back together in order to identify and perceive the odorant.

Odorants that are similar in structure activate similar patterns of glomeruli, which lead to a similar perception in the brain. Data from animal models, suggests that the brain may have a chemotopic map. A chemotopic map is an area in the brain, specifically the olfactory bulb, in which glomeruli project their signals onto the brain in a specific pattern. The idea of the chemotopic map has been supported by the observation that chemicals containing similar functional groups have similar responses with overlapped areas in the brain. This is important because it allows the possibility to predict the neural activation pattern from an odorant and vice versa.

Olfaction and taste


Olfaction, taste
Taste
Taste is one of the traditional five senses. It refers to the ability to detect the flavor of substances such as food, certain minerals, and poisons, etc....

 and trigeminal
Trigeminal nerve
The trigeminal nerve contains both sensory and motor fibres. It is responsible for sensation in the face and certain motor functions such as biting, chewing, and swallowing. Sensory information from the face and body is processed by parallel pathways in the central nervous system...

 receptors (also called Chemesthesis
Chemesthesis
Chemesthesis is defined as the chemical sensibility of the skin and mucus membranes. Chemesthetic sensations arise when chemical compounds activate receptors associated with other senses that mediate pain, touch, and thermal perception. These chemical-induced reactions do not fit into the...

) together contribute to flavor
Flavor
Flavor or flavour is the sensory impression of a food or other substance, and is determined mainly by the chemical senses of taste and smell. The "trigeminal senses", which detect chemical irritants in the mouth and throat as well as temperature and texture, are also very important to the overall...

. The human tongue
Tongue
The tongue is a muscular hydrostat on the floors of the mouths of most vertebrates which manipulates food for mastication. It is the primary organ of taste , as much of the upper surface of the tongue is covered in papillae and taste buds. It is sensitive and kept moist by saliva, and is richly...

 can distinguish only among five distinct qualities of taste
Taste
Taste is one of the traditional five senses. It refers to the ability to detect the flavor of substances such as food, certain minerals, and poisons, etc....

, while the nose can distinguish among hundreds of substances, even in minute quantities. It is during exhalation
Exhalation
Exhalation is the movement of air out of the bronchial tubes, through the airways, to the external environment during breathing....

 that the olfaction contribution to flavor
Flavor
Flavor or flavour is the sensory impression of a food or other substance, and is determined mainly by the chemical senses of taste and smell. The "trigeminal senses", which detect chemical irritants in the mouth and throat as well as temperature and texture, are also very important to the overall...

 occurs in contrast to that of proper smell which occurs during the inhalation
Inhalation
Inhalation is the movement of air from the external environment, through the air ways, and into the alveoli....

 phase

Olfaction and audition


Olfaction and sound
Sound
Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...

 information has been shown to converge in the olfactory tubercles of rodents. This neural convergence is proposed to give rise to a percept termed smound
Smound
Smound is a term used to describe a purported perception or sense experience created from the convergence of scents, olfaction and sounds in the brain...

. Whereas a flavor
Flavor
Flavor or flavour is the sensory impression of a food or other substance, and is determined mainly by the chemical senses of taste and smell. The "trigeminal senses", which detect chemical irritants in the mouth and throat as well as temperature and texture, are also very important to the overall...

 results from interactions between smell and taste, a smound may result from interactions between smell and sound.

Disorders of olfaction


The following are disorders of olfaction:
  • Anosmia
    Anosmia
    Anosmia is a lack of functioning olfaction, or in other words, an inability to perceive odors. Anosmia may be either temporary or permanent. A related term, hyposmia, refers to a decreased ability to smell, while hyperosmia refers to an increased ability to smell. Some people may be anosmic for one...

     – inability to smell
  • Cacosmia
    Cacosmia
    Cacosmia is a genus of flowering plant in the Asteraceae family.It contains the following species:* Cacosmia harlingii* Cacosmia hieronymi...

     – things smell like feces
    Feces
    Feces, faeces, or fæces is a waste product from an animal's digestive tract expelled through the anus or cloaca during defecation.-Etymology:...

  • Dysosmia
    Dysosmia
    Dysosmia, also known as olfactory dysfunction, is the impairment of olfactory stimuli processing leading to an altered sense of smell.These dysfunctions can present in a variety of ways, such as the stimuli not activating the olfactory bulb, some odors being interpreted as other odors, or...

     – things smell different than they should
  • Hyperosmia
    Hyperosmia
    Hyperosmia is an increased ability to smell - for example, being able to identify the perfume of the previous occupant of a chair. It is seen in patients with cluster headaches, migraines, and adrenal cortical insufficiency , although some people possess it naturally...

     – an abnormally acute sense of smell.
  • Hyposmia
    Hyposmia
    Hyposmia is a reduced ability to smell and to detect odours. A related condition is anosmia, in which no odours can be detected. Some of the causes of olfaction problems are allergies, nasal polyps, viral infections and head trauma...

     – decreased ability to smell
  • Olfactory Reference Syndrome
    Olfactory Reference Syndrome
    Olfactory Reference Syndrome is a psychiatric condition in which the affected person is excessively preoccupied by the concern that one's body odor is foul or unpleasant. This disorder is often accompanied by shame, embarrassment, significant distress, avoidance behavior, social phobia and social...

     – psychological disorder which causes the patient to imagine he or she has strong body odor
    Body odor
    Body odor or body odour, sometimes colloquially abbreviated as B.O., is the smell of bacteria growing on the body. The bacteria multiply rapidly in the presence of sweat, but sweat itself is almost completely odorless to humans....

  • Parosmia
    Parosmia
    Parosmia, also known as troposmia or cacosmia, is an olfactory dysfunction that is characterized by the inability of the brain to properly identify an odor’s “natural” smell....

     – things smell worse than they should
  • Phantosmia
    Phantosmia
    Phantosmia, or olfactory hallucinations, involves smelling odors that are not derived from any physical stimulus. These phantom odors can range from rotting flesh to a spring meadow, though most cases report unpleasant aromas...

     – "hallucinated smell," often unpleasant in nature

Quantifying olfaction in industry


Scientists have devised methods for quantifying the intensity of odors, particularly for the purpose of analyzing unpleasant or objectionable odors released by an industrial source into a community. Since the 1800s, industrial countries have encountered incidents where proximity of an industrial source or landfill produced adverse reactions to nearby residents regarding airborne odor. The basic theory of odor analysis is to measure what extent of dilution with "pure" air is required before the sample in question is rendered indistinguishable from the "pure" or reference standard. Since each person perceives odor differently, an "odor panel" composed of several different people is assembled, each sniffing the same sample of diluted specimen air. A field olfactometer
Olfactometer
An olfactometer is an instrument typically used to detect and measure ambient odor dilution. Olfactometers are used in conjunction with human subjects in laboratory settings, most often in market research, to quantify and qualify human olfaction. Olfactometers are used to gauge the odor detection...

 can be utilized to determine the magnitude of an odor.

Many air management districts in the USA 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
Bay Area Air Quality Management District
The Bay Area Air Quality Management District is a public agency that regulates the stationary sources of air pollution in the nine counties of California's San Francisco Bay Area: Alameda, Contra Costa, Marin, Napa, San Francisco, San Mateo, Santa Clara, southwestern Solano, and southern Sonoma...

 has applied its standard in regulating numerous industries, landfills, and sewage treatment plants. Example applications this district has engaged are the San Mateo, California
San Mateo, California
San Mateo is a city in San Mateo County, California, United States, in the San Francisco Bay Area. With a population of approximately 100,000 , it is one of the larger suburbs on the San Francisco Peninsula, located between Burlingame to the north, Foster City to the east, Belmont to the south,...

 wastewater treatment plant; the Shoreline Amphitheatre
Shoreline Amphitheatre
Shoreline Amphitheatre is an outdoor amphitheater, in Mountain View, California, USA, in the San Francisco Bay Area. Inside the venue it has a capacity of 22,500, with 6,500 reserved seats and 16,000 general admission on the lawn...

 in Mountain View, California
Mountain View, California
-Downtown:Mountain View has a pedestrian-friendly downtown centered on Castro Street. The downtown area consists of the seven blocks of Castro Street from the Downtown Mountain View Station transit center in the north to the intersection with El Camino Real in the south...

; and the IT Corporation
IT Corporation
IT Corporation was a United States industrial company whose principal business was the disposal of industrial hazardous waste. At least as early as the 1970s the company was one of the largest market share holders of the liquid hazardous waste disposal sector in the western U.S...

 waste ponds, Martinez, California
Martinez, California
Martinez is a city and the county seat of Contra Costa County, California, United States. The population was 35,824 at the 2010 census. The downtown is notable for its large number of preserved old buildings...

.

Olfaction in plants and animals


The tendrils of plants are especially sensitive to airborne volatile organic compound
Volatile organic compound
Volatile organic compounds are organic chemicals that have a high vapor pressure at ordinary, room-temperature conditions. Their high vapor pressure results from a low boiling point, which causes large numbers of molecules to evaporate or sublimate from the liquid or solid form of the compound and...

s. Parasites such as dodder make use of this in locating their preferred hosts and locking on to them. The emission of volatile compounds is detected when foliage is browsed by animals. Threatened plants are then able to take defensive chemical measures, such as moving tannin
Tannin
A tannin is an astringent, bitter plant polyphenolic compound that binds to and precipitates proteins and various other organic compounds including amino acids and alkaloids.The term tannin refers to the use of...

 compounds to their foliage. (see Plant perception).

The importance and sensitivity of smell varies among different organisms; most mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...

s have a good sense of smell, whereas most bird
Bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...

s do not, except the tubenoses
Procellariiformes
Procellariiformes is an order of seabirds that comprises four families: the albatrosses, petrels and shearwaters, storm petrels, and diving petrels...

 (e.g., petrel
Petrel
Petrels are tube-nosed seabirds in the bird order Procellariiformes. The common name does not indicate relationship beyond that point, as "petrels" occur in three of the four families within that group...

s and albatross
Albatross
Albatrosses, of the biological family Diomedeidae, are large seabirds allied to the procellariids, storm-petrels and diving-petrels in the order Procellariiformes . They range widely in the Southern Ocean and the North Pacific...

es), certain species of vultures and the kiwi
Kiwi
Kiwi are flightless birds endemic to New Zealand, in the genus Apteryx and family Apterygidae.At around the size of a domestic chicken, kiwi are by far the smallest living ratites and lay the largest egg in relation to their body size of any species of bird in the world...

s. Among mammals, it is well-developed in the carnivore
Carnivore
A carnivore meaning 'meat eater' is an organism that derives its energy and nutrient requirements from a diet consisting mainly or exclusively of animal tissue, whether through predation or scavenging...

s and ungulate
Ungulate
Ungulates are several groups of mammals, most of which use the tips of their toes, usually hoofed, to sustain their whole body weight while moving. They make up several orders of mammals, of which six to eight survive...

s, who must always be aware of each other, and in those that smell for their food, like mole
Mole (animal)
Moles are small cylindrical mammals adapted to a subterranean lifestyle. They have velvety fur; tiny or invisible ears and eyes; and short, powerful limbs with large paws oriented for digging. The term is especially and most properly used for the true moles, those of the Talpidae family in the...

s. Having a strong sense of smell is referred to as macrosmatic.

Figures suggesting greater or lesser sensitivity in various species reflect experimental findings from the reactions of animals exposed to aromas in known extreme dilutions. These are, therefore, based on perceptions by these animals, rather than mere nasal function. That is, the brain's smell-recognizing centers must react to the stimulus detected, for the animal to show a response to the smell in question. It is estimated that dogs in general have an olfactory sense approximately a hundred thousand to a million times more acute than a human's. This does not mean they are overwhelmed by smells our noses can detect; rather, it means they can discern a molecular presence when it is in much greater dilution in the carrier, air.
Scenthounds as a group can smell one- to ten-million times more acutely than a human, and Bloodhound
Bloodhound
The Bloodhound is a large breed of dog which, while originally bred to hunt deer and wild boar, was later bred specifically to track human beings. It is a scenthound, tracking by smell, as opposed to a sighthound, which tracks using vision. It is famed for its ability to discern human odors even...

s, which have the keenest sense of smell of any dogs , have noses ten- to one-hundred-million times more sensitive than a human's. They were bred for the specific purpose of tracking humans, and can detect a scent trail a few days old. The second-most-sensitive nose is possessed by the Basset Hound
Basset Hound
The Basset Hound is a short-legged breed of dog of the hound family. They are scent hounds, bred to hunt rabbits and hare by scent. Their sense of smell for tracking is second only to that of the Bloodhound....

, which was bred to track and hunt rabbits and other small animals.

Bears, such as the Silvertip Grizzly found in parts of North America, have a sense of smell seven times stronger than 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 18 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.

The sense of smell is less-developed in the catarrhine primate
Primate
A primate is a mammal of the order Primates , which contains prosimians and simians. Primates arose from ancestors that lived in the trees of tropical forests; many primate characteristics represent adaptations to life in this challenging three-dimensional environment...

s (Catarrhini
Catarrhini
Catarrhini is one of the two subdivisions of the higher primates . It contains the Old World monkeys and the apes, which in turn are further divided into the lesser apes or gibbons and the great apes, consisting of the orangutans, gorillas, chimpanzees, bonobos, and humans...

), and apparently nonexistent in cetacea
Cetacea
The order Cetacea includes the marine mammals commonly known as whales, dolphins, and porpoises. Cetus is Latin and is used in biological names to mean "whale"; its original meaning, "large sea animal", was more general. It comes from Ancient Greek , meaning "whale" or "any huge fish or sea...

ns, which compensate with a well-developed sense of taste
Taste
Taste is one of the traditional five senses. It refers to the ability to detect the flavor of substances such as food, certain minerals, and poisons, etc....

. In some prosimian
Prosimian
Prosimians are a grouping of mammals defined as being primates, but not monkeys or apes. They include, among others, lemurs, bushbabies, and tarsiers. They are considered to have characteristics that are more primitive than those of monkeys and apes. Prosimians are the only primates native to...

s, such as the Red-bellied Lemur
Red-bellied Lemur
The Red-bellied Lemur is a medium sized prosimian with a luxuriant chestnut brown coat. This lemur is endemic to eastern Madagascan rainforests and is distinguished by patches of white skin below the eyes, giving rise to a "teardrop" effect, particularly conspicuous in the male.The species, first...

, scent glands occur atop the head. In many species, olfaction is highly tuned to pheromone
Pheromone
A pheromone is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting outside the body of the secreting individual to impact the behavior of the receiving individual...

s; a male silkworm moth, for example, can sense a single molecule of bombykol
Bombykol
Bombykol is a pheromone released by the female silkworm moth to attract mates. Discovered by Adolf Butenandt in 1959, it was the first pheromone to be characterized chemically...

.

Fish too have a well-developed sense of smell, even though they inhabit an aquatic environment. Salmon utilize their sense of smell to identify and return to their home stream waters. Catfish use their sense of smell to identify other individual catfish and to maintain a social hierarchy. Many fishes use the sense of smell to identify mating partners or to alert to the presence of food.

Insects primarily use their antennae
Antenna (biology)
Antennae in biology have historically been paired appendages used for sensing in arthropods. More recently, the term has also been applied to cilium structures present in most cell types of eukaryotes....

 for olfaction. Sensory neurons in the antenna generate odor-specific electrical signals called spikes in response to odor. They process these signals from the sensory neurons in the antennal lobe
Antennal lobe
The antennal lobe is the deutocerebral neuropil of insect which receives the input from the olfactory sensory neurons on the antenna. Functionally, it shares some similarities with the olfactory bulb in vertebrates....

  followed by the mushroom bodies and lateral horn of the brain. The antennae have the sensory neurons in the sensilla and they have their axons terminating in the antennal lobes where they synapse with other neurons there in semidelineated (with membrane boundaries) called glomeruli. These antennal lobes have two kinds of neurons, projection neurons (excitatory) and local neurons (inhibitory). The projection neurons send their axon terminals to mushroom body and lateral horn (both of which are part of the protocerebrum of the insects), and local neurons have no axons. Recordings from projection neurons show in some insects strong specialization and discrimination for the odors presented (especially for the projection neurons of the macroglomeruli, a specialized complex of glomeruli responsible for the pheromones detection). Processing beyond this level is not exactly known though some preliminary results are available.

See also


  • Electronic nose
    Electronic nose
    An electronic nose is a device intended to detect odors or flavors.Over the last decade, “electronic sensing” or “e-sensing” technologies have undergone important developments from a technical and commercial point of view...

  • Machine olfaction
    Machine olfaction
    Machine olfaction is the automated simulation of the sense of smell. It is an emerging requirement of modern robotics where robots or other automated systems are needed to measure the existence of a particular chemical concentration in air...

  • Nasal administration
    Nasal administration
    Nasal administration can be used to deliver drugs for either local or systemic effect. Locally acting drugs are for example decongestants and allergy treatments...

     olfactory transfer
  • Odor
    Odor
    An odor or odour is caused by one or more volatilized chemical compounds, generally at a very low concentration, that humans or other animals perceive by the sense of olfaction. Odors are also commonly called scents, which can refer to both pleasant and unpleasant odors...

  • Olfactometer
    Olfactometer
    An olfactometer is an instrument typically used to detect and measure ambient odor dilution. Olfactometers are used in conjunction with human subjects in laboratory settings, most often in market research, to quantify and qualify human olfaction. Olfactometers are used to gauge the odor detection...

  • Olfactory fatigue
    Olfactory fatigue
    Olfactory fatigue or adaptation is the temporary, normal inability to distinguish a particular odor after a prolonged exposure to that airborne compound. For example, when entering a restaurant initially the odor of food is often perceived as being very strong, but after time the awareness of the...

  • Chemesthesis
    Chemesthesis
    Chemesthesis is defined as the chemical sensibility of the skin and mucus membranes. Chemesthetic sensations arise when chemical compounds activate receptors associated with other senses that mediate pain, touch, and thermal perception. These chemical-induced reactions do not fit into the...

  • Major Histocompatibility Complex and Sexual Selection
    Major Histocompatibility Complex and Sexual Selection
    A pathogen is any agent that causes disease. The body has two defense mechanisms for dealing with pathogenic microorganisms and other harmful substances: the inflammatory reaction and the development of an acquired immunity...


External links