Olfaction (also known as olfactics or smell) refers to the sense of smell. Senses are the physiological methods of Perception. The senses and their operation classification and theory are overlapping topics studied by a variety of fields This sense is mediated by specialized sensory cells of the nasal cavity of vertebrates, and, by analogy, sensory cells of the antennae of invertebrates. For air-breathing animals, the olfactory system detects volatile or, in the case of the accessory olfactory system, fluid-phase chemicals. The Accessory olfactory system (AOS is one of the two Olfactory systems commonly found in Vertebrates. For water-dwelling organisms, e. g. , fish or crustaceans, the chemicals are present in the surrounding aqueous medium. Olfaction, along with taste, is a form of chemoreception. Taste (or more formally gustation) is a form of direct Chemoreception and is one of the traditional five Senses A chemosensor, also known as chemoreceptor, is a Sensory receptor that transduces a chemical signal into an Action potential. The chemicals themselves which activate the olfactory system, generally at very low concentrations, are called odors.
As described by the Roman philosopher Lucretius (1st Century BCE), different odors are attributed to different shapes and sizes of odor molecules that stimulate the olfactory organ. Titus Lucretius Carus (ca 99 BC- ca 55 BC was a Roman Poet and Philosopher. The modern counterpart to that theory was the cloning of olfactory receptor proteins by Linda B. Buck and Richard Axel (who were awarded the Nobel Prize in 2004), and subsequent pairing of odor molecules to specific receptor proteins. Linda B Buck, PhD, (born January 29, 1947) is an American Biologist best known for her work on the Olfactory system Richard Axel (born July 2, 1946, New York City) is an American Neuroscientist whose work on the Olfactory system won him The Nobel Prize (Nobelpriset (Nobelprisen is a Swedish prize established in the 1895 will of Swedish chemist Alfred Nobel; it was first awarded in Peace, Literature Each odor receptor molecule recognizes only a particular molecular feature or class of odor molecules. Mammals have about a thousand genes expressing for odor reception. Mammals ( class Mammalia) are a class of Vertebrate Animals characterized by the presence of Sweat glands, including sweat glands History See also History of genetics The existence of genes was first suggested by Gregor Mendel (1822-1884 who in the 1860s studied inheritance Olfactory receptors expressed in the Cell membranes of Olfactory receptor neurons are responsible for the detection of Odor molecules  Of these genes, only a portion are functional odor receptors. Humans have far fewer active odor receptor genes than other mammals and primates
Each olfactory receptor neuron expresses only one functional odor receptor. History See also History of genetics The existence of genes was first suggested by Gregor Mendel (1822-1884 who in the 1860s studied inheritance An olfactory receptor neuron also called an olfactory sensory neuron is the primary transduction cell in the Olfactory system. 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 shape theory, each receptor detects a feature of the odor molecule. 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 Chemistry, a molecule is defined as a sufficiently stable electrically neutral group of at least two Atoms in a definite arrangement held together by Weak-shape theory, known as odotope theory, 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). Odotope Theory, also known as Weak-Shape Theory is a leading neurophysiological theory of how the sense of smell functions An alternative theory, the vibration theory proposed by Luca Turin, posits that odor receptors detect the frequencies of vibrations of odor molecules in the infrared range by electron tunnelling. The Vibration theory of smell proposes that a molecule's smell character is due to its vibrational frequency in the infrared range Luca Turin ( 1953 -) is a biophysicist with a long-standing interest in the sense of smell the art of perfume and the Fragrance industry In Quantum mechanics, quantum tunnelling is a nanoscopic phenomenon in which a particle violates the principles of Classical mechanics by penetrating a However, the behavioral predictions of this theory have been called into question.  As of yet, there is no theory that explains olfactory perception completely.
In vertebrates smells are sensed by olfactory sensory neurons in the olfactory epithelium. Vertebrates are members of the Subphylum Vertebrata, Chordates with backbones or spinal columns The grouping sometimes includes An olfactory receptor neuron also called an olfactory sensory neuron is the primary transduction cell in the Olfactory system. The olfactory epithelium is a specialized epithelial tissue inside the Nasal cavity that is involved in smell. The proportion of olfactory epithelium compared to respiratory epithelium (not innervated) gives an indication of the animal's olfactory sensitivity. In biology and medicine epithelium is a tissue composed of cells that line the cavities and surfaces of structures throughout the body Humans have about 10 cm² of olfactory epithelium, whereas some dogs have 170 cm2. 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 of the nasal passages dissolve in the mucus lining the superior portion of the cavity and are detected by olfactory receptors on the dendrites of the olfactory sensory neurons. 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 In vertebrates mucus is a slippery secretion produced by and covering Mucous membranes It is a viscous Colloid containing Antiseptic enzymes (such as Olfactory receptors expressed in the Cell membranes of Olfactory receptor neurons are responsible for the detection of Odor molecules Dendrites (from Greek δένδρον déndron, “tree” are the branched projections of a Neuron that act to conduct the electrochemical This may occur by diffusion or by the binding of the odorant to odorant binding proteins. 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 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). Glycosaminoglycans (GAGs or mucopolysaccharides are long unbranched Polysaccharides consisting of a repeating Disaccharide unit Enzymes are Biomolecules that catalyze ( ie increase the rates of Chemical reactions Almost all enzymes are Proteins Antibodies (also known as immunoglobulins, abbreviated Ig) are Gamma globulin Proteins that are found in Blood or other Bodily The brain is the center of the Nervous system in animals All Vertebrates and the majority of Invertebrates have a brain
In insects smells are sensed by olfactory sensory neurons in the chemosensory sensilla, which are present in insect antenna, palps and tarsa, but also on other parts of the insect body. Insects ( Class Insecta) are a major group of Arthropods and the most diverse group of Animals on the Earth with over a million described An olfactory receptor neuron also called an olfactory sensory neuron is the primary transduction cell in the Olfactory system. Sensilla ( sg.= sensillum are Insect Sensory organs protruding from the Cuticle, or sometime lie within or beneath it Odorants penetrate into the cuticle pores of chemosensory sensilla and get in contact with insect Odorant binding proteins (OBPs) or Chemosensory proteins (CSPs), before activating the sensory neurons. 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 Chemosensory proteins are a class of small (10-15 kDa soluble proteins characterised for the first time by Angeli et al
The process of how the binding of the ligand (odor molecule or odorant) to the receptor leads to an action potential in the receptor neuron is via a second messenger pathway depending on the organism. In Chemistry, a ligand is either an Atom, Ion, or Molecule (see also Functional group) that bonds to a central metal generally In Neurophysiology, the action potential is a self-regenerating Wave of Electrochemical activity that allows Nerve cells to carry a signal In Cell physiology, a secondary messenger system (also known as a second messenger system) is a method of cellular signaling whereby a diffusable signaling molecule In mammals the odorants stimulate adenylate cyclase to synthesize cAMP via a G protein called Golf. Adenylate cyclase ( also known as adenylyl cyclase or AC) is a Lyase Enzyme. Cyclic adenosine monophosphate ( cAMP, cyclic AMP or 3'-5'-cyclic Adenosine monophosphate) is a Second messenger that is important in many biological GTP chemical structurepng|thumb|180px| Guanosine triphosphate]] G proteins short for guanine nucleotide-binding proteins, are a family of Proteins involved cAMP, which is the second messenger here, opens a cyclic nucleotide-gated ion channel (CNG) producing an influx of cations (largely Ca2+ with some Na+) into the cell, slightly depolarising it. A cyclic nucleotide-gated (CNG ion channel is any Ion channel that opens in the presence of Cyclic nucleotides Mechanism The channels are gated by An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge Calcium (ˈkælsiəm is the Chemical element with the symbol Ca and Atomic number 20 Sodium (ˈsoʊdiəm is an element which has the symbol Na( Latin natrium, from Arabic natrun) atomic number 11 atomic mass 22 The Ca2+ in turn opens a Ca2+-activated chloride channel, leading to efflux of Cl-, further depolarising the cell and triggering an action potential. Chloride channels are a superfamily of poorly understood Ion channels consisting of approximately 13 members Chlorine (ˈklɔriːn from the Greek word 'χλωρóς' ( khlôros, meaning 'pale green' is the Chemical element with Atomic number 17 and In Neurophysiology, the action potential is a self-regenerating Wave of Electrochemical activity that allows Nerve cells to carry a signal Ca2+ is then extruded through a sodium-calcium exchanger. The sodium-calcium exchanger (often denoted Na+/Ca2+ exchanger, NCX, or exchange protein) is an Antiporter Membrane A calcium-calmodulin complex also acts to inhibit the binding of cAMP to the cAMP-dependent channel, thus contributing to olfactory adaptation. Calmodulin ( CaM) (an abbreviation for CAL cium MODUL ated prote' IN') is a calcium-binding protein expressed in all Eukaryotic cells This mechanism of transduction is somewhat unique, in that cAMP works by directly binding to the ion channel rather than through activation of protein kinase A. Ion channels are pore-forming Proteins that help establish and control the small Voltage Gradient across the Plasma membrane of all living In Cell biology, protein kinase A, refers to a family of Enzymes whose activity is dependent on the level of Cyclic AMP (cAMP in the cell It is similar to the transduction mechanism for photoreceptors, in which the second messenger cGMP works by directly binding to ion channels, suggesting that maybe one of these receptors was evolutionarily adapted into the other. Photoreceptor can refer to In anatomy/cell biology Photoreceptor cell: a photosensitive cell most commonly referring to a specialized type of neuron Cyclic guanosine monophosphate ( cGMP) is a Cyclic nucleotide derived from Guanosine triphosphate (GTP There are also considerable similarities in the immediate processing of stimuli by lateral inhibition. In Neurobiology, lateral inhibition is the capacity of an excited Neuron to reduce the activity of its neighbors
Averaged activity of the receptor neuron to an odor can be measured by an electroolfactogram in vertebrates or an electroantenogram in insects. Electroantennogram or EAG is a technique by which we measure the average output of the antenna to the Brain for a given Odor.
The receptor neurons in the nose are particularly interesting because they are the only direct recipient of stimuli in all of the senses which are nerves. Senses like hearing, tasting, and, to some extent, touch use cilia or other indirect pressure to stimulate nerves, and sight uses the chemical Rhodopsin to stimulate the mind.
Olfactory sensory neurons project axons to the brain within the olfactory nerve, (cranial nerve I). An axon or nerve fiber is a long slender projectionof a nerve cell or Neuron, that conducts electrical impulses away from the neuron's Cell The olfactory nerve, or cranial nerve I, is the first of twelve Cranial nerves The specialized Olfactory receptor neurons of the olfactory nerve are located Cranial nerves are Nerves that emerge directly from the Brain stem in contrast to Spinal nerves which emerge from segments of the Spinal cord. These axons pass to the olfactory bulb through the cribriform plate, which in-turn projects olfactory information to the olfactory cortex and other areas. The olfactory bulb is a structure of the Vertebrate Forebrain involved in Olfaction, the perception of Odors. The cribriform plate of the Ethmoid bone ( horizontal lamina is received into the ethmoidal notch of the frontal bone and roofs in the nasal cavities. The Olfactory helps and relates sense of smell. The olfactory system is the Sensory system used for Olfaction. The axons from the olfactory receptors converge in the olfactory bulb within small (~50 micrometers in diameter) structures called glomeruli. Olfactory receptors expressed in the Cell membranes of Olfactory receptor neurons are responsible for the detection of Odor molecules The olfactory bulb is a structure of the Vertebrate Forebrain involved in Olfaction, the perception of Odors. A micrometre ( American spelling: micrometer; symbol µm) is one millionth of a Metre, or equivalently one thousandth of a Millimetre A glomerulus is a Capillary tuft surrounded by Bowman's capsule in Nephrons of the vertebrate Kidney. Mitral cells in the olfactory bulb form synapses with the axons within glomeruli and send the information about the odor to multiple other parts of the olfactory system in the brain, where multiple signals may be processed to form a synthesized olfactory perception. Mitral cells are Neurons that are part of the Olfactory system. There is a large degree of convergence here, with twenty-five thousand axons synapsing on one hundred or so mitral cells, and with each of these mitral cells projecting to multiple glomeruli. Mitral cells also project to periglomerular cells and granular cells that inhibit the mitral cells surrounding it (lateral inhibition). Granular cell also refers to Juxtaglomerular cell in the kidney In Neuroscience, granule cells refer to tiny neurons (a type of In Neurobiology, lateral inhibition is the capacity of an excited Neuron to reduce the activity of its neighbors Granular cells also mediate inhibition and excitation of mitral cells through pathways from centrifugal fibres 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, the olfactory tubercle, the amygdala, the piriform cortex, and the entorhinal cortex. The olfactory tract is a narrow white band triangular on Coronal section the apex being directed upward The anterior olfactory nucleus (AON also called the anterior olfactory cortex is a portion of the forebrain of vertebrates Olfaction (also known as olfactics or smell) refers to the Sense of smell. The la amygdalae ( Latin, also la corpus amygdaloideum, singular la amygdala, from Greek el αμυγδαλή grc-Latn amygdalē, 'almond' In Anatomy of Animals the piriform cortex, or pyriform cortex is a region in the Brain. The entorhinal cortex (EC is an important Memory center in the Brain. The anterior olfactory nucleus projects, via the anterior commissure, to the contralateral olfactory bulb, inhibiting it. The Anterior Commissure (precommissure is a bundle of white fibers connecting the two Cerebral hemispheres across the middle line and placed in front of the columns of the The piriform cortex projects to the medial dorsal nucleus of the thalamus, which then projects to the orbitofrontal cortex. The medial dorsal nucleus (or dorsomedial nucleus of thalamus) is a large nucleus in the Thalamus. 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 and is involved in emotional and autonomic responses to odor. The la amygdalae ( Latin, also la corpus amygdaloideum, singular la amygdala, from Greek el αμυγδαλή grc-Latn amygdalē, 'almond' It also projects to the hippocampus and is involved in motivation and memory. Odor information is easily stored in long-term memory and has strong connections to emotional memory. Long-term memory ( LTM) is Memory that can last as little as a few days or as long as decades Emotion can have a powerful impact on Memory. Numerous studies have shown that the most vivid autobiographical memories tend to be of emotional events which This is possibly due to the olfactory system's close anatomical ties to the limbic system and hippocampus, areas of the brain that have long been known to be involved in emotion and place memory, respectively. The limbic system, or Paleomammalian brain is a term for a set of brain structures including the Hippocampus and Amygdala and anterior thalamic nuclei and a limbic The hippocampus is a part of the Forebrain, located in the medial Temporal lobe.
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.
In insects smells are sensed by sensilla located on the antenna and first processed by the antennal lobe (analogous to the olfactory bulb), and next by the mushroom bodies. Insects ( Class Insecta) are a major group of Arthropods and the most diverse group of Animals on the Earth with over a million described Antennal lobe is the deutocerebral Neuropil of the insect which receives the input from the Olfactory sensory neurons on the antenna. The olfactory bulb is a structure of the Vertebrate Forebrain involved in Olfaction, the perception of Odors.
Many animals, including most mammals and reptiles, 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. The Accessory olfactory system (AOS is one of the two Olfactory systems commonly found in Vertebrates. Behavioral evidence suggests that these fluid-phase stimuli often function as pheromones, although pheromones can also be detected by the main olfactory system. A pheromone (from Greek φέρω phero "to bear" + ‘ορμόνη " Hormone " is a Chemical that triggers a natural In the accessory olfactory system, stimuli are detected by the vomeronasal organ, located in the vomer, between the nose and the mouth. The Accessory olfactory system (AOS is one of the two Olfactory systems commonly found in Vertebrates. The vomeronasal organ ( VNO) or Jacobson's organ, is an auxiliary Olfactory Sense organ that is found in many Animals It was 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 The mouth, buccal cavity, or oral cavity is the first portion of the Alimentary canal that receives food and begins digestion by mechanically breaking up 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 flehmen response, ˈflɛːmən or fleɪmən also called the flehmen position, flehmen reaction, flehming, or flehmening (from German
In women, the sense of olfaction is strongest around the time of ovulation, significantly stronger than during other phases of the menstrual cycle and also stronger than the sense in males. Note This article deals primarily with Human ovulation nonhuman Animal ovulation is touched on briefly at the conclusion The menstrual cycle is a recurring cycle of physiologic changes that occurs in reproductive-age Females Overt menstruation (where there is blood flow from the 
The MHC genes (known as HLA in humans) are a group of genes present in many animals and important for the immune system; in general, offspring from parents with differing MHC genes have a stronger immune system. The major histocompatibility complex ( MHC) is a large genomic region or Gene family found in most Vertebrates It is the most gene-dense region The human leukocyte antigen system ( HLA) is the name of the Major histocompatibility complex (MHC in humans An immune system is a collection of mechanisms within an Organism that protects against Disease by identifying and killing Pathogens and Tumor 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. 
Olfaction, taste and trigeminal receptors together contribute to flavor. Taste (or more formally gustation) is a form of direct Chemoreception and is one of the traditional five Senses The trigeminal nerve (the fifth Cranial nerve, also called the fifth nerve or simply V) is responsible for sensation in the face Flavor or flavour is the sensory impression of a Food or other substance, and is determined The human tongue can distinguish only among five distinct qualities of taste, while the nose can distinguish among hundreds of substances, even in minute quantities. The tongue is the large bundle of Skeletal muscles on the floor of the Mouth that manipulates Food for chewing and swallowing (deglutition Taste (or more formally gustation) is a form of direct Chemoreception and is one of the traditional five Senses
The following are disorders of olfaction:
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 can be utilized to determine the magnitude of an odor. One example is the Nasal Ranger field olfactometer, which is often utilized in odor studies. 
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. The United States of America —commonly referred to as the For example, the Bay Area Air Quality Management District has applied its standard in regulating numerous industries, landfills, and sewage treatment plants. History The first meeting of the Bay Area Air Pollution Control District (as it was initially known board of directors was on November 16 1955, possessing Example applications this district has engaged are the San Mateo, California wastewater treatment plant; the Shoreline Amphitheatre in Mountain View, California; and the IT Corporation waste ponds, Martinez, California. San Mateo ( "san muh-TAY-o") is a city in San Mateo County, California, United States Shoreline Amphitheatre is an outdoor Amphitheater in Mountain View California, USA, in the San Francisco Bay Area. IT Corporation was a United States industrial company whose principal business was the disposal of industrial Hazardous waste. Martinez ( "mar-TEE-niss" or "mar-TEE-nezz") is the County seat of Contra Costa County California, United States
The importance and sensitivity of smell varies among different organisms; most mammals have a good sense of smell, whereas most birds do not, except the tubenoses (e. Mammals ( class Mammalia) are a class of Vertebrate Animals characterized by the presence of Sweat glands, including sweat glands Birds ( class Aves) are bipedal endothermic ( Warm-blooded) Vertebrate animals that lay eggs. g. , petrels and albatrosses), and the kiwis. This article is about the petrel seabirds For other uses see Petrel (disambiguation. Albatrosses, of the biological family Diomedeidae, are large Seabirds allied to the procellariids, Storm-petrels and Diving-petrels KIWI (1029 FM, "Radio Lobo" is a commercial Radio station located in McFarland California, broadcasting to the Bakersfield California Among mammals, it is well-developed in the carnivores and ungulates, who must always be aware of each other, and in those, such as the moles, that smell for their food. A carnivore (ˈkɑrnɪvɔər meaning 'meat eater' ( Latin carne meaning 'flesh' and vorare meaning 'to devour' is any animal with a diet consisting Ungulates (meaning roughly "being Hoofed quot or "hoofed animal" are several groups of Mammals most of which use the tips of their toes usually
Dogs in general have a nose approximately a hundred thousand to a million times more sensitive than a human's. Scenthounds as a group can smell one to ten million times more acutely than a human, and Bloodhounds, which have the keenest sense of smell of any dogs, have noses ten to a hundred million times more sensitive than a human's. Scent hounds (Scenthounds are a type of Hound that primarily hunts by Scent rather than Sight. A bloodhound (also known as the St Hubert hound) is a large breed of Dog bred for the specific purpose of tracking human beings 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, which was bred to track and hunt rabbits and other small animals. The Basset Hound is a short-legged breed of Dog of the Hound family
The sense of smell is less-developed in the catarrhine primates (Catarrhini), and nonexistent in cetaceans, which compensate with a well-developed sense of taste. A primate is a member of the biological order Primates ( Latin: "prime first rank" the group that contains Lemurs the Aye-aye Catarrhini is a Parvorder of the Primates, one of the three major divisions of the Suborder Haplorrhini. The Order Cetacea (sɪˈteɪʃiə L cetus, whale includes Whales Dolphins and Porpoises Cetus is Taste (or more formally gustation) is a form of direct Chemoreception and is one of the traditional five Senses In some prosimians, such as the Red-bellied Lemur, scent glands occur atop the head. Prosimians are the most primitive extant Primates they have characteristics similar to forms that were ancestral to Monkeys Apes and The Red-bellied Lemur ( Eulemur rubriventer) is a medium sized Prosimian with a luxuriant chestnut brown coat In many species, olfaction is highly tuned to pheromones; a male silkworm moth, for example, can sense a single molecule of bombykol. A pheromone (from Greek φέρω phero "to bear" + ‘ορμόνη " Hormone " is a Chemical that triggers a natural Bombykol is a Pheromone released by the female Silkworm moth to attract mates
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 for olfaction. Antennae (singular antenna) are paired Appendages connected to the front-most segments of Arthropods In Crustaceans they are 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 followed by the mushroom bodies and lateral horn of the brain. Antennal lobe is the deutocerebral Neuropil of the insect which receives the input from the Olfactory sensory neurons on the antenna. The mushroom bodies or corpora pedunculata are a pair of structures in the Brain of Insects and other Arthropods They are usually In the Thoracic region the postero-lateral part of the Anterior column projects lateralward as a triangular field which is named the lateral column ( lateral 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. An axon or nerve fiber is a long slender projectionof a nerve cell or Neuron, that conducts electrical impulses away from the neuron's Cell 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.