of flavor. often, land organisms will have separate olfaction systems for smell and taste (orthonasal smell and retronasal smell), but water-dwelling organisms usually have only one system. molecules of odorants passing through the superior nasal concha of the nasal passages dissolve in the mucus that lines the superior portion of the cavity and are detected by olfactory receptors on the dendrites of the olfactory sensory neurons. this may occur by diffusion or by the binding of the odorant to odorant-binding proteins. 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). this mucus acts as a solvent for odor molecules, flows constantly, and is replaced approximately every ten minutes. 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. 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. receptor neuron the binding of the ligand (odor molecule or odorant) to the receptor leads to an action potential in the receptor neuron, via a second messenger pathway, depending on the organism. in mammals, the odorants stimulate adenylate cyclase to synthesize camp via a g protein called golf. 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. the ca2+ in turn opens a ca2+-activated chloride channel, leading to efflux of cl−, further depolarizing the cell and triggering an action potential. ca2+ is then extruded through a sodium-calcium exchanger. a calcium-calmodulin complex also acts to inhibit the binding of camp to the camp-dependent channel, thus contributing to olfactory adaptation. the main olfactory system of some mammals also contains small subpopulations of olfactory sensory neurons that detect and transduce odors somewhat differently. olfactory sensory neurons that use trace amine-associated receptors (taars) to detect odors use the same second messenger signaling cascade as do the canonical olfactory sensory neurons. other subpopulations, such as those that express the receptor guanylyl cyclase gc-d (gucy2d) or the soluble guanylyl cyclase gucy1b2, use a cgmp cascade to transduce their odorant ligands. these distinct subpopulations (olfactory subsystems) appear specialized for the detection of small groups of chemical stimuli. this mechanism of transduction is somewhat unusual, in that camp works by directly binding to the ion channel rather than through activation of protein kinase a. 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. there are also considerable similarities in the immediate processing of stimuli by lateral 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 electroantennography or calcium imaging within the olfactory bulb. olfactory bulb projections schematic of the early olfactory system including the olfactory epithelium and bulb. each orn expresses one or that responds to different odorants. odorant molecules bind to ors on cilia. ors activate orns that transduce the input signal into action potentials. in general, glomeruli receive input from ors of one specific type and connect to the principal neurons of the ob, mitral and tufted cells (mt cells). olfactory sensory neurons project axons to the brain within the olfactory nerve, (cranial nerve i). these nerve fibers, lacking myelin sheaths, pass to the olfactory bulb of the brain through perforations in the cribriform plate, which in turn projects olfactory information to the olfactory cortex and other areas. the axons from the olfactory receptors converge in the outer layer of the olfactory bulb within small (≈50 micrometers in diameter) structures called glomeruli. mitral cells, 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 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 25,000 axons synapsing on 25 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 cells also mediate inhibition and excitation of mitral cells through pathways from centrifugal fibers and the anterior olfactory nuclei. neuromodulators like acetylcholine, serotonin and norepinephrine all send axons to the olfactory bulb and have been implicated in gain modulation, pattern separation, and memory functions, respectively. 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 anterior olfactory nucleus projects, via the anterior commissure, to the contralateral olfactory bulb, inhibiting it. the piriform cortex has two major divisions with anatomically distinct organizations and functions. the anterior piriform cortex (apc) appears to be better at determining the chemical structure of the odorant molecules, and the posterior piriform cortex (ppc) has a strong role in categorizing odors and assessing similarities between odors (e.g. minty, woody, and citrus are odors that can, despite being highly variant chemicals, be distinguished via the ppc in a concentration-independent manner). the piriform cortex projects to the medial dorsal nucleus of the thalamus, which then projects to the orbitofrontal cortex. the orbitofrontal cortex mediates conscious perception of the odor. the three-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. it also projects to the hippocampus and is involved in motivation and memory. odor information is stored in long-term memory and has strong connections to emotional memory. 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. 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 may seem strange that human beings are able to distinguish so many different odors. it seems that a highly complex form of processing must be 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 to only one odor, and the rest to only a few. it has been shown through microelectrode studies that each individual odor gives a particular spatial map of excitation in the olfactory bulb. it is possible that the brain is able to distinguish specific odors through spatial encoding, but temporal coding must also 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 nostrils have separate inputs to the brain, with the result that, when each nostril takes up a different odorant, a person may experience perceptual rivalry in the olfactory sense akin to that of binocular rivalry. in insects, smells are sensed by sensilla located on the antenna and maxillary palp and first processed by the antennal lobe (analogous to the olfactory bulb), and next by the mushroom bodies and lateral horn. coding and perception the process by which olfactory information is coded in the brain to allow for proper perception is still being researched, and is not completely understood. when an odorant is detected by receptors, they 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 that recognize only 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 in the olfactory bulb. each glomerulus receives signals from multiple receptors that detect similar odorant features. because several receptor types are activated due to the different chemical features of the odorant, several glomeruli are activated as well. the signals from the glomeruli are transformed to a pattern of oscillations of neural activities of the mitral cells, the output neurons from the olfactory bulb. olfactory bulb sends this pattern to the olfactory cortex. olfactory cortex is thought to have associative memories, so that it resonates to this bulbar pattern when the odor object is recognized. the cortex sends centrifugal feedback to the bulb. this feedback could suppress bulbar responses to the recognized odor objects, causing olfactory adaptation to background odors, so that the newly arrived foreground odor objects could be singled out for better recognition. during odor search, feedback could also be used to enhance odor detection. the distributed code allows the brain to detect specific odors in mixtures of many background odors. it is a general idea that the layout of brain structures corresponds to physical features of stimuli (called topographic coding), and similar analogies have been made in smell with concepts such as a layout corresponding to chemical features (called chemotopy) or perceptual features. while chemotopy remains a highly controversial concept, evidence exists for perceptual information implemented in the spatial dimensions of olfactory networks. 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 pheromones, although pheromones can also be detected by the main olfactory system. in the accessory olfactory system, stimuli are detected by the vomeronasal organ, located in the vomer, between the nose and the mouth. snakes use it to smell prey, sticking their tongue out and touching it to the organ. some mammals make a facial expression called flehmen to direct stimuli to this organ. the sensory receptors of the accessory olfactory system are located in the vomeronasal organ. as in the main olfactory system, the axons of these sensory neurons project from the vomeronasal organ to the accessory olfactory bulb, which in the mouse is located on the dorsal-posterior portion of the main olfactory bulb. 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 and bed nucleus of the stria terminalis, and from there to the hypothalamus, where they may influence aggression and mating behavior. in insects insect olfaction refers to the function of chemical receptors that enable insects to detect and identify volatile compounds for foraging, predator avoidance, finding mating partners (via pheromones) and locating oviposition habitats. thus, it is the most important sensation for insects. most important insect behaviors must be timed perfectly which is dependent on what they smell and when they smell it. for example, smell is essential for hunting in many species of wasps, including polybia sericea. the two organs insects primarily use for detecting odors are the antennae and specialized mouth parts called the maxillary palps. however, a recent study has demonstrated the olfactory role of ovipositor in fig wasps. inside of these olfactory organs there are neurons called olfactory receptor neurons which, as the name implies, house receptors for scent molecules in their cell membranes. the majority of olfactory receptor neurons typically reside in the antenna. these neurons can be very abundant, for example drosophila flies have 2,600 olfactory sensory neurons. insects are capable of smelling and differentiating between thousands of volatile compounds both sensitively and selectively. sensitivity is how attuned the insect is to very small amounts of an odorant or small changes in the concentration