PHYSIO P12 – Olfaction and Taste PDF

PHYSIO P12 – Olfaction and taste I.​ Taste 1.​ Anatomy of the Taste bud Taste buds are the sensory organs having sensory receptors found in the tongue, pharynx, epiglottis and upper 1/3 of oesophagus. On the tongue, they are clustered in papillae. They are organised having:...

PHYSIO P12 – Olfaction and taste I.​ Taste 1.​ Anatomy of the Taste bud Taste buds are the sensory organs having sensory receptors found in the tongue, pharynx, epiglottis and upper 1/3 of oesophagus. On the tongue, they are clustered in papillae. They are organised having: -​ basal cells -​ taste cells (the receptors) -​ supporting cells -​ taste pore (where the apex of taste cells reach towards the oral cavity) a.​ Types There are different types of papillae: -​ fungiform 25% have mushroom shape, a low concentration (1-5) of taste buds and are mainly found in anterior 2/3 of tongue (tip) -​ circumvallate 50% are large round structures surrounded by grooves, found in posterior 1/3 of tongue and they contain high amounts of taste buds -​ foliate 25% 2 foliate papillae are present on the posterolateral tongue, each having about 20 parallel ridges with about 600 taste buds in their walls These papillae differ in shape but not the type of taste buds they contain. Chemical stimuli on the tongue first stimulate receptors in the fungiform papillae then the foliate papillae and finally the circumvallate papillae. On the papillae surfaces there are the taste buds, which can be more or less exposed to, hence higher and lower probability to be in contact with, the fluids on the mucosa of the oral cavity. b.​ Structure Taste buds have an onion shape, having an apical portion which is the only part in contact with fluid found in the oral cavity (the rest is embedded in the mucus). There are afferent fibres nerves, taste cells - the sensory second type receptors, supporting cells, and basal cells – are a reservoir of taste receptors. Sensory cells are second type receptors as they are not neurons (they do not form the action potential), but release neurotransmitters which activate afferent fibres in the nerve. Basal cells can differentiate into new sensory receptor taste cells, hence there is a renewal of tissue every 10-15 days and regeneration of taste buds (the taste cell receptors and not afferent neurons are regenerated). Supporting cells secrete substances into the lumen of the taste bud. c.​ Substrates The receptors are chemoreceptors, sensitive to classes of different substrates, but not an infinite number of different substrates. The classes of substrates trigger the receptors, inducing at a conscious sensory experience of different tastes. There is a definite number of different tastes and a precise sensory experience for each class of substrates. The primary sensations of taste have been grouped into five general categories—sour, salty, sweet, bitter, and umami: -​ Sodium corresponds to taste of salt: there is a conscious experience to the content of sodium in food. -​ Hydrogen corresponds to sour -​ Glucose corresponds to sweet -​ Glutamate corresponds to umami -​ A wider family of substrates correspond to bitter 2.​ Defining taste Taste receptor cells transduce soluble chemical stimuli into electrical signals that can be transmitted to the brain resulting in the different submodalities of taste. Taste cells are chemoreceptors, hence their mechanism is a ligand to receptors mechanism: there must be the binding of the substrate to the receptor or the passage through a channel (sodium is not a ligand as it does not bind to a receptor, but passes through a channel: the high specificity of a channel to the substrate able to diffuse through it allows us to consider the receptor as a chemical ligand dependent receptor). In the case of bitter, a ligand binds to a G protein-coupled receptor, triggering a secondary messenger and metabolic transduction pathway which eventually leads to the release of calcium from an internal storage within the cell. The same applies for sweet and umami. For sour and salt, there are channel dependent mechanisms. -​ In sour, hydrogen ion binds to the receptors which results in the closing of the K+ channel, which results in depolarisation. -​ In salt, there is the entrance of sodium into the cell through a leak channel, leading to depolarisation of the cell. Depolarisation in the taste cell results in the opening of a calcium voltage-gated channel in the base of the cell, allowing calcium to enter the cell (for all different pathways of different tastes, the final aim is to increase the concentration of calcium). Calcium is needed to allow the release of neurotransmitter from the basal portion of the cell. Whatever is the ligand, the excitation of the receptor can be an electrical/ionotropic excitation (where there is depolarisation) or metabolic excitation (in the case of bitter, sweet and umami where there is no depolarisation but the release of calcium from an internal storage). In all cases there is the eventual increase in calcium within the cell which stimulates the release of neurotransmitter 3.​ Mechanisms of taste There are 5 specific families of substrates chosen taste cell receptors because taste is the special sense of the autonomic system, not a special sense contributing to somatic sensitivity: taste is a sensory system used to explore the environment in order to identify features of food that must be/must not be introduced. Taste transduction in response to the different compounds is activated by means of various channels and G-protein-coupled receptors. Neurotransmitter is released via either direct changes in depolarization or second messenger-mediated changes in IC Ca2+ concentration. a.​ Salty Adequate concentrations of different ions and molecules keep the gradients and currents across excitable cells stable, in order to prevent electrical problems of excitable tissues (e.g. overexcitation). The concentration of electrolytes is important as it guarantees the correct functioning of excitable tissues (which in turn coordinate other tissues). Secondly, the electrolytes play an important role in the osmotic equilibrium. Imbalances of sodium are not severe as sodium does not freely pass through membranes (due to its low conductance). It causes only an acute electrical problem. Imbalances of sodium have therefore a low risk of imbalances in membrane potentials in exposed tissues (e.g. heart, affecting the activity of pacemaker and myocardial cells), but there is a higher risk involving imbalances of potassium, as it has a higher conductance. Sodium is the ion for osmotic equilibrium of extracellular fluids. Extracellular and intracellular fluids have similar osmolarity. Changes in ion concentration affecting ion osmolarity can lead to shrinking or swallowing of cells: an imbalance of sodium is just as dangerous as an imbalance of potassium (sodium is the main ion responsible for osmolarity). Some tissues are able to tolerate different osmolarities as they are able to equilibrate osmotic gradients by the exchange water and ions. Some other tissues, such as CNS, are not able to tolerate different osmolarities, hence imbalance of sodium can lead to swallowing or shrinking of the brain. Correcting this equilibrium is very difficult in terms of therapy, hence these ion concentrations must be monitored. Sodium concentration hence matters a lot and animals hence have a taste receptors dedicated solely to salt. There are 2 ways of controlling salts: -​ feeding -​ kidneys. There are sensory receptors sensitive to sodium and potassium ions in the kidney; the kidney and hypothalamus can drive the animals behaviour to find (search for food) or secrete the ions when there is an imbalance. The only way for an animal to understand if they need the food (e.g. salt) is at the entrance of oral cavity by taste. Taste is an external monitor of the main parameters of homeostasis, hence can be considered a special sensory organ for the autonomic system. Salty sensation is mediated by passive movement of Na+ through Na+-selective channels in the microvilli down its concentration gradient between outside and inside, thereby depolarizing the cell membrane. It is important in homeostatic control of electrolytes concentration. b.​ Sour Sour monitors the concentration of hydrogen (pH, degree of acidity). Acid transduction is mediated in part by a pH-sensitive K+ channel. Competitive blockade of these K-channels produces a depolarizing receptor potential. A very acid solution can be dangerous for mucosae of the oral cavity and mainly of the gastrointestinal tract. Hence when monitoring hydrogen concentration by taste, there is an autonomic reflex involving secretion of saliva containing bicarbonate ion from salivary glands, used to dilute excess acid: this is a defensive mechanism which, even just by looking at food, can cause salivation in order to prevent having a solution which is too acid entering the digestive system. It is accompanied by a facial muscle contraction (leading to the typical facial expression) that directs salivary fluid onto tongue. c.​ Sweet Glucose is one of the most important nutrients for animals. The sweet sensation is mediated by a G-protein – second messenger system, in which the sugar molecule first binds to the membrane receptor that in turn leads to the production of cAMP. cAMP causes the PKA-mediated closure of K+ channels on the basolateral surface of the cell. Another pathway stimulates IP3 production, leading to the release of IC stores of Ca2+. The T1R3 and T1R2 G-protein coupled receptor genes code for the subunits that form an heterometric sweet receptors These receptors are sensitive to glucose and sweeteners (molecules with less calories than glucose that can bind to these receptors and give the same taste, e.g. fructose). d.​ Bitter Bitter is the most complex taste quality. Many types of substances produce bitter taste, including salts, acids, peptides, complex sugars, alkaloids and carbamates: the transduction pathways are correspondingly diverse. Transduction mechanisms include G-protein mediated activation of: -​ Phosphodiesterase (PDE) or phospholipase C (PLC) -​ Direct occlusion of K+ channels T2R genes encode bitter receptors, of which about 25 functional genes have been identified in humans. Bitter in nature is associated to poisons, but not everything that is bitter is a poison (everything can be a position depending on its dosage, “Omnia venenum…”). However, there are some molecules in nature that can be dangerous in low amounts. Animals are able to smell and taste the poison with the tip of their tongue. Bitter is the taste which has the lowest threshold, meaning the receptors are very sensitive: this is an evolutionary advantage, as it allows animals to monitor a small proportion of something that is potentially dangerous, without needing to eat a large amount of it.  e.​ Umami Umami is the most recently discovered/identified taste. It involves binding of glutamate to receptors. In image A, the glutamate receptor is ionotropic, involving the opening of calcium and sodium channels which enter the cell, depolarising it. In image B, the glutamate binds to a metabotropic receptor, where cAMP secondary messenger induces the closure of potassium channels or opening of calcium or sodium channels, depolarising the cell. Umami is the tase of taste. Some foods contain more glutamate than others (e.g. parmesan, soy sauce). Glutamate is important as a neurotransmitter, and as an amino acid for protein Transduction process has many different possibilities, but in all cases eventually there is the release of the neurotransmitter serotonin. On afferent fibres are present receptors to which serotonin binds, and if the amount of stimulation is enough, an action potential is made. 4.​ Transduction process Within the taste buds, only the taste cells are specialized for sensory transduction, and their basic structure and function are uniform across all classes of papillae and their constituent taste buds. Taste cells have distinct apical and basal domains, reflecting their epithelial character. Chemosensory transduction: (1)​ It is initiated in the apical domain of the taste cells (2)​ electrical signals are generated at the basal domain via graded receptor potentials and corresponding secretion of neurotransmitters: specific neurotransmitters released by taste cells remain uncertain but include serotonin, ATP, and GABA. Taste receptor proteins and related signalling molecules, like those in ORNs, are concentrated on microvilli that emerge from the taste cell apical surface. The basal domain is specialized for synaptic activation in response to tastant binding on apical receptor proteins. There are voltage-regulated ion channels as well as channels controlled by second messengers—especially members of the transient receptor potential, or TRP, family. In addition, local endoplasmic reticulum acts as a store that provides Ca2+ to facilitate synaptic vesicle fusion and neurotransmitter release at synapses made onto gustatory afferents at the basal surface. These synapses are made onto primary afferent axons from branches of three cranial nerves: the facial (VII), glossopharyngeal (IX), and vagus (X) nerves. Five distinct classes of taste receptor molecules represent tastants in the major perceptual categories—salty, sour, sweet, bitter, and umami. These receptor molecules are thought to be concentrated primarily in the apical microvilli of taste cells. ` 5.​ Regions of taste Previously there was the idea that different regions of the tongue were dedicated to different tastes (taste regions), in order to encode taste: -​ sweet at the tip -​ salt at anterolateral -​ sour at posterolateral -​ bitter in posterior -​ umami in middle This is true in terms of distribution of probability, but it is not true in terms of presence of only certain taste buds in certain regions of the tongue. Analysing different regions of the tongue (and different parts of oral cavity having taste buds, e.g. epiglottis), despite the type of papillae, sensitivity to every taste is found in every region: the relative proportion characterises the different areas: -​ in the tip of the tongue there is higher sensitivity for sucrose -​ the posterior portion of tongue is more sensitive to quinine (a substrate for bitter receptor) and less to sucrose -​ the epiglottis is sensitive to water, which is important as it must close when swallowing. In the image, the sensitivity of the area to that substrate is given by dimensions of the circle  6.​ Labelled line & Across neuron theories The “Labelled line” theory (shown in image A) suggests that each receptor is dedicated to one taste. It categorizes individual taste cells and associated neurons into classes such as “NaCl-best”, “sugar-best”, … One of the stimulus categories will evoke the greatest number of action potentials per unit time in any given gustatory neuron. This implies that the activity of one neuron type is both necessary and sufficient to represent a given sensory attribute: if the sugar best cell is activated, it is enough to have a sensory experience of sweet. Gustatory information are transmitted throughout the central components of the taste system along pathways that preserve the “best” fibre categories. The brain will collect all the sweet fibres (red) together, all the sour fibres (green) together and the final computation is based on who is firing most among the different categories. The “Across neuron” hypothesis (shown in images B and C) proposes that the pattern of responses to a particular stimulus across all fibres is the central feature coding. In image B, each receptor is sensitive to more than 1 substrate, but is tuned more to 1 or 2 substrates with respect to others (the threshold to activate the receptor is lower for those substrates and higher for the others). In this case each cell can respond to more than 1 taste but is differently tuned for different substrates. In image C, each cell is sensitive to only 1 substrate, but each afferent fibre innervates more than 1 taste cell, meaning that at the end the discharge of the fibre will depend on the alchemy of the receptor. In the across neuron hypothesis, the pattern of responses to a particular stimulus across the fibre is the central, feature of coding: it is not the receptor which is classified into “sugar”, “bitter” or “umami”, but instead it is the final computation of the matrix that the brain receives which gives out the perception. The 2 theories are completely different: -​ the label line theory bases everything on the identity of the receptor -​ the across neuron hypothesis it is defined by the alchemy, what the brain receives and the overall output of the computational analysis of the brain, that receives from taste buds the information regarding which receptors are discharging more or less. Both these theories are valid. 7.​ Encoding Taste Experiment: by submitting the same stimulus (e.g.: sodium chloride, quinine, acid and sucrose) and recording the membrane potential of cells 1, 2 and 3 in a taste bud, it will be visible that: -​ Cell 1 responds to sodium chloride and, to a lesser extent, to hydrochloric acid; no response is detected for quinine or sucrose. -​ Cell 2 reacts to sodium chloride and strongly to acid, but also in small part to quinine. -​ Cell 3 seems instead to be tuned for sucrose. By looking at the afferent fibers, their discharge is clearly proportional to the graded potential, guaranteeing different discharge intensities for different tastes: -​ Sweet taste is detected through the activation of Axons 3 -​ Salty is the consequence of a strong activation in Axons 1 and 2 -​ Sour is detected by a high activity in Axons 2 and a mild activity in Axons 1 -​ Bitter by a mild activation of Axons 2 The same fibers of chorda tympani respond in a different way to different stimuli: background discharging is almost completely absent, but the responses vary basing on tastes and on temperature. This fiber comes from a bud connected to receptor that is very sensitive to salt and Saccharin, and not very sensitive to the other tastes. So, these are the responses of a single fiber of chorda timpani when the bud, connected to the fiber, is exposed to flavours. Different tuning curves exist for different types of chorda tympani exposed to stimulations. Fibers are subject to the threshold phenomenon. The amount of substrate required for the different fibers to activate is not the same: -​ in the first fiber, sensitive to sucrose and NaCl, the reaction to a certain amount of sodium chloride is way stronger than in the case of sucrose, which is poorly activated; the bud innervated by this fiber is therefore tuned for salty tastes rather than for sweet ones -​ opposite is true for fiber 2, in which sucrose is preferred to sodium chloride. -​ in the third case, the fiber is sensitive for both salty, bitter and sour tastes; although, the threshold for sodium chloride is higher, indicating tuning for bitter and sour. In the image, are shown the response profiles of 40 individual chorda timpani axons to the four different stimuli (all but Umami). The different substrates are represented by different colours. -​ from 1 to 9 the fibers can be defined as sugar-best -​ from 11 to 21 they are sodium chloride-best (she actually defined them as both Salty and Sour-best); -​ from 22 to 32 it seems like there is not a highly defined favoured taste -​ from 33 to 38 they are sour-best Quinine appears in part of the Sour-best fibers, but it is not very present in the selection of receptors for the chorda timpani. By analysing the populations, the fibers are clearly showing consistency with the “Labelled line” scheme of taste coding, because of the preferential discharges for specific stimuli. The responses of these same 40 fibers are also consistent with the “Across neuron” pattern: by looking at the fibers one by one, observing their complex discharge, it is evident that there is a tuning curve for every receptor, but what the brain does is the analysis of the complex picture, given by the sum of all the different tastes recognized by each receptor. The Labelled line theory is therefore useful to understand the tuning of the receptor, while the Across neuron explain the overall flavour. Fibers react to particular tasting molecules together with the controlled stimulations that respond to temperature and the spontaneous activity. Because of the spontaneous activity, it is impossible for the brain to rely on just one type of receptor; a large number of different receptors will be necessary to have a correct overall picture of the flavour. The brain receives and computes the whole excitation of the fibers. Based on the matrix, the mixing of different tastes is carried out, leading to the perception of flavours. When eating food (unless in the case of pure composts, which are usually only found in experiments), the tongue is continuously exposed to different substrates. The activity of chewing helps in the tasting process: one proof of this is that, when inserting some food in the mouth, the taste can be perceived as a bit different before and after chewing. 8.​ Central processing of taste perception Taste sensory cells have no axons. They transmit taste information via synapses onto the terminals of sensory fibers within the taste bud. The fibers arise from the ganglion cells of the cranial nerve VII (corda tympani of facial nerve VII), IX (lingual branch of the glossopharyngeal nerve) and X (superior laryngeal branch of the vagus nerve). These innervate respectively, the anterior 2/3 of the tongue, posterior 1/3 of the tongue and palate and the epiglottis and esophagus. Fibers carrying taste information make their synapses centrally in the medulla, in the nucleus of the solitary tract. There is a specific taste pathway which connect via the parvocellular region of the VPM of the thalamus to the cortical taste areas in the anterior insula and frontal operculum, which mediates the conscious perceptions of taste. The peripheral cells are equipped with receptors tuned for one (or more) specific molecules, but sensitive for most of the substrate. It is the brain that computes the actual sensory experience. This mechanism is different form the one of Merkel cells receptors, in which an increase in pressure leads to an increased frequency of discharge, leading to the sensation of “increasing pressure”; it is also different from the mechanism of Meissner cells, that detect and transmit “increased frequency of vibration”. These types of sensations and responses are perfectly coded in separated families, while the receptors for taste can be considered polymodal (in terms of substrates). Taste sensory cells have no axons. They transmit taste information via synapses onto the terminals of sensory fibers within the taste bud. Transmission of taste signals into CNS: (1)​ The fibers arise from ganglion cells of cranial nerves: -​ VII (chorda tympani branch of facial nerve) innervates anterior 2/3 tongue. Its motor component is also the one that innervates mimic muscles (muscles of the face responsible for face expression): when eating something very acid, the expression of the face helps to convey the saliva released through a reflex due to the afferent information coming from sour receptors. It helps diluting the food as soon as possible, and swallow it in case it is not recognized as dangerous. -​ IX (glossopharyngeal nerve) innervates posterior 1/3 tongue -​ X (superior laryngeal branch of vagus nerve) innervates the epiglottis and esophagus  (2)​ The nucleus to which the fibers are projecting is the solitary tract in the medulla. (3)​ From the solitary tract, information will be transmitted to the VPM of thalamus (4)​ and then to cortical areas of the primary gustative cortex and anterior insula (opercular insular area). In the picture, it is shown the response of a solitary tract neuron when receiving information given by stimulation of peripheral receptors for sucrose, fructose and other components. The pattern of discharge changes since information is received from different buds. This scheme, shows neurons in solitary tract (5S, 5H and 5N). Here the convergence matters a lot for the brain, since the central computation relies on it; taste buds with a similar tuning converge to get to the brain. In the image, different concentrations of substrate react with receptors to converge in the chorda tympani and get to the solitary tract. Thanks to high overlapping, what happens in the nucleus reflects the overall situation in the periphery. In the picture, red represents acid while green corresponds to sweet, open circles are the chorda tympani and filled circles are the nucleus; the perfect overlapping is shown. In the picture below, are shown recordings from the primary gustative cortex, secondary gustative cortex and their behaviour in response to glucose intake. Regions of the primary gustative area are not affected at all by the eaten volume, and they continuously react independently from the amount of ingested food. This happens because the primary area is responsible for controlling the quality and not the quantity of food. The secondary area, instead, adapts, showing that its cortex behaves in a volume-adapting way, decreasing the frequency of discharge as the eaten volume increases. There must in fact be a connection between the blood glucose, the hypothalamus (which drives the behaviour) and this area of the cortex. The secondary cortex computes the same information of the first one in a different way (it does not behave as a backup!) and is needed for different functions. 9.​ Taste perception varies with age The taste perception varies with age: the acuity of this sense starts declining after 60 years of age. The receptors that remain the most reactive are the ones for bitter. 10.​Taste synergies In such an architecture as the one of taste buds, synergies, meaning that one taste can enhance or depress the perception of another are unavoidable. By looking at the graph, it is visible that the synergies vary when increasing the amounts of substrates (low, medium or high concentrations), that correspond to stimulations of the tongue with different compounds. These relationships are not fixed: -​ Salty depresses bitter but bitter can both increase and decrease the effect of salty -​ Salty enhances sweet, while the opposite has a milder effect -​ Bitter and sour enhance each other -​ Sweet enhances bitter, but bitter doesn’t always enhance sweet. By changing the amounts some effects are maintained, for example salty depresses bitter at any concentration. In other cases, the relationships change: high concentrations of bitter suppress sour, while mild concentrations enhance it. This mechanism is fundamental for the final perception of flavour, which depends on the ingredients and on temperature. This complex elaboration is the reason of the cultural interest in food: the interactions between different tastes are mixed at best, modulating the tastes to obtain the wanted flavour. The importance of temperature is not irrelevant since it influences the flavour of dishes. Temperature matters for two reasons: it increases the relationships between two tastes, and (mainly) it increases the molecules of odorants coming from the food. Therefore, more molecules are able to enter the nasal cavity contributing for a big component of the perceived flavour. A simple proof in favour of the relationship between olfaction and taste is that when people are affected by a cold, they are less able to feel the flavour. This occurs since the olfactory component is lacking. Some tastes, if intense, cause the person to manifest a certain facial expression. In this picture babies and newborns have been chosen since they don’t have any cultural sovra-structure, and express freely in response to stimuli. Babies are also less prone to like some foods with particular tastes. II.​ Smell 1.​ Olfactory mucosa The substrate molecules that are the origin of smells are odorants, which react with chemoreceptors that are basically appendices of the brain outside. The olfactory cells are actually bipolar nerve cells derived originally from the CNS. There are about 100 million of these cells in the olfactory epithelium interspersed among sustentacular cells. The mucosal end of the olfactory cell forms a knob from which 4 to 25 olfactory hairs (also called olfactory cilia), measuring 0.3 micrometer in diameter and up to 200 micrometers in length, project into the mucus that coats the inner surface of the nasal cavity. Spaced among the olfactory cells in the olfactory membrane are many small Bowman glands that secrete mucus onto the surface of the olfactory membrane. Dendrites are present, since the receptors for odorants are neurons themselves (not secondary!). The axon of these neurons goes up through the holes of the cribriform plate of ethmoid bone and terminates in the glomeruli of the olfactory bulb. Mitral cells, that are the output of the glomeruli, exit in the olfactory tract. Air is the vehicle for odorants, entering the nasal cavity in the nasal conchae and acquiring turbulence: it is the turbulence that increases the probability for these molecules to enter in contact with the olfactory mucosa. When exploring, animals increase the inflow of air to increase the turbulence; in this way, the probability for odorants to hit the mucosa will also increase. Mucus is very important too: it is produced by submucosal cells and goblet cells in the respiratory system (in the upper airways, where it helps the movement of the cilia in order to get rid of waste products). Mucus increases the possibility to bind the odorants to the receptors in the mucosa. It has also a defensive role, capturing unwanted substances that should not proceed to the lower airways. There are two types of mucus: -​ one watery, more superficial -​ one more viscous, covers the cilia and olfactory epithelium. 2.​ Olfactory receptor neurons The neural portion of the olfactory epithelium is defined by the olfactory receptor neurons (ORNs), bipolar cells which give rise to: -​ small-diameter, unmyelinated axons at their basal surface transmit olfactory information centrally -​ a single dendritic process on the apical surface that expands into a knoblike protrusion from which several microvilli, called olfactory cilia, extend into a thick layer of mucus (actin based cellular protrusions that have scaffolding proteins that localize odorant receptors and signal transduction molecules within the ORN apical dendrite) ORNs have direct access to odorant molecules as air is inspired through the nose into the lungs; however, this access exposes these neurons to airborne pollutants, allergens, microorganisms, and other potentially harmful substances, subjecting them to more or less continual damage. The ultimate solution to the vulnerability of ORNs is to maintain a healthy population by a normal cycle of degeneration and regeneration: ORN regeneration relies on maintaining among the basal cells in the mature olfactory epithelium a population of neural stem cells. Olfactory receptors are structured as four rings connected through gap junctions in the olfactory mucosa. This means that our body needs the activation of a certain number of neurons responding to a stimulation in order to have a relevant signal to be transmitted. The gap junctions can be at the level of the spines or at the level of the bodies, depending on the receptors. The receptor process occurs at the level of the cilia: the odorants bump into the floating cilia, causing the formation of an inward current. If the receptors in an experimental setup are stimulated at the level of the bodies, no current is generated; this means that the active portion is only at the level of the cilia 3.​ Olfactory sense Odor is a lock and key association. Differently from taste, in the receptors for olfaction there are no channels; this means that there are different receptor types that react or not with the odorant depending on its shape. A same odorant can stimulate different receptors depending on the sterical arrangement. The receptors are selective for more than one substrate. The nose shape helps in its own function since the turbinate bones increase the turbulence of air. In case of a mild inspiration flow, the probability for air flow to go up to the higher portions reaching the receptor is low. In case of forced inspiration, instead, the probability is higher. (1)​ Odorants in the mucus bind directly (or are shuttled via odorant binding proteins) to one of many receptor molecules located in the membranes of the cilia. (2)​ Upon odorant binding, a G-Protein metabotropic receptor increases the production of cAMP, (3)​ which activates the channels for sodium, calcium, chloride (the less understood, since it hyperpolarizes the membrane instead of depolarizing it) or exchange of sodium-calcium. Receptor proteins have: -​ 7 transmembrane domains -​ a variable cell surface region -​ a cytoplasmic tail that interacts with G proteins. They are encoded by as many as 1000 genes, each gene presumably encodes an odorant receptor that detects sets of odorants. As shown in the picture, receptors work in a similar way to taste buds, activating strongly, mildly or not at all in response to the odorants. Each receptor expresses a single olfactory receptor protein, being therefore sensitive for only one set of molecules. By stimulating the different receptors (green, blue, red) with different odorants, the profile of firing shows a different response of each receptor to each substrate, with preferences (different tuning). Receptors in the nasal mucosa are distributed everywhere, not only in a precise sector. They project up with their axons through the ethmoid bone to reach the olfactory bulb. In the olfactory bulb each family of receptor clusters composing a matrix; the Mitral cells, by means of a sort of labelled line, will then converge the nerve fibers in the olfactory tract. The glomerular activity can be recorded with optical imaging. In an experiment, a rat was stimulated with increasing concentrations of the same substrate (Amyl Acetate): at the beginning (low concentration) the activation of receptors is selective; by increasing the concentration, also the intensity of activity in the particular glomeruli responding to the odor increases; by increasing too much, the activation will spread because at very high concentrations almost all receptors are at least mildly reactive. The tuning of a receptor (valid for both taste and olfaction) is based on concentration and identity of the molecule. To understand which is the preferred substrate, it has to be found the one with the lowest threshold. The olfactory bulb is organized in different zones sensitive to different families of odorants, but, at the end, the brain will furnish the final mixed solution that is the flavour. Mitral cells in the olfactory tract are going on different structures, such as: -​ the contralateral olfactory bulb through the anterior olfactory nerve -​ the olfactory tubercle -​ the thalamus -​ the orbito-frontal cortex -​ the frontal cortex -​ the enthorinal and pyriform cortex -​ the amygdala - part of the limbic system going to the hypothalamus. Animals use odorants to explore the environment, looking for foods but also to detect dangers in the surroundings, modifying their behaviour: -​ the frontal cortex in primates and humans is the one associated to decision making -​ the hippocampus is responsible for olfactory memory (mixtures of odorants can trigger far memories) As already mentioned, the perception of flavours, also from the olfactory point of view, decreases with aging.  4.​ Possible deficits of tasting and olfaction One example of deficit affecting taste and smell is that of Covid-19, which leads to loss in these two senses. Ageusia and Anosmia are the inabilities to detect respectively gustatory and olfactory stimulants, leading to complete absence of the affected sensory experience. -​ Hypogeusia and hyposmia mean a diminished ability to detect stimulants; vice versa, -​ Hypergeusia and Hyperosmia indicate increased sensitivity to stimulants. In pregnancy some odorants that were previously neutral can be perceived as disgusting or associated with discomfort. -​ Dysgeusia and Dysosmia are the distorted perception of gustatory and olfactory stimulants. -​ Agnosia is the inability to identify a taste or odor. There are some tumors that grow in the insular lobe and can manifest with seizures; a signal that anticipates the occurrence of seizures is the recognition of odorants that are not present in the surrounding environment. Another sign is headache.