{"id":4458,"date":"2018-10-07T00:46:11","date_gmt":"2018-10-07T04:46:11","guid":{"rendered":"https:\/\/www.amyork.ca\/academic\/zz\/?p=4458"},"modified":"2018-10-07T00:47:21","modified_gmt":"2018-10-07T04:47:21","slug":"taste","status":"publish","type":"post","link":"https:\/\/www.amyork.ca\/academic\/zz\/sensation-and-perception\/taste\/","title":{"rendered":"Taste"},"content":{"rendered":"<p><em>Taste versus flavour<\/em><\/p>\n<ul>\n<li>Retronasal olfactory sensation: the sensation of an odour that is perceived when chewing and swallowing force an odorant in the mouth up behind the palate into the nose. Such odour sensations are perceived as originating from the mouth, even though the actual contact of odorant and receptor occurs at the olfactory mucosa<\/li>\n<li>Flavour: the combination of true taste and retronasal olfaction; because you taste and fell the food only in your mouth the brain concludes that the sensations must have arisen entirely from the mouth<\/li>\n<\/ul>\n<p><em>Localizing flavour sensations<\/em><\/p>\n<ul>\n<li>When the left chorda tympani, a branch of the facial nerve that carries info from taste receptors to the brain, is anesthetized while eating blueberry yogurt, the blueberry sensation, which is due entirely to retronasal olfaction, seems to come only from the right side of the mouth. Moreover the intensity of the blueberry sensation was reduced, and this intensity was reduced even further when both taste nerves were blocked<\/li>\n<li>The brain processes odours differently, depending on whether they come from the mouth or through the nostrils. Without the proper cues to tell us where an odorant is coming from, input from the olfactory receptors apparently cannot be routed to the proper brain area to connect the smell sensation with the food stimulus<\/li>\n<\/ul>\n<p><em>\u00a0<\/em><\/p>\n<p><em>\u00a0<\/em><\/p>\n<p><em>Anatomy and physiology of the gustatory system<\/em><\/p>\n<ul>\n<li>Chewing breaks down food substances into molecules, which are dissolved in salvia. The salvia-borne food molecules flow into a taste pore that leads to the taste buds housed in papillae that are located mostly on the edges of the tongue in a rough oval. Taste buds contain multiple taste receptor cells, each of which responds to a limited number of molecule types. When it comes in contact with one of its preferred molecules, the taste receptor cell produces action potentials that send info along one of the cranial nerves to the brain (figure 15.2 page 435)<\/li>\n<\/ul>\n<p><em>Papillae<\/em><\/p>\n<ul>\n<li>Four major varieties of papillae:\n<ol>\n<li>Filiform papillae: do not have any taste function, are located on the anterior portion of the tongue and come in different shapes in different species<\/li>\n<li>Fungiform papillae: resemble tiny mushrooms, located on the anterior part of the tongue, blue food colouring makes them particularly easy to see; contains, on average, six taste buds. The taste nerves that enter that taste buds, branch so that an individual cell can be innervated by more than one taste fibre and an individual taste fibre can innervate more than one cell<\/li>\n<li>Foliate papillae: located on the sides of the tongue at the point where the tongue is attached. Under magnification the look as a series of folds, in which the taste buds are buried 4. Circumvallate papillae: relatively large, circular structures forming an inverted V on the rear of the tongue. Look like tiny islands surrounded by moats, in which the taste buds are buried<\/li>\n<\/ol>\n<\/li>\n<\/ul>\n<p><em>Taste buds and taste receptor cells<\/em><\/p>\n<ul>\n<li>Each taste bud is a cluster of elongated cells, the tips of some cells, the taste receptor cells, end in slender microvilli containing the sites that bind to taste substances. The microvilli are extensions of the cell membrane<\/li>\n<li>Taste bud cells fall into three different groups that appear to mediate different taste qualities (figure 15.4 page 437):\n<ol>\n<li>Type I: may respond to salt, do not have synapses<\/li>\n<li>Type II: respond to sweet, bitter or umami, have receptor sites on the microvilli but do not have synapses<\/li>\n<li>Type III: carry sourness and saltiness, have synapses<\/li>\n<\/ol>\n<\/li>\n<li>Tastant: any stimulus that can be tasted<\/li>\n<li>Two mechanisms that permit a taste cell to recognise a tastant that is contacting its microvillus:\n<ol>\n<li>One class of tastants is made up of small, charged molecules that taste salty or sour. Ion channels in microvilli membranes allow some types of charged particles to enter cells but bar others. When the charged particles in salty and sour foods enter salty and sour receptor cells, these cells signal their respective tastes<\/li>\n<li>The second class of tastants, which produces sweet and bitter sensations, are perceived via a mechanism similar to that in the olfactory system, using G-protein-coupled receptors (GCPRs). The GCPRs wind back and forth across the microvilli membranes, and when a particular tastant molecule key is fitted into the lock portion of a GCPR on the outside of the membrane, the portion of the GCPR inside the cell starts a cascade of molecular events that eventually causes an action potential to be sent to the brain<\/li>\n<\/ol>\n<\/li>\n<\/ul>\n<p><em>Taste processing in the central nervous system<\/em><\/p>\n<ul>\n<li>Insula\/insular cortex\/gustatory cortex: primary cortical processing area for taste<\/li>\n<li>Orbitofrontal cortex: receives projections from the insular cortex. The OFC is responsible for the conscious experience of olfaction, as well as the integration of pleasure and displeasure from food; may be an integration area<\/li>\n<li>Inhibition protects our whole-mouth perception of taste in the face of injuries to the taste system. Inhibitory signals from the taste cortex normally help preventing eating-disruptive symptoms<\/li>\n<\/ul>\n<p><em>The four basic tastes<\/em><\/p>\n<ul>\n<li>The liking or disliking of salty, sour, sweet and bitter is hardwired in the brain<\/li>\n<\/ul>\n<p><em>Salty<\/em><\/p>\n<ul>\n<li>Salts are made up of two charged particles: a cation (+) and an anion (-), the cation is the source of the salty taste<\/li>\n<li>Reduced sodium intake increases the intensity of saltiness over time, this adjustment in perception helps people on a low-sodium diet to keep their sodium intake down<\/li>\n<\/ul>\n<p><em>Sour<\/em><\/p>\n<ul>\n<li>Sour taste is produced by hydrogen ions (H<sup>+<\/sup>). Hydrogen ions enter the receptor cell through ion channels, however an additional mechanism for sour allows undissociated acid molecules to enter as well. The undissociated acid molecules dissociate inside the cell. Ultimately, the stimulus that triggers sour taste is the hydrogen concentration inside the receptor cell<\/li>\n<\/ul>\n<p><em>Bitter<\/em><\/p>\n<ul>\n<li>There are 25 different bitter receptors. The bitter gene family is TAS2R (TAS=taste, 2=bitter), numbers following the R indicate the particular gene that is a member of that family. The 25 bitter genes are located on three different chromosomes: 5,7, and 12.<\/li>\n<\/ul>\n<p>o Meyerhof: there are thousands of bitter molecules that can be detected by the 25 bitter receptors, because some of the T2 receptors respond only to specific compounds, but others respond to many compounds<\/p>\n<ul>\n<li>Sweet and bitter tastes inhibit one another<\/li>\n<li>We are born with an aversion for bitter, because poisons are bitter. Our biological task is not to distinguish between different bitter molecules, but rather to avoid them all. Thus we have multiple bitter receptors to encompass the chemical diversity of poisons, but they all feed into a common line leading to rejection<\/li>\n<\/ul>\n<p><em>Sweet<\/em><\/p>\n<ul>\n<li>Sweetness is evoked by sugars, which are simple carbohydrates<\/li>\n<li>Some biologically useless sugars have structures very similar to those of glucose, fructose and sucrose. The task of the taste system is to tune receptors so specifically that the biologically important sugars stimulate sweet taste but the others do not<\/li>\n<li>The gene family of expressing sweet receptors consist of TAS1R1, TAS1R2 and TAS1R3, which express three different G-protein-coupled receptors: T1R1, T1R2 and T1R3.<\/li>\n<li>Dimer: a chain of two molecules<\/li>\n<li>Heterodimer: a chain of two molecules that are different from each other. Receptors T1R2 and T1R3 combine to form the heterodimer that is known to be a sweet receptor. The complex extracellular portion of the receptors that form this combination provides a variety of positions at which sweeteners with very different chemical structures can bind. No matter how the heterodimer is stimulated, the receptor produces only one signal (figure 15.9 p443)<\/li>\n<li>The receptor T1R3 appears to be able to function alone to respond only to high concentrations of sucrose. In addition, another mechanism in addition to the heterodimer can mediate sweetness, which explains why many of us perceive differences between the sweetness of sucrose and artificial sweeteners<\/li>\n<\/ul>\n<p><em>The survival value of taste<\/em><\/p>\n<ul>\n<li>The gustatory system responds to a fixed and much smaller set of molecules that nature knows we will encounter. This precise tuning is consistent with the role of taste as a system for detecting nutrients and antinutrients before we ingest them.\n<ul>\n<li>Bitter taste subsystem is nature\u2019s poison detector, poisons are diverse thus bitter receptors must be diverse as well. We do not really care if we can discriminate among poisons, thus all of those receptors are hooked up to a few common lines to the brain<\/li>\n<li>Sour subsystem is configured to reject highly acidic solutions without distinguishing exactly what is causing the acidity of the solution to be so high<\/li>\n<li>Salty and sweet subsystems enable us to detect, and therefor selectively ingest, foods that our bodies need: sodium and sugars<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n<p><em>The pleasure of taste<\/em><\/p>\n<ul>\n<li>Steiner: infants respond with stereotyped facial expressions when sweet, salty, sour and bitter solutions are applied to their tongue. Sweet evokes a smilelike expression followed by sucking, sour produces pursing and protrusion of the lips, bitter produces gaping, movements of spitting and sometimes vomiting movements. Even infants born without cerebral hemispheres showed the same facial expressions<\/li>\n<\/ul>\n<p><em>Specific hungers<\/em><\/p>\n<ul>\n<li>Specific hungers theory: the need for a nutrition causes the body to crave it, is limited to sweet and salty \u2192 for craving to cause an animal to seek out and take in the needed nutrient, a sensory cue would have to be unambiguously associated with the nutrient<\/li>\n<li>Our likes and dislikes of food depend very much on our likes and dislikes of the retronasal olfactory sensations associated with foods. Thus our affect towards food is made up of the hardwired affect contributed by taste combined with the learned affect contributed by retronasal smell<\/li>\n<\/ul>\n<p><em>The special case of umami<\/em><\/p>\n<ul>\n<li>Umami: the taste sensation produced by monosodium glutamate(MSG) o Stimulates the T1R1-T1R3 heterodimer<\/li>\n<li>Protein molecules are too large to be sensed by taste or smell, but proteins are made up of amino acids, including glutamic acid. When eaten, proteins are broken down into their constituent amino acids, providing stimuli for the glutamate receptors in the gut. These receptors can signal the brain that protein has been consumed. In this way, glutamate receptors can fulfil the function attributed to umami, but this is done in the gut, not in the mouth<\/li>\n<\/ul>\n<p><em>The special case of fat<\/em><\/p>\n<ul>\n<li>Fat molecules are made up of fatty acids attached to a support structure. Whole fat molecules stimulate the trigeminal nerve in the mouth, evoking tactile sensations like oily, viscous, creamy and so on; but some fat molecules may be partially digested while still in the mouth, thus releasing fatty acids<\/li>\n<li>Fat in the gut produces conditioned preferences for the sensory properties of the food containing fat<\/li>\n<\/ul>\n<p><em>Coding of taste quality<\/em><\/p>\n<ul>\n<li>Labelled lines: a theory of taste coding in which each taste neuron would unambiguously signal the presence of a certain basic taste<\/li>\n<li>Or tastes are coded via patterns of activity across many different taste neurons -In taste there is a lot of evidence preferring the labelled lines theory:\n<ul>\n<li>The functions of the four tastes are well served by their independence from each other<\/li>\n<li>We are very good at analysing taste mixtures o Most taste nerve fibres have a clear favourite stimulus<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n<p><em>Supertasters<\/em><\/p>\n<ul>\n<li>Bitterness grows more slowly with concentration than sweetness does<\/li>\n<li>People with the most numerous fungiform papillae experience not only the most intense taste sensations in general, but also the most intense sensations of oral burn and oral touch, because fungiform papillae are innervated by nerve fibres that convey burn and touch sensations, as well as those that convey taste sensations<\/li>\n<li>Removing input from one of the taste nerves, can intensify taste sensations. This intensification results because inputs from the different taste nerves inhibit one another in the brain. Loss of input from one nerve releases others from that inhibition<\/li>\n<\/ul>\n","protected":false},"excerpt":{"rendered":"<p>Taste versus flavour Retronasal olfactory sensation: the sensation of an odour that is perceived when chewing and swallowing force an odorant in the mouth up behind the palate into&hellip; <a class=\"continue\" href=\"https:\/\/www.amyork.ca\/academic\/zz\/sensation-and-perception\/taste\/\">Continue Reading<span> Taste<\/span><\/a><\/p>\n","protected":false},"author":4,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":[],"categories":[112],"tags":[],"_links":{"self":[{"href":"https:\/\/www.amyork.ca\/academic\/zz\/wp-json\/wp\/v2\/posts\/4458"}],"collection":[{"href":"https:\/\/www.amyork.ca\/academic\/zz\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.amyork.ca\/academic\/zz\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.amyork.ca\/academic\/zz\/wp-json\/wp\/v2\/users\/4"}],"replies":[{"embeddable":true,"href":"https:\/\/www.amyork.ca\/academic\/zz\/wp-json\/wp\/v2\/comments?post=4458"}],"version-history":[{"count":0,"href":"https:\/\/www.amyork.ca\/academic\/zz\/wp-json\/wp\/v2\/posts\/4458\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.amyork.ca\/academic\/zz\/wp-json\/wp\/v2\/media?parent=4458"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.amyork.ca\/academic\/zz\/wp-json\/wp\/v2\/categories?post=4458"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.amyork.ca\/academic\/zz\/wp-json\/wp\/v2\/tags?post=4458"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}