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PSYC 3450
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INTRODUCTION Gustation and olfaction have a similar task: the detection of environmental chemicals. In fact, only by using both sense can the nervous system perceive flavour. However, the systems of gestation and olfaction are separate and different –from the structures and mechanisms of their chemoreceptors, to the gross organization of their central connections, to their effects on behaviour. The neural information from each system is processed in parallel and is merged at rather high levels in the cerebral cortex. TASTE The Basic Tastes It is likely that we can recognize only a few basic tastes. Most neuroscientists put the number at five. The four obvious taste qualities are:  Saltiness  Sourness  Sweetness  Bitterness  Umami How do we perceive the countless flavours of food?  First, each food activates a different combination of the basic tastes.  Second, most foods have a distinctive flavour as a result of their taste and smell occurring simultaneously  Third, other sensory modalities contribute to a unique food-tasting experience (e.g. texture, temperature, and pain sensations) The Organs of Taste Although we taste with our tongue, there are other areas of the mouth (e.g. palate, pharynx, and epiglottis) that are also involved. Odours from the food pass, via the pharynx, into the nasal cavity, where they can be detected by olfactory receptors. The tip of the tongue is most sensitive to sweetness, the back to bitterness and the sides to saltiness and sourness. However, most of the tongue is sensitive to all basic tastes. The surface of the tongue is scattered with small projections called papillae. Each papilla has from one to several hundred taste buds (see Fig. 8.2) and each of these have 50 – 150 taste receptor cells. Taste cells make about 1% of the tongue epithelium. Taste buds also have basal cells that surround the taste cells and a set of gustatory afferent axons. A person typically has 2000-5000 taste buds. Tastants at very low concentrations will not be tasted, but at some critical concentration, the stimulus will evoke a perception of taste. This is the threshold concentration. At levels just above threshold, most papillae tend to be sensitive to only one basic taste. However, when the concentrations of the Tastants are increased, most papillae become less selective. For example, a papilla might have responded only to sweet when all stimuli were weak but also responds to sour and salt if they are made stronger. This relative lack of specificity is a common phenomenon in sensory systems. Many sensory receptors are surprisingly indiscriminate about the things that excite them. This presents a paradox: how can we distinguish reliably between differences as subtle as two kinds of chocolate? The answer lies in the brain. Taste Receptor Cells The chemically sensitive part of a taste receptor cell is called the apical end. These ends have thin extensions called microvilli that project into the taste pore. Taste receptor cells are not neurons but do form synapses with the endings of the gustatory afferent axons near the bottom of the taste bud. Taste receptor cells also make electrical and chemical synapses onto some of the basal cells. When a taste receptor cell is activated by an appropriate chemical, its membrane potential changes (i.e. depolarization). This shift is called the receptor potential (see Fig. 8.3). If receptor potential is depolarizing and large enough, most taste receptor cells may fire action potentials. More than 90% of the receptor cells respond to two or more of the basic tastes. Figure 8.4 shows the results of recordings from four gustatory axons in a rat. One responds strongly only to salt and one only to sweet. Two respond to all but sweet. Why? This is because the responses depend on the particular transduction mechanisms present in each cell. Mechanisms of Taste Transduction The process by which an environmental stimulus causes an electrical response in a sensory receptor cell is called transduction. Taste transduction involves several different processes and each basic taste uses one or more of these mechanisms:  Directly pass through ion channels (salt and sour)  Bind and block ion channels (sour)  Bind to G-protein-coupled receptors in the membrane that activate second messenger systems Saltiness + The taste of salt is mostly the taste of the cation Na and its concentration must be quite high in order to taste it (at least 10 nM). Salt-sensitive taste cells have a special Na -selective channel that is blocked by the drug amiloride (see Fig. 8.5). This chan+el is insensitive to voltage and is always open. When you sip chick soup, for example, the Na concentration rises outside the receptor cell. Na then diffuses down its concentration gradient which results in an inward current and depolarization (receptor potential) of the membrane. The receptor potential causes the voltage-gated sodium and calcium channels to open and trigger the release of neurotransmitter onto the gustatory afferent axon. Sourness Protons are the causative agents of acidity and sourness. They are known to affect sensitive taste receptors in at least two ways (see Fig. 8.5):  First, H can permeate the amiloride-sensitive sodium channel and cause an inward H + current and depolarize the cell.  Second, hydrogen ions can bind to and block K -selective channels. When the K + permeability of a membrane is decreased, it depolarizes. Bitterness There are two families of taste receptor genes (T1R and T2R) which encode for a variety of G- protein-coupled taste receptors. Bitter substances are detected by the 30 or so different types of T2R receptors, however, animals are not very good at telling different bitter tastants apart because each bitter taste cell expresses many, and perhaps all, of the 30 bitter receptor proteins. Because each taste cell can send only one type of signal to its afferent nerve, a chemical binding to one of the 30 receptors will trigger the same response as a different chemical that binds to another bitter receptor. This is important because it conveys to the brain that a bitter substance is poisonous and should be avoided regardless of how bitter it is. Bitter receptors use a second messenger pathway to carry their signal to the gustatory afferent axon. Bitter, sweet, and Umami receptors all seem to use exactly the same second messenger pathway to carry their signals to the afferent axons (see Fig. 8.6).  When a tastants binds to a bitter (or sweet or umami) receptor, it activates its G-proteins  The G-proteins stimulate the enzyme phospholipase C and thereby increasing the production of inositol triphosphate (IP ). 3  IP3activates a special type of ion channel that is unique to taste cells o This allows Na to enter and depolarizing the taste cell 2+  The voltage-gate calcium channels open and allow Ca to enter the cell o IP3also triggers the release of Ca from intracellular storage sites 2+ o These two sources of Ca help trigger neurotransmitter release Sweetness Sweet receptors resemble bitter receptors, in that they are both G-protein-coupled receptors, but they are different in that sweet receptors are formed from two such proteins bound tightly together (the bitter receptor is only a single protein) (see Fig. 8.7). A functioning sweet receptor requires two very particular members of the T1R receptor family: T1R2 and T1R3. Chemicals binding to the T1R2 + T1R3 receptor activate exactly the same second messenger system as the bitter receptors. Why don’t we confuse bitter chemicals with sweet ones then?  Bitter receptor proteins and sweet receptor proteins are expressed in different taste cells and also connect to different gustatory axons. Umami The transduction process for umami is identical to that for sweetness, with one exception. The umami receptor is comprised of two members of the T1R protein family: T1R1 and T1R3. Similar to sweet and bitter receptors, umami receptors use the exact same second messenger pathways. So why don’t we confuse the taste of amino acids with sweet or bitter chemicals?  The taste cells selectively express only one class taste receptor proteins Central Taste Pathways The main flow of taste information: taste buds  primary gustatory axons  brain stem  thalamus  cerebral cortex (see Fig. 8.8)  Three cranial nerves carry primary gustatory axons and bring taste info to the brain o The anterior two-thirds of the tongue and the palate send axons into a branch of cranial nerve VII (facial nerve) o The posterior third of the tongue is innervated by cranial nerve IX (glossopharyngeal nerve) o The regions around the throat (e.g. glottis, epiglottis, and pharynx) send taste axons to cranial nerve X (vagus nerve)  These nerves synapse within the slender gustatory nucleus From the gustatory nucleus, taste pathways diverge. The conscious experience of taste is presumably mediated by the cerebral cortex. Neurons of the gustatory nucleus synapse on a subset of small neurons in the ventral posterior medial (VPM) nucleus. These VPM taste neurons then send axons to the primary gustatory cortex. The taste pathways to the thalamus and cortex are primarily ipsilateral to the cranial nerves that supply them. Lesions within the VPM thalamus or the gustatory cortex can cause ageusia. The Neural Coding of Taste Labelled line hypothesis: If you were going to design a system for coding tastes, you might begin with many specific taste receptors for many basic tastes. Each receptor type will be connected by separate axons to neurons in the brain that also responded to only one specific taste. Therefore, there will be specific neurons responding
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