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Chapter 8

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Neuroscience : Chapter 8: Chemical Senses (Taste and Smell) Chemical sensation is the oldest and most common of the sensory systems. Taste Humans have an innate preference for sweetness, and rejection of bitterness; however these preferences can be modified with practice We can recognize only a few basic tastes: saltiness, sweetness, sourness, bitterness, and umami (glutamate) K+ and Mg2+ are ions that contribute to the taste of bitterness, as well as more complex structures like coffee and quinine We perceive different flavours by the activation of different combination of basic tastes, as well as the use of smell + taste, and other sensory modalities (texture, temperature, pain etc.) The organs of taste include the tongue, palate, pharynx, epiglottis, and nasal cavity The tip of the tongue is most sensitive to sweetness, the back of the tongue to bitterness, and the sides to saltiness and sourness. However in high concentrations, all basic tastes can be detected by any region of the mouth with varying sensitivity Papillae: Small projections scattered around the tongue where taste buds, and taste cells are located There are three different types of papillae:  Folliate Papillae are shaped like ridges and are located on the side of the tongue  Vallate Papillae are shaped like pimples and are located at the black of the tongue  Fungiform Papillae are shaped like mushrooms and are located anterior 2/3rds of the tongue Within each papillae, there are between 1 and several hundred taste buds; and within each taste bud there is between 50-150 taste receptor cells like the sections of an orangeHumans have around 2000-5000 taste buds, although extreme cases have shown as few as 500 to as many as 20,000 To be tasted, concentrations of a substance must be higher than a threshold concentration. At concentrations just above the threshold, most papillae are only sensitive to 1 of the five basic tastes. However, at higher concentrations, the papillae become less sensitive and may respond to multiple basic tastes. At these higher concentrations, more than 90% of taste receptor cells are sensitive to two or more of the basic tastes Taste Receptor Cell Within a taste cell, the chemically sensitive part is called the apical end which is located at the surface of the tongue. The apical end includes microvilli that lead into the taste pore, which is located at the surface of the tongue and is exposed to the contents of the mouth Taste receptor cells are not neurons but they form synapses with afferent axons (to the brain) at the bottom of the taste bud, as well as electric and chemical synapses with basal cells A taste bud has a lifespan of about 2 weeks before it is replaced When a taste bud is activated by the appropriate chemical, it caused the membrane to depolarize. This is called the receptor potential; if this potential is large enough the cell will fire action potentials which will cause Ca to enter the cell and release neurotransmitters onto the afferent axons Taste Transduction Transduction: The process by which an environmental stimulus causes an electrical response in a receptor cell Saltiness The most common salty chemical is NaCl; the salty taste comes from the cation Na+, however its concentration must be quite high to actually taste it (at least 10 mM) In salty compounds, the size of the anion affects the salty taste of the cation; in general, the larger the anion, the more it inhibits the salty taste of the anion. As the anion becomes large, it may even take over the cation in influencing the taste. E.g. the compound sodium saccharin tastes sweet because the sweet tasting saccharin dominates the salty sodium In salt-sensitive taste cells, there are Na+ sensitive ion channels (not to be confused with voltage gated Na+ channels) that always remain open and are not sensitive to voltage When you eat salty foods, the concentration of Na rises outside the taste cell, which causes the concentration gradient across the membrane to become steeper. Na then diffuses into the taste receptor cell through the Na+ channels, which causes the membrane to depolarize. This depolarization (aka receptor potential), in turn causes the voltage gated Na and Ca channels to open, triggering the influx of calcium and the release of neurotransmitters from the synaptic vesicles onto the gustatory afferent axons Sourness Sourness results from the high acidity of the food (low pH). The proton H+ is the agent that causes foods to be salty. The transduction mechanism of sourness is very similar to that of saltiness; In sour-sensitive taste cells, acidic compounds dissolve in water and H+ concentrations rise outside the taste receptor cell. The concentration gradient across the membrane for H+ increases, and H+ begins to diffuse down the amiloride-senstive Na channels into the cell (same as salt). This causes the membrane to depolarize, sending a receptor potential down the taste cell, and causing voltage gated Na and Ca channels to open. This triggers the influx of Ca, and the release of neurotransmitters from the synaptic vesicles. Another way H+ affects the taste cell is by binding to, and blocking K+ channels. By blocking these K+ channels, it prevents K+ from leaving the cell (because K+ concentrations are higher in the cell). This contributes to the depolarization of the membrane. Bitterness There are two families of taste receptor genes, called T1R and T2R. Bitterness receptor cells come from the T2R gene (there are about 30 different ones) and are all in form of G-protein-coupled receptors. Bitter receptors are poison receptors, and although we have so many, we are not good at differentiating between different bitter tastants. This is because each bitter taste cell may have most, if not all, of the 30 different receptors on it. And because all 30 receptors cause the same series of events inside the cell, the afferent axons are not able to tell which bitter receptor caused the cell to excite. So instead of differentiating between different bitter substances, the brain sends a general message that all bitter substances should be avoided.When a bitter tastants activates one of the 30 receptors, it activates G-proteins which, in turn, stimulate the enzyme Phosphokinase C. This increases the levels of IP (i3ositol trophosphate). IP do3s two things: First, it activates special types of ion channels unique to taste cells, allowing Na to enter the cell (thus depolarization and the opening of Ca channels). Secondly, IP also causes the release of Ca from 3 intracellular storage sites, which trigger the release of neurotransmitters. This second messenger pathway is the exact same for sweet and umami substances; the only difference is the gene family of the receptor, and the tastants molecule that binds to the receptors Sweetness Unlike bitterness, sweet-sensitive taste receptors are from the T1R family. In addition sweet receptors are actually two tightly bound G-proteins whereas bitterness is only one The two proteins are: T1R2 and T1R3. If either one of these proteins are missing or dysfunctional, a person may not be able to taste sweetness Once a sweet tastants binds to one this protein complex, the second messenger system that occurs within the cell is identical to that of bitterness (see above). Umami Umami receptors, like sweetness, are part of the T1R family; however the two proteins that make up the umami receptors are different from sweetness receptors The two proteins are: T1R1 and T1R3. Because Umami and Sweetness share the T1R3 protein, what differentiates the two is the first
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