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Chapter 8

Chapter 8 The Chemical Senses

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Matthias Niemeier

NROC64 Chapter 8The Chemical Senses Animals including humans depend on the chemical senses to help identify nourishment noxious substances or the suitability of a potential mate chemical sensation is the oldest and most common of sensory systemsEverything serves sensorimotor transformations autonomic circuits memory cognitions consciousness and conscious and otherwise functional androids Chemoreceptors chemically sensitive cells eg nerve endings of skin and mucous membranesFlavour can be perceived by the nervous system only if BOTH gustation and olfaction were usedTASTE The system of taste is necessary to distinguish between new sources of food and potential toxins the body has the capacity to recognize a deficiency of certain key nutrients and develop appetite for them eg cravings for salty foodsThe Basic Tastesy Five basic tastessaltiness sourness sweetness bitterness umami savoury taste of amino acid glutamate monosodium glutamate or MSGy Correspondence between chemistry and taste sweetsugars like fructose sucrose artificial sweeteners saccharin aspartame 2etc bitterions like K and Mg quinine and caffeiney Many bitter organic compounds can be tasted even at very low concentrations which is an advantage in detecting poisonous substances set of receptors for mainly poisonous substances lumped togetherperceived as bitterHow do we perceive countless favours of food 1 Food activates a different combination of basic tastes2 Most foods have a distinctive flavour as a result of taste and smell at the same time eg biting into an onion vs an apple 3 Other sensory modalities contribute to unique foodtasting experience eg texture and temperature The Organs of Taste We taste with our tongue palate pharynx and epiglottis odours pass via pharynx nasal cavitydetected by olfactory receptors y Areas of sensitivity of the tongue tip of the tongue sweetness back of the tongue bitterness sides of tongue saltiness and sourness most of the tongue is sensitive to all basic tastes y Papillae small projections scattered on the surface of the tongue each having several hundred taste buds y Ridgeshaped foliate papillae Pimples vallate papillae mushrooms fungiform papillae y Each tastebud has 50150 taste receptor cells a typical person has 20005000 taste buds y Critical concentrations in which the stimulus will evoke a perception of tastethreshold concentration y At concentrations just above threshold most papillae are sensitive to just one basic tastey When concentrations of stimuli increase most papillae become less selective Taste Receptor CellsApical endschemically sensitive part of a taste receptor cell small membrane region near the surface of the tonguey Apical ends have microvilli that project into the taste porea small opening on the surface of the tongue where the taste cell is exposed to contents of the mouth y Cells of the taste bud undergo a constant cycle of growth death and regeneration the lifespan of one taste cell is about 2 weeks y Receptor cell activatedmembrane potential changes via depolarizationvoltage shiftreceptor potential y If the depolarization is large enough an action potential is fired which ultimately causes transmitter substance identity unknown to be released which excites postsynaptic sensory neurons to fire action potentials to communicate taste signal y BROAD TUNING 2 or more tastants Tuningproperty to selectively represent a particular kind of sensory information
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