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PSYC 385
Sidney Eve Matrix

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 tastant
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