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University of Toronto Scarborough
Matthias Niemeier

Chapter 8 Notes: The Chemical Senses 03:27 Introduction: In this chapter, we will basically discuss gustation (taste) and olfaction (smell). However, there are other chemical sensors such as chemoreceptors, which warn us of irritating chemicals in our system and they are distributed throughout the body. Gustation and Olfaction have similar tasks: to detect chemicals in our environment but they are structurally different and have different effects on our behavior. TASTE: As humans we have an innate preference for sweetness but can grow accustomed to bitter food as well such as coffee. Also the body has the ability to recognize the deficiency of certain key nutrients and develop a craving for them. Most neuroscientists have determined that there are five main categories of taste known to mankind and those are saltiness, sourness, sweetness, bitterness and umami (Japanese for delicious). Umami is defined by the taste of amino acid glutamate and MSG. Most acids taste sour, most salts taste salty, sugars taste sweet and bitter substances can be detected even in very small amounts (down to nanomolar range) and is beneficial to mankind because most poisons are bitter. In order to perceive a wide variety of foods, each food activates a range of the basic tastes to make it unique. Also, most foods have a specific combination of taste and smell that also help make it unique. Texture and temperature of the food as well as other modalities help contribute to the uniqueness of the food. We experience the taste of foods with different areas of the mouth such as the palate, the pharynx and the epiglottis. On the surface of the tongue are small projections called papillae which contains 1 to several hundred taste buds. Each taste bud has 50-150 taste receptor cells but they comprise of only 1% of the tongue epithelium. When concentrations are just above the threshold, most papillae become sensitive to only one basic taste; however, when the concentration of the taste stimuli are increased, the papillae become less selective. The chemically sensitive part of a taste receptor cell is its apical end (a small membrane region), near the surface of the tongue. Taste-receptor cells are not neurons by histological criteria; but they do form synapses on afferent gustatory axons and make electrical and chemical synapses onto some basal cells. Depolarization of the receptor membrane causes voltage-gated calcium channels open; calcium enters the cytoplasm which triggers the release of transmitter molecules. www.notesolution.com How is there distinction between the number of chemicals that a taste receptor cell will respond to? It is because of the particular transduction mechanisms present in each cell MECHANISMS OF TASTE TRANSDUCTION: Transduction- the process by which an environmental stimulus causes an electrical response in a sensory receptor cell. Taste transduction can involve several processes to detect taste such as: Tastants directly pass through ion channels (salt and sour) Bind to G-protein coupled receptors in the membrane that activate second messengers and in turn open ion channels (bitter, sweet and umami) Bind to and block ion channels (sour) Saltiness- salt-sensitive taste cells have a special Na+-selective channel (amiloride- sensitive sodium channel). Unlike voltage-gated channels this taste channel is not voltage sensitive; it stays open all the time. When concentration of Na+ rises outside the receptor cell the Na+ ions move inside the receptor cells down its concentration gradient. Once inside, the cell is depolarized which triggers the opening of voltage- gated sodium and calcium channels to open near synaptic vesicles, which causes the release of neurotransmitters on to gustatory afferent axons. Sourness- Foods taste sour due to their high acidity. These types of tastes can affect sensitive taste receptors in two ways: 1) H+ can permeate the amiloride- sensitive sodium channel, which causes an influx of H+ and depolarized the cell; 2) hydrogen ions can bind to and block K+-selective channels. When the permeability of K+ is decreased, the cell depolarizes. Bitterness- bitter substances are detect by 30 or so different types of T2R receptors. Bitter receptors in animals cant really tell the difference between different bitter tastes because each cell can only send one type of signal to its afferent nerve. Different chemicals that bind to the different receptors will still generate the same response. Bitter receptors use the secondary messenger pathway in order to signal their afferent gustatory axon. The pathway for signaling in bitterness are similar to those of sweetness and umami: G-protein coupled receptor stimulated stimulation of phospholipase C increased production of IP3 activation of special taste ion channel opening of Na+ channels(depolarization) voltage-gated calcium channels to open. Sweetness- sweet receptors are formed by two g-protein coupled receptors as opposed to bitter receptors which contain one. Sweet receptor cells require T1R2 and T1R3 receptors in order to perceive sweetness. These receptors activate the same www.notesolution.com second messenger system as bitter receptors. The reason why we dont confuse bitter and sweet taste is because these tastes occur in different taste cells. Umami(amino acids)- The transduction process for umami is identical to that of sweetness with one exception: the two protein complex consists of T1R1 an T1R3 receptors. We dont confuse umami taste with bitter or sweet for same reason that there are different taste cells for the different tastes. CENTRAL TASTE PATHWAYS: Flow of taste information: taste budsprimary gustatory axonsbrain stemthalamuscerebral cortex. There are three cranial nerves that relay taste information: facial nerve (receives info from the anterior 2/3 of the tongue and palate), glossopharyngeal nerve (receives info from posterior 1/3of the tongue) and the vagus nerve (receives info from regions around the throat). These nerves all bundle together in the brain stem and synapse on the gustatory nucleus. From the gustatory nucleussynapse on to the ventral posterior medial nucleus (VPM)primary gustatory cortex (this pathway is ipsilateral to the cranial nerves that supply them) Lesions in the VPM thalamus and gustatory cortex can cause ageusia, the loss of taste perception. Labeled line hypothesis- shows how the modality of stimulus are encoded in the nervous system. Population coding- a scheme in which the responses of a large number of broadly tuned neurons, rather than a small number of precisely tunes neurons, are used to specify the properties of a particular stimulus, such as a taste. SMELL: Smell is important for distinguishing between good and bad smells which could be harmful. It is also a mode of communication. Pheromones, which are chemicals released by the body, are important signals for reproductive behavior. We smell with a thing sheet of cells called the olfactory epithelium which have three main cell types: Olfactory receptor cells- the site of transduction; they are genuine neurons that have axons of their own that penetrate in to the CNS. Supporting cells- similar to glia; they help produce mucus among other things. Basal cells- the source of new receptor cells. www.notesolution.com Sniffing brings air in to the nasal passage and only a smell percentage of that air passes over the olfactory epithelium. The epithelium then secretes mucus, which is replaced every 10 minutes and is continuously flowing. The mucus is water based and contains several mucopolysaccharides, antibodies, and odorant binding proteins. The odorants dissolve in the mucus before they get to the receptor cells. It is important to have antibodies in case viruses are inhaled and odorant binding proteins to concentrate the smell. OLFACTORY PATHWAY: Odorants Binding to membrane odorant receptor proteins G-protein stimulation Activation of adenalyl cyclase Formation of cAMP Binding of cAMP to specific cation channel Opening of cation channels and influx of Na+ and Ca2+ Opening of Ca2+-activated chloride channels Current flow and membrane depolarization Olfactory axons do not come together as a single nerve like other cranial nerves. Rather they form small clusters of axons which penetrate the cribriform plate and then enter the olfactory bulb. When olfactory axons are severed, it causes anosmia, which is the inability to smell. Same as in taste, olfaction uses a similar sort of population coding scheme in which receptors are more or less sensitive to certain odorants. The concentration of the odorant as well as the odorant itself are important for generating stronger responses. Glomeruli- spherical structures that are the input layer for axons of olfactory receptor neurons on the bulbs. It seems that each glomerulus receives input from a large area of the epithelium and the arrangement of input is symmetrical on both bulbs. It also seems that glomerulus receive input from only receptor cells of one type; basically making it a map of odor information. How does the whole brain discriminate between odors when the single neurons cant? : Each odor is represented by the activity of a large population of neurons the neurons responsive to particular odors may be organized in to spatial maps the timing of action potentials may be an essential code for particular odors. With the use of population coding the brain can use the information provided by a large array of neurons to determine the smell being perceived which would be unique to only that odor. A sensory map is an orderly arrangement of neurons that co
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