GSU NEUR 3000 - NEUR 3000 - Chapter 8 (34 pages)

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NEUR 3000 - Chapter 8



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NEUR 3000 - Chapter 8

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Pages:
34
School:
Georgia State University
Course:
Neur 3000 - Hon Principles of Neuroscience
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THE CHEMICAL SENSES NEUR 3000 Dr Joseph J Normandin CHEMOSENSATION From the simplest of organisms to the amazing human the detection of chemicals in the environment in ubiquitous We will focus on the two familiar chemical senses taste smell but we must remember that there are chemical sensors within our body that are critical to our bodies function The sense of taste is called gustation and the sense of smell is called olfaction In both systems chemoreceptors transduce the presence of a chemical signal into an electrical signal GUSTATION The common idea of taste is surprisingly complicated How does that burger taste Juicy yummy smoky Our common experience of taste is actually combination of gustation olfaction and somatosensation A better way to describe this might be flavor Here we consider the chemical components of flavor transduced in the gustatory system or what a neuroscientist would call taste The five basic tastes Sweet Salty Bitter Sour Umami GUSTATION The chemistry of the 5 tastes and their adaptive value Sweet Sugars carbohydrates artificial sweeteners some proteins Signals the presence of a high calorie food source Salty Table salt NaCl Signals the presence of salts necessary for metabolism Bitter Caffeine medicine K Mg2 Signals the presence of a possible toxin Sour Acids Presence of food source or chemical needed for metabolism Umami Glutamate some amino acids and proteins Presence of a food source GUSTATION The organs of taste Chemoreceptors can be found on the tongue pharynx and epiglottis Tongue contains the largest abundance of chemoreceptors The tongue Papillae small round protrusions of varying morphology cover the tongue Each papilla can contain several hundred taste buds Each taste bud can contain up to 150 taste receptor cells Gustatory sensory neurons innervate the taste buds in apposition to taste receptor cells An individual taste bud tends to respond to a particular taste i e a salty taste bud though at high concentrations of a given chemical this selectivity weakens GUSTATION GUSTATION Taste receptor cells The apical end of a taste receptor cell contains chemoreceptors Although not technically neurons they are excitable If a taste receptor cell is activated the cell will depolarize producing a receptor potential Depolarization opens voltage gates Ca2 channels Neurotransmitter of unknown type is released to gustatory sensory neurons nearby and produces EPSPs in the sensory neuron Taste receptor cells are constantly regenerating When you burn your tongue and kill taste receptor cells because you could not wait to eat your pizza you may still be able to enjoy the leftovers GUSTATION GUSTATION Mechanisms of taste transduction Tastants taste stimuli are transduced by taste receptor cells in a number of different ways Direct passage through ion channels salty sour Block ion channels sour Binding to G protein coupled receptors sweet bitter umami GUSTATION Salty Salty is the tasting of Na Salt sensitive taste receptor cells express the amiloride sensitive Na channel Constantly open NaCl table salt when dissolved in water or saliva dissociates into Na and Cl As you eat something salty Na moves inside the cell through the amiloridesensitive Na channel along it s concentration gradient GUSTATION Sour Sour is the tasting of H Sour sensitive taste receptor cells express the amiloride sensitive Na channel Also permeable to H Acidic compounds release H when dissolved in water As you eat something sour H will move through the amiloride sensitive Na channel AND block K channels The combination of H effects produces sour instead of salty Likely to be other mechanisms as well GUSTATION Bitter Bitter is the tasting of many substances that are body recognizes as toxic Bitter sensitive taste receptor cells express T2R receptors taste type 2 receptor a host of 30 subtypes Bitter substances bind to this metabotropic receptor Activation of T2Rs and other receptors of this family results in the activation of the IP3 pathway that opens Na channels and intracellular Ca2 channels GUSTATION Sweet Sweet is the tasting of many substances that are carbohydrates or similar in structure Sweet sensitive taste receptor cells express a T1R2 T1R3 receptor complex Sweet substances bind to this metabotropic receptor complex Also activates the IP3 pathway Strangely felines do not have this receptor Evidence that cats are evil GUSTATION EVIL Can t taste sweet GUSTATION Umami Umami is the tasting of amino acids Umami sensitive taste receptor cells express a T1R1 T1R3 receptor complex Umami substances bind to this metabotropic receptor complex Also activates the IP3 pathway GUSTATION The gustatory pathway Taste receptor cells form synapses with nearby gustatory sensory neurons Gustatory sensory neuron axons from different parts of the tongue form the facial nerve CN VII anterior 2 3 tongue and the glossopharyngeal nerve CN IX posterior 1 3 tongue These axons synapse with the gustatory nucleus in the medulla Gustatory nucleus neurons project to the ventral posterior medial nucleus VPM of the thalamus VPM neurons project to primary gustatory cortex of the parietal lobe Taste perception Gustatory nucleus neurons also project to other brainstem regions involved in swallowing salivation gagging and vomiting or to the hypothalamus regulating motivation for feeding GUSTATION GUSTATION Neural coding of taste Some taste receptor cells are quite specific i e respond to one taste Only have chemoreceptors for one taste Many are broadly tuned and respond to multiple tastes Have chemoreceptors for more than one taste Gustatory sensory neurons responding to multiple taste cells are also broadly tuned This broad tuning occur throughout the gustatory pathway into the cortex These responses exhibit population coding where the activity of large number of broadly tuned neurons reveals the code in this case what the taste is The aggregate activity of salty sour sweet sour sour sweet salty gives the compound being tasted a distinct taste identity This combined in the cortex with olfactory and somatosensory information for flavor OLFACTION The sense of smell is called olfaction Our common experience of taste is actually combination of gustation olfaction and somatosensation A better way to describe this might be flavor Here we consider the chemical components transduced by the olfactory system Detection of odors can contribute to flavor Detection of odors can be a signal of something harmful Detection of odors can


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