Chapter 8: Chemical Senses
Chapter 8: Chemical Senses NEUROSC 3000 - 020
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This 0 page Class Notes was uploaded by Victoria Gonzalez on Wednesday November 4, 2015. The Class Notes belongs to NEUROSC 3000 - 020 at Ohio State University taught by Robert Boyd in Summer 2015. Since its upload, it has received 16 views. For similar materials see Introduction to Neuroscience in Neuroscience at Ohio State University.
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Date Created: 11/04/15
Chapter 8 Chemical Senses Victoria Gonzalez Learning objectives 0 Understand basic anatomy of taste receptors 0 Know basic signal transduction mechanisms for each taste 0 Know basic anatomy of olfactory receptors 0 Understand olfactory signal transduction Understand chemical sensory pathways to the brain 1 Introduction a b C d 2 Taste a b no Of all sensory systems chemical sensation is the oldest and most pervasive across species universal Taste gustation and smell olfaction both work together for avor to detect the environment i Signals are integrated in the orbitofrontal area Chemoreceptors monitor internal environment chemical communication and integration Senses are important for hunger emotion sex and memory Taste is needed to determine what is food from what is poison Enjoy sweet not bitter things i The threshold for salty and sweet is high ii The threshold for bitter is low We crave nutrients that are lacking Only 5 basic tastes sweet salty bitter sour umami Acids are usually sour ii Salts are usually salty iii Different structures are sweet aspartame made of amino acids is sweeter than sucrose table sugar Magnesium potassium and caffeine are bitter Smell combinations of receptors pain texture visual cues and temperature all contribute to avour iv v Organs of taste i We taste with our tongue palate pharynx and epiglottis Nasal cavity Palate Tongue I 39 4 n quotV H I 39 l Eplgl ttis ii Taste buds are scattered on the sides of the tongue so that all parts with taste buds are sensitive to all basic tastes 1 Taste bunds are located on papillae bumps on tongue 2 Each papillae has from 1 to hundreds of taste buds 3 Each taste bud has 50 to 150 taste bud cells 4 Taste buds account for only 1 of tongue epithelium 5 People have between 500 and 20000 taste buds Fmglzi39rn I 1 mm milli litwhee iii Taste bud cells are sometimes speci c to taste sometimes not iv The sensory part of taste receptor cell is at the apical end v These microvilli stick out into the taste pore and are exposed to tastants in the mouth vi Taste cells are replaced every two weeks vii Taste receptor cells are not genuine neurons viii Types of taste bud cells 1 Type I Na sensing like glial cells 2 Type II known as receptor cells use G protein coupled receptors a Detect either bitter sweet or umami only one b Have no synapse 3 Type III known as presynaptic cells form synapses vesicles use voltage gated calcium channels VGCC a Respond to sour taste 4 Type IV known as basal cells progenitors of other taste sensing ces f Response of taste ces i When exposed to chemicals taste cells generate receptor potentials and the cell depolarizes ii Sour and salty release serotonin iii Sweet umami and bitter release ATP iv Most ces respond strongly to one taste g Mechanisms of taste transduction i Transduction an environmental stimulus causes an electrical response in a sensory receptor cell ii Each basic taste uses one mechanism not all understood iii Many animals are used for studies iv Salty and sour pass through ion channels v Sour bind to block ion channels vi Bitter sweet umami use Gproteincoupled receptors h Saltiness i Sodium ions enter through amiloridesensitive sodium channels 1 Open all the time depolarizes the taste cell when sodium enters in the cell insensitive to vo age ii Type cells involved iii Anions affect the taste of salts i Sourness i Low pH acids ii Type III presynaptic cells iii Acids dissolve in water and produce H iv Protons enter proton sensitive TRP transient receptor potential channels v Bind to block K selective channels to cause depolarization vi Selectively expressed in a unique population of taste bud cells vii May also be used to detect pH in spinal cord j Bitterness i Use Type II cells no synapse ii Two families of taste receptor genes T1R and T2R 1 They are Gprotein coupled 2 Dimers two T2R proteins bound to each other iii Many poisons are bitter so there are many genes used to recognize bitter at least 30 T2R 1 Multiple T2R genes in each taste cell iv Some taste cells only express bitter receptors some communication to speci c gustatory axons v When a tastant binds to a bitter receptor it activates a G protein which stimulates the enzyme PLC vi PLC and taste cell speci c cationic channel k Sweetness i Use Type II cells no synapse ii There are many different sweet tastants but are all detected by the same receptor 1 Dimer T1R2T1R3 Same second messenger system as bitter iv Sweet receptors are expressed in speci c cells connected to the sweet gustatory axons l Umami amino acids Use Type II cells no synapse All detected by the same receptor 1 Dimer T1R1T1R3 2 T1R3 is in sweet too 3 T1R1 is what determines umami Activate the same second messenger system as for bitter iv Umami receptors connect to speci c gustatory axons m Central taste pathways Flow of taste information taste buds primary gustatory axons brain stem medulla thalamus cortex ipsiatera o W Gustatory axons carried by 1 Cranial nerve Vll facial 2 Cranial nerve IX glossopharyngeal 3 Cranial nerve X vagus Cranial nerves synapse in the gustatory nucleus in the medulla Neurons of the gustatory nucleus synapse on the ventral posterior medial VPM nucleus in the thalamus Axons are then sent to the primary gustatory cortex 1 Located on Brodmann s area 36 of the cortex vi vii viii Ageusia a loss of taste perception 1 Caused by lesions to VPM thalamus or gustatory cortex Gustation is important for vomiting swallowing digestion breathing Gustatory information is distributed to the hypothalamus and the medulla n Neural coding of taste Some afferent neurons are tightly tuned speci c others are broadly tuned not speci c Afferent neurons show response pro les similar to narrowly tuned taste bud receptor cells and broadly tuned presynaptic cells but this is not clear yet Gustatory nucleus axons are broadly tuned all the way to the cortex Are there more tastes Perhaps fats 1 Fatty acids are potent stimuli 2 There are membrane receptors for fatty acids on taste bud cells 3 Smell olfaction a We can smell 100000s of different substances most are unpleasant b Other animals use pheromones to communicate Detected by vomeronasal organ vestigial in humans c Olfactory organs We do not smell with our nose we smell with the olfactory epithelium thin sheet of cells high up in the nasal cavity Olfactory epithelium has three cell types 1 Olfactory receptor cells sites of transduction a Genuine neurons b Have a 48 week life cycle 2 Supporting cells similar to glia a Produce mucus 3 Basal cells source of new receptor cells Odorants dissolve in mucus 1 Mucus mixture of antibodies proteins and odorant binding proteins which concentrate odorants Sensitivity to smell is dependent on the size of the olfactory epithelium and the number of receptors 1 Dogs have more receptors and a larger epithelium surface area Gilliam AH bum W mamKc r i a fr j Uli g rynew 7 Eribril39nrrmplate plate ii aluminumdd Ema can epithelium Gillan Rapier call Euil P rting cell Inhaled 7 39 quot ciiiaur air quot 39 V quot olfactory39oelle 37quotquot Mucuslearar v Olfactory Receptor neurons 1 Have only one dendrite 2 Have an unmyelinated axon 3 Axons from olfactory nerve make up cranial nerve I 4 Cranial nerve l connects to the olfactory bulb 5 Axons are fragile and can be easily damaged producing anosmia inability to smell d Olfactory signal transduction i Olfactory receptor neurons have a single dendrite that ends with a small knob on the surface of the epithelium on the knob are cilia in the mucus ii Odorants dissolve in the mucus and bind to the cilia to activate the transduction process by G proteins Golf iii Golf activates adenylyl cyclase iv Adenylyl cylase forms cAMP v cAMP binds to a cation channel vi Cation channel opens and there is an in ux of Na and Ca2 vii Ca2 opens Cl39channels Cl39 leaves the cell viii Depolarization receptor potential occurs and an action potential occurs if the threshold is reached ix Signal fades quickly 1 Adaptation decreased response despite the continuing presence of a stimulus 39l39 i ll l1 nll39IEIJEil bun l 139 f l i r EIEEFIEET nail l i ll a 39 H i39 l i lquoti Mums i 2h A i Fai tgn i39 j Er Pi Membrane iem39 nmtinm emnte n lming 53E l tteriyl 915 L L f lr 39l39 Fill aw f receptor 1 r 1 WEIElli Esuur itili Ecru 539 olfactory can 1a d il l minimums e Olfactory receptor genes i There are many different types of odorant receptor proteins each cell expresses only one type The different receptor proteins are organized into zones Vomeronasal organ expresses its own receptors few functional proteins 1 VI vii Olfactory receptor proteins are Gprotein coupled Have 7 transmembrane proteins All receptors are linked to Golf cAMP is the second messenger Population coding combination of responses from many cells in uences transduction 1 2 Receptors are broadly tuned The amount of odorant in uences response f Central olfactory pathways i Olfactory receptor neurons synapse on the glomeruli in the olfactory bulbs 1 2 Each glomerulus receives input from a broad area of olfactory epithelium Each glomerulus receives input only from receptor cells expressing the same gene The array of glomeruli is a map of genes map of odor information 4 Temporal patterns of neuron ring may represent odor qualities Glomeruli and bulbs communicate to modify the input to brain Higher brain areas also connect to bulbs Olfactory tracts connect to cortex before the thalamus 1 Different than for other systems Olfactory connections to forebrain areas are involved in memory motivation and emotion