MIT OpenCourseWare http://ocw.mit.edu 9.01 Introduction to Neuroscience Fall 2007 For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms.Taste and smell Sebastian SeungSensory transduction • How is the receptor potential generated? – ion channel – GPCRPsychology of taste • What is taste for? – Distinguish between food and poison – Distinguish between types of food • How many basic tastes are there? – salt, sour, sweet, bitter – umamiCentral taste pathways • Three cranial nerves from tongue • Medulla: gustatory nucleus – common pathway • Thalamocortical pathway – VPM – Gustatory cortex – Thought to be responsible for conscious perceptionMost gustatory axons respond to more than one basic taste • A distributed neural code 100500Sucrose NaCl HCl QuinineAction potentials / 5 secFigure by MIT OpenCourseWare. After Figure 8.4 in Bear, Mark F., Barry W.Connors, and Michael A. Paradiso. Neuroscience: Exploring the Brain. 3rd ed.Baltimore, MD: Lippincott Williams & Wilkins, 2007.Taste receptor cells • 50-150 in a taste bud • Synapses onto gustatory afferents Lingual epitheliumTaste receptor cellTaste poreGustatory afferent axonsBasal cell Connective tissueMicrovilliSynapseFigure by MIT OpenCourseWare.Taste receptors sweet T1R2+T1R3 GPCR umami T1R1+T1R3 bitter T2R (~30 types) sour PKD2L1 ion channel salt ?Genetic manipulations • Knockout – heterozygous – homozygous • TransgenicAn alternate reality: labeled line encoding • Different tastes are represented by the activation of nonoverlapping sets of neurons. • A single neuron can unambiguously signal the presence of a taste. cell type salt sour sweet bitter salt sour sweet bitter stimulusMost receptor cells respond to more than one basic taste. Figure by MIT OpenCourseWare. After Figure 8.3 in Bear, Mark F., Barry W. Connors,and Michael A. Paradiso. Neuroscience: Exploring the Brain. 3rd ed. Baltimore, MD:Lippincott Williams & Wilkins, 2007. NaCl Quinine HCl SucroseNaCl Quinine HCl SucroseAxon 1Cell 1Cell 2Cell 3Axon 2Axon 3VmVmVmTaste budGustatory afferent axonsCell 1Cell 2Cell3Genetic variation in taste • Phenylthiocarbamide (PTC) – supertasters: extremely bitter – medium tasters: bitter – nontasters: no taste • TAS2R38 – nontasters and tasters differ in three amino acidsPerception of flavor is complex • combination of basic • other sensory tastes modalities • smell – texture – temperature – pain – visionWhat is smell for? Identify foods CommunicateHow many smells are there? • Professional “noses” can distinguish between thousands of scents. • Are there basic smells?Olfactory epithelium Image removed due to copyright restrictions.See Figure 8.9 in Bear, Mark F., Barry W. Connors,and Michael A. Paradiso. Neuroscience: Exploring the Brain.3rd ed. Baltimore, MD: Lippincott Williams & Wilkins, 2007.Olfactory receptor neuron • the axon projects to the olfactory bulb via the cribriform plate • the dendrite sends cilia into the epithelium • odorants bind to the cilia Figure by MIT OpenCourseWare. After Figure 8.10 in Bear, Mark F., Barry W.Connors, and Michael A. Paradiso. Neuroscience: Exploring the Brain. 3rd ed.Baltimore, MD: Lippincott Williams & Wilkins, 2007.Olfactory transduction • odorant binds to receptor • G-protein is activated • adenylyl cyclase is activated • cAMP binds to cation channel • influx of Na and Ca causes depolarization • amplified by Ca-activated Cl channels Image removed due to copyright restrictions.See Figure 8.10 in Bear, Mark F., Barry W. Connors,and Michael A. Paradiso. Neuroscience: Exploring the Brain.3rd ed. Baltimore, MD: Lippincott Williams & Wilkins, 2007.Odorant receptor genes • roughly 1000 genes in rodents • each receptor cell expresses only one gene • 2004 Nobel prizeOlfactory glomeruli • 2000 glomeruli in the bulb • axons of receptor neurons meet dendrites of second-order neurons Image removed due to copyright restrictions.See Figure 8.14 in Bear, Mark F., Barry W. Connors,and Michael A. Paradiso. Neuroscience: Exploring the Brain.3rd ed. Baltimore, MD: Lippincott Williams & Wilkins, 2007.Each glomerulus receives input from one type of ORN Image removed due to copyright restrictions.See Figure 8.16 in Bear, Mark F., Barry W. Connors,and Michael A. Paradiso. Neuroscience: Exploring the Brain.3rd ed. Baltimore, MD: Lippincott Williams & Wilkins, 2007.Central olfactory pathways • direct pathway to olfactory cortex • thalamocortical pathway to neocortex Image removed due to copyright restrictions.See Figure 8.17 in Bear, Mark F., Barry W. Connors,and Michael A. Paradiso. Neuroscience: Exploring the Brain.3rd ed. Baltimore, MD: Lippincott Williams & Wilkins, 2007.Broad tuning of ORNs Image removed due to copyright restrictions.See Figure 8.13 in Bear, Mark F., Barry W. Connors,and Michael A. Paradiso. Neuroscience: Exploring the Brain.3rd ed. Baltimore, MD: Lippincott Williams & Wilkins, 2007.Calcium imaging Friedrich & Korsching (1997) Courtesy Elsevier, Inc., http://www.sciencedirect.com. Used with permission.Pheromones • Secreted chemicals for communication • Reproductive behaviors • Territorial markings • Identification of individuals • Social hierarchyAccessory olfactory system • vomeronasal organ (VNO) • to accessory olfactory bulb • to hypothalamus Image removed due to copyright