Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins Neuroscience: Exploring the Brain, 3e Chapter 8: The Chemical Senses

Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins Introduction Animals depend on the chemical senses to identify nourishment, poison, potential mate Chemical sensation –Oldest and most common sensory system Chemical senses –Gustation –Olfaction –Chemoreceptors

Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins Taste The Basics Tastes –Saltiness, sourness, sweetness, bitterness, and umami –Examples of correspondence between chemistry Sweet—sugars like fructose, sucrose, artificial sweeteners (saccharin and aspartame) Bitter—ions like K + and Mg 2+, quinine, and caffeine –Advantage – Survival Poisonous substances - often bitter

Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins Taste The Basic Tastes –Steps to distinguish the countless unique flavors of a food Each food activates a different combination of taste receptors Distinctive smell Other sensory modalities

Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins Taste The Organs of Taste –Tongue, mouth, palate, pharynx, and epiglottis

Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins Taste The Organs of Taste –Areas of sensitivity on the tongue Tip of the tongue Sweetness Back of the tongue Bitterness Sides of tongues Saltiness and sourness

Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins Taste The Organs of Tastes –Papillae Foliate papillae Vallate papillae Fungiform papillae –Threshold concentration Just enough exposure of single papilla to detect taste

Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins Taste Tastes Receptor Cells –Apical ends  Microvilli  Taste pore –Receptor potential: Voltage shift

Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins Taste Taste Receptor Cells

Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins Taste Mechanisms of Taste Transduction –Transduction process Taste stimuli (tastants) Pass directly through ion channels Bind to and block ion channels Bind to G-protein-coupled receptors

Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins Taste Mechanisms of Taste Transduction –Saltiness Salt-sensitive taste cells Special Na + selective channel Blocked by the drug amiloride

Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins Taste Mechanisms of Taste Transduction –Sourness Sourness- acidity – low pH Protons causative agents of acidity and sourness Depolarize by Blocking K channels, pass through amiloride-sens Na channels + +

Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins Taste Mechanisms of Taste Transduction –Bitterness Families of taste receptor genes - TIR and T2R Block K channels (e.g., quinine), Bind to G- protein coupled receptors. +

Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins Taste Mechanisms of Taste Transduction –Sweetness Sweet tastants natural and artificial Sweet receptors T1R2+T1R3 Expressed in different taste cells G protein-coupled receptors, 2 nd messenger- mediated transduction (cAMP, IP3) cAMP  phosphorylates (& closes) K channel

Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins Taste Mechanisms of Taste Transduction –Umami Umami receptors: Detect amino acids T1R1+T1R3 Ionotropic & metabotropic

Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins Taste Central Taste Pathways

Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins Taste Central Taste Pathways (Cont’d) –Localized lesions Ageusia- the loss of taste perception –Gustation Important to the control of feeding and digestion Hypothalamus Basal telencephalon

Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins Taste The Neural Coding of Taste –Labeled line hypothesis Individual taste receptor cells for each stimuli In reality, neurons broadly tuned Population coding Roughly labeled lines Temperature Textural features of food

Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins Smell Pheromones –Smell— a mode of communication –Important signals Reproductive behavior Territorial boundaries Identification Aggression –Role of human pheromones

Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins Smell The Organs of Smell –Olfactory epithelium Olfactory receptor cells, supporting cells, and basal cells

Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins Smell The Organs of Smell –Odorants: Activate transduction processes in neurons –Olfactory axons constitute olfactory nerve –Cribriform plate: A thin sheet of bone through which small clusters of axons penetrate, coursing to the olfactory bulb –Anosmia: Inability to smell –Humans: Weak smellers Due to small surface area of olfactory epithelium

Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins Smell Olfactory Receptor Neurons –Olfactory Transduction

Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins Smell Olfactory Receptor Neurons (Cont’d) –Olfactory Transduction

Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins Olfactory Transduction –Adaptation: Decreased response despite continuous stimulus Smell

Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins Central Olfactory Pathways (Cont’d) Smell

Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins Central Olfactory Pathways (Cont’d) Smell

Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins Central Olfactory Pathways (Cont’d) –Axons of the olfactory tract: Branch and enter the forebrain –Neocortex: Reached by a pathway that synapses in the medial dorsal nucleus Smell

Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins Spatial and Temporal Representations of Olfactory Information –Olfactory Population Coding –Olfactory Maps (sensory maps) Smell

Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins Spatial and Temporal Representations of Olfactory Information (cont’d) –Temporal Coding in the Olfactory System Smell

Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins Concluding Remarks Transduction mechanisms – Gustation and olfaction Similar to the signaling systems used in every cell of the body Common sensory principles - broadly tuned cells –Population coding –Sensory maps in brain Timing of action potentials –May represent sensory information in ways not yet understood

Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins End of Presentation