Neuroscience Exploring the Brain 3 e Chapter 8

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

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:

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

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 • Serotonin released (new) Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins

Taste • Mechanisms of Taste Transduction – Sourness • Sourness- acidity – low p. H • 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 – T 1 R and T 2 R + • Block K channels (e. g. , quinine), Bind to G-protein coupled receptors (T 2 R class). • ATP = Transmitter (new) Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins

Taste • Mechanisms of Taste Transduction – Sweetness • Sweet tastants natural and artificial • Sweet receptors • T 1 R 2+T 1 R 3 • Expressed in different taste cells • G protein-coupled receptors, 2 nd messengermediated transduction (c. AMP, IP 3) c. AMP phosphorylates (& closes) K channel Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins

Taste • Mechanisms of Taste Transduction – Umami • Umami receptors: • Detect amino acids • T 1 R 1+T 1 R 3 • Ionotropic & metabotropic Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins

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

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

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

Smell • Olfactory Receptor Neurons (Cont’d) – Olfactory Transduction Copyright © 2007 Wolters Kluwer

Smell • Olfactory Transduction – Adaptation: Decreased response despite continuous stimulus Copyright © 2007

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

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

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

Smell • Spatial and Temporal Representations of Olfactory Information (cont’d) – Temporal Coding in

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