CHEMICAL SENSES TASTE

Neuroscience: Exploring the Brain, 3rd Ed, Bear, Connors, and Paradiso Copyright © 2007 Lippincott Williams & Wilkins Slide 1

Introduction

Animals depend on the chemical senses to identify i) nourishment, ii) noxious substance, and iii) potential mate Chemical sensation Oldest and most common sensory system Essential to monitor changing environment both internally and externally Chemical senses Gustation Olfaction - Mostly for sensing outside environment, reporting to our basic instincts such as thirst, hunger, emotion, sex Chemoreceptors internal probes (gut, muscle, blood..) Slide 2

Taste

Inborn vs. Learned preference The Basics Tastes Saltiness, sourness, sweetness, bitterness, and umami Simple or complex relationship between chemicals and taste Sour-acids, Salty-salts Sweet sugars like fructose and sucrose Proteins such as monellin artificial sweeteners (saccharin and aspartame) Bitter ions like K + and Mg often bitter) 2+ , quinine, and caffeine Mostly perceived at a very low concentrations : survival advantage (many poisonous substances are Slide 3

Taste The Basic tastes and Unique Tastes

A different combination of basic taste Distinctive flavor (taste + smell) Other sensory modalities (texture, temperature, pain…) Slide 4

Taste The Organs of Taste

Tongue, palate, pharynx, and epiglottis Slide 5

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 Most of the tongue are sensitive to all basic tastes Slide 6

Taste

•One to several hundred taste buds/papilla •50-150 taste receptor cells / taste bud • 2000-5000 taste buds / person • Taste tuning (specificity) at papilla level is present but not absolute exclusiveness (at higher concentrations of taste stimuli) Slide 7

Taste Tastes Receptor Cells

Apical ends’ membrane extensions : Microvilli project to taste pore Not neurons by standard histological criteria but do form synapses to gustatory ganglion neurons (as well as to basal cells) Receptor potential: Voltage shift occur when taste cells are activated by chemicals Slide 8

Taste Taste Receptor Cells

- More than 90% of receptor cells respond to two or more of the basic tastes Slide 9

Taste Mechanisms of Taste Transduction

Transduction Process by which an environmental stimulus causes an electriacl response in a sensory receptor cell Taste stimuli (tastants) Pass directly through ion channels (salt and sour) Bind to and block ion channels (sour) Bind to G-protein-coupled receptors (bitter, sweet, umami) Slide 10

Taste

Mechanisms of Taste Transduction Saltiness Salt-sensitive taste cells have a special Na + selective channel • • • Blocked by the drug amiloride Open all the time Requires the concentration to be quite high (at least 10mM) Anions of salts affect saltiness - anion inhibition (the larger anions inhibit the salt taste of cations) Some anions could produce their own taste Slide 11

Taste

Mechanisms of Taste Transduction Sourness Sourness- acidity – low pH Protons causative agents of acidity and sourness Amiloride-sensitive sodium channels allow the influx of protons - inward current produce membrane depolarization Hydrogen ions bind and block K+ selective channels - depolarization Slide 12

Taste

Mechanisms of Taste Transduction Families of taste receptor genes - TIR and T2R discoveries shone a light into the study of bitterness, sweetness and umami GPCRs- seem to use the same secondary messenger pathway Slide 13

Taste Mechanisms of Taste Transduction

Bitterness ~30 T2R receptors ; vast array of bitterness detection = for safer life Distinguishing among bitterness is hard because many (most) T2Rs are expressed in the same cell Slide 14

Taste Mechanisms of Taste Transduction

Sweetness Sweet tastants natural and artificial Sweet receptors T1R2+T1R3 Expressed in different taste cells from bitter receptors Slide 15

Taste Mechanisms of Taste Transduction

Umami Umami receptors: Detect amino acids (monosodium glutamate; MSG) T1R1+T1R3 Slide 16

Taste Central Taste Pathways

VII; facial nerve IX; glossopharyngeal nerve X; vagus nerve Mostly ipsilateral!

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Taste

Central Taste Pathways Localized lesions to VPM or gustatory cortex Ageusia- the loss of taste perception Conscious experience of taste Gustation and behavior Important to the control of feeding and digestion – involve additional taste pathways Intramedullar projections : swallowing, salivation, gagging, vomiting… Hypothalamus Basal telencephalon Motivation to eat Slide 18

Taste

The Neural Coding of Taste Labeled line hypothesis Individual taste receptor cells for each stimuli In reality, neurons broadly tuned Multiple transduction systems in a receptor cell Convergence of receptor cell inputs onto afferent axons Population coding Roughly labeled lines Activation of different sets of neurons encode different taste Slide 19