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
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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 Mg2+, quinine, and caffeine
– Advantage – Survival
• Poisonous substances - often bitter
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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
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Taste
• The Organs of Taste
– Tongue, mouth, palate, pharynx, and epiglottis
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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
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Taste
• The Organs of Tastes
– Papillae
• Foliate papillae
• Vallate papillae
• Fungiform papillae
– Threshold concentration
• Just enough exposure of single papilla to detect
taste
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Taste
• Tastes Receptor Cells
– Apical ends Microvilli Taste pore
– Receptor potential: Voltage shift
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Taste
• Taste Receptor Cells
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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
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Taste
• Mechanisms of Taste
Transduction
– Saltiness
• Salt-sensitive taste cells
• Special Na+ selective
channel
• Blocked by the drug
amiloride
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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
+
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Taste
• Mechanisms of Taste
Transduction
– Bitterness
• Families of taste receptor
genes - TIR and T2R
• Block K + channels (e.g.,
quinine), Bind to Gprotein coupled
receptors.
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Taste
• Mechanisms of Taste Transduction
– Sweetness
• Sweet tastants natural and artificial
• Sweet receptors
• T1R2+T1R3
• Expressed in different taste cells
• G protein-coupled receptors, 2nd messengermediated transduction (cAMP, IP3)
cAMP phosphorylates (& closes) K channel
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Taste
• Mechanisms of Taste
Transduction
– Umami
• Umami receptors:
• Detect amino acids
• T1R1+T1R3
• Ionotropic &
metabotropic
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Taste
• Central Taste Pathways
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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
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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
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Smell
• Pheromones
– Smell— a mode of communication
– Important signals
• Reproductive behavior
• Territorial boundaries
• Identification
• Aggression
– Role of human pheromones
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Smell
• The Organs of Smell
– Olfactory epithelium
• Olfactory receptor cells, supporting cells, and
basal cells
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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
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Smell
• Olfactory Receptor Neurons
– Olfactory Transduction
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Smell
• Olfactory Receptor Neurons (Cont’d)
– Olfactory Transduction
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Smell
• Olfactory Transduction
– Adaptation: Decreased
response despite
continuous stimulus
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Smell
• Central Olfactory Pathways (Cont’d)
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Smell
• Central Olfactory Pathways (Cont’d)
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Smell
• 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
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Smell
• Spatial and Temporal Representations of Olfactory
Information
– Olfactory Population Coding
– Olfactory Maps (sensory maps)
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Smell
• Spatial and Temporal
Representations of
Olfactory Information
(cont’d)
– Temporal Coding in
the Olfactory System
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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
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End of Presentation
Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins