Psychology 355

04 The Chemical Senses

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Introduction

I. Animals depend on the chemical senses to identify nourishment

II. Chemical sensationA. Oldest and most common sensory

systemIII. Chemical senses

A. GustationB. OlfactionC. Chemoreceptors

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TasteThe Basics Tastes

1. Saltiness2. sourness,3. sweetness,4. bitterness, and5. Umami

Examples of correspondence between chemistry1. Sweet—sugars like fructose, sucrose, artificial

sweeteners (saccharin and aspartame)2. Bitter—ions like K+ and Mg2+, quinine, and

caffeine3. Sour— Acidity (low Ph), H+4. Salt—Na+

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Taste

Advantage – Survival: Poisonous substances - often bitter

Single-Trial Learning; Self-Balancing Diet

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TasteThe Basics Tastes

1. Saltiness2. sourness,3. sweetness,4. bitterness, and5. Umami

chemistry1. Sweet—sugars like fructose,

sucrose, artificial sweeteners (saccharin and aspartame)

2. Bitter—ions like K+ and Mg2+, quinine, and caffeine

3. Sour— Acidity (low Ph), H+4. Salt—Na+

Advantage – Survival: Poisonous substances - often bitter

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TasteThe Basics Tastes

A. SaltinessB. sourness,C. sweetness,D. bitterness, andE. Umami

chemistryA. Sweet—sugars like

fructose, sucrose, artificial sweeteners (saccharin and aspartame)

B. Bitter—ions like K+ and Mg2+, quinine, and caffeine

C. Sour— Acidity (low Ph), H+

D. Salt—Na+ Advantage – Survival: Poisonous

substances - often bitter

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Taste

The Basic TastesA. Steps to distinguish the countless

unique flavors of a food1. Each food activates a different

combination of taste receptors2. Distinctive smell3. Other sensory modalities

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Taste

The Organs of TasteTongue, mouth, palate, pharynx, and

epiglottis

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Taste

Areas of sensitivity on the tongue1. Tip of the tongue

Sweetness2. Back of the tongue

Bitterness3. Sides of tongues

Saltiness and sourness

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Taste

The Organs of TastesA. Papillae

1. Foliate papillae2. Vallate papillae3. Fungiform papillae

Threshold concentrationJust enough exposure of single

papilla to detect taste

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TasteI. Tastes

Receptor CellsA. Apical

ends Microvilli Taste pore

B. Receptor potential: Voltage shift

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Taste

Mechanisms of Taste TransductionA. Transduction process

1. Taste stimuli (tastants)i. Pass directly through ion

channels (Na+)ii. Bind to and block ion channels

(sour-H+)iii. Bind to G-protein-coupled

receptors (bitter, sweet, umami)

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Taste

Mechanisms of Taste Transduction

Saltiness1. Salt-sensitive taste

cellsi. Special Na+

selective channel

2. Blocked by the drug amiloride

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Taste

Mechanisms of Taste Transduction

Sourness1. Sourness-

acidity – low pH2. Protons

causative agents of acidity and sourness

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TasteMechanisms of Taste

TransductionBitter, Sweet, UmamiA. G-protein coupled

receptorB. Activates

Phospholipase CC. Increases messenger

inositol triphosphate (IP3)

D. CA2+

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Taste

Mechanisms of Taste Transduction

Bitterness1. Families of

taste receptor genes - TIR and T2R

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Taste

Mechanisms of Taste TransductionSweetness

1. Sweet tastants natural and artificial

2. Sweet receptorsi. T1R2+T1R3ii. Expressed in different taste

cells

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Taste

Mechanisms of Taste Transduction

A. Umami1. Umami

receptors:i. Detect

amino acidsii. T1R1+T1R3

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Taste

Central Taste PathwaysA. Gustatory nucleus

Point where taste axons bundle and synapse

B. Ventral posterior medial nucleus (VPM) of the thalamus

C. Primary gustatory cortexReceives axons from VPM taste

neurons

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Taste

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Taste

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Taste

Central Taste Pathways (Cont’d)A. Localized lesions

1. Ageusia- the loss of taste perceptionB. Gustation

1. Important to the control of feeding and digestion

i. Hypothalamus ii. Basal telencephalon

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Taste

The Neural Coding of TasteA. Labeled line hypothesis

1. Individual taste receptor cells for each stimuli

2. In reality, neurons broadly tuned3. Population coding

i. Roughly labeled linesii. Temperatureiii. Textural features of food

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Smell

PheromonesA. Smell— a mode of communicationB. Important signals

1. Reproductive behavior2. Territorial boundaries3. Identification4. Aggression

C. Role of human pheromones

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Smell

The Organs of SmellOlfactory epithelium

Olfactory receptor cells, supporting cells, and basal cells

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Smell

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SmellThe Organs of Smell

A. Odorants: Activate transduction processes in neurons

B. Olfactory axons constitute olfactory nerveC. Cribriform plate: A thin sheet of bone through

which small clusters of axons penetrate, coursing to the olfactory bulb

D. Anosmia: Inability to smellE. Humans: Weak smellers

Due to small surface area of olfactory epithelium: Dogs have about 170 cm2 compared to 10 cm2 in humans, and about 100 times more receptors per unit area

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Perception of Smell

The dimensions of smell

Flowery

Foul

Fruity

Spicy

Burnt

Resinous

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Perception of Smell (cont.)

I. We can distinguish between about 10,000 different smellsDifferent threshold levels for different smells

II. Two thresholds for each smell -low threshold for the existence of a chemical, somewhat higher threshold to discriminate one smell from another

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Perception of Smell

I. Adaptation - Olfactory fatigue - cross-adaptation

II. Smell Constancy - receptors are more stimulated during a deep sniff than a shallow one - the judgment of odor intensity does not change -

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Smell

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Pheromones: Mammals

Powerful effects on behavior, specifically sexual behavior, territorial behavior and identification of kin

MammalsI. Most mammals only become sexually aroused in

the presence of pheromones

II. Increased likelihood of pregnancy

III. Synchronization of estrus cycles

IV. Mutual recognition of mother and offspring

V. Territory marking (e.g. dogs)

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Pheromones: MammalsI. Releasers - trigger a specific behavioral response

II. Primers - trigger a hormone response which increases the likelihood of certain types of behaviors

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Pheromones: HumansHumans I. infants can correctly identify their own mother's

milk and are much more likely to nurse when its their own mother

II. female menstrual cycles can be altered by pheromones - the sorority effect

III. male and female behavior is highly influenced by pheromonest-shirt experiment - musky versus sweet -

IV. the musky odor is rated by males and females as unpleasant and is thought to serve as a territorial marker among males

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Pheromones: HumansIncrease sexual arousal in males

I. Increases male perception of female attractiveness

• women in photographs were rated as significantly more sexually

attractive when judges were first exposed to alpha androstenol

II. Increases willingness of females to initiate social contact with males

• females exposed to alpha androstenol were much more

receptive to male-initiated contact

• more likely to seek out male company

• less likely to seek female company

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Smell

Olfactory Receptor NeuronsA. Olfactory Transduction

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Smell

Olfactory Receptor NeuronsOlfactory Transduction

Oderant to receptor proteinStimulates G(olf)-protein

Activates adenylyl cyclasecAMPOpens Na+ Ca2+ channelsOpens Cl- channels (out)Depolarization

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Smell

Olfactory Receptor Neurons

Olfactory Transduction

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Olfactory TransductionAdaptation:

Decreased response despite continuous stimulus

Smell

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Central Olfactory Pathways

Smell

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Olfactory Bulb

Olfactory bulb - organ which houses all the nerves which receive inputs from the olfactory receptors

Limbic and Thalamic connections

Olfactory cortex (frontal lobe)

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Central Olfactory Pathways

Smell

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Central Olfactory PathwaysA. Axons of the olfactory tract: Branch and

enter the forebrainB. Neocortex: Reached by a pathway that

synapses in the medial dorsal nucleus

Smell

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Central Olfactory PathwaysA. Axons of the olfactory tract: Branch and enter the

forebrainB. Neocortex: Reached by a pathway that synapses in the

medial dorsal nucleus

Smell

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I. Spatial and Temporal Representations of Olfactory Information

A. Olfactory Population CodingB. Olfactory Maps (sensory

maps)C. Temporal Coding in the

Olfactory System

Smell

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Concluding Remarks

I. Transduction mechanismsA. Gustation and olfaction

II. Similar to the signaling systems used in every cell of the body

III. Common sensory principles - broadly tuned cells

A. Population coding B. Sensory maps in brain

IV. Timing of action potentialsA. May represent sensory information

in ways not yet understood

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End of Presentation