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PSYCHOLOGY 135

CFTIANGCO

UPDEPP

Chemical Senses II:GUSTATION

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Whats your flavor?

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Purposes of Taste

Evaluate nutritious content of food

Prevent the ingestion of toxic substances

Sweet: Identify energy-rich nutrientsUmami: Recognize amino acidsSalt: Ensures proper dietary electrolyte balanceSour and bitter: Warn against the intake of

potentiallynoxious or poisonous chemicals

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Taste takes time

Taste experience changes from moment to moment

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The Stimulus for TASTE

To be tasted, a substance must besolublesome of its molecules mustdissolve in saliva

Saliva transports taste solutions totaste receptors within the tongue andthe mouth

Besides aiding in digestion, saliva

contains ingredients that preventerosion of teeth enamel and eliminate

bacteria in the oral cavity

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The Classification of Tastes

Aristotle : sweet, sour, salty, bitter, pungent, harsh,astringent

19th century: sweet, sour, salty, bitter (Henning)

1909 Ikeda of the Imperial University - umami

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Anatomy and Physiology of Taste

The Taste Receptors- The tongue consists mainly of

muscle covered with mucousmembrane

- Covered with little bumps(papillae), which walls are linedwith tiny structures (taste buds)

- Taste buds house the receptorcells responsible for registeringthe presence of chemicalsubstances

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Taste buds degenerate over time andare replaced by new ones (10 day lifeexpectancy)

Taste buds do not have axons that

project to the brainEach tiny bud contains an average of

50 individual taste receptor cells

Microvillas upon contact with

saliva-borne chemical stimuli,change in taste buds electrical stateresults

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Sensory transduction involves mechanisms that varywith stimulus type:

- Some ions enter the receptor membrane directly and

alter the cells electrical potential- When an amino acid or a complex molecule is the

chemical stimulus, the movement of that chemicalstimulus to a buds microvilli triggers a cascade of

events inside the cell

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Taste qualities and receptortypes mediating thosequalities are fairly wellintermingled over the tongue

However, different areas ofthe tongue tend to bedifferentially sensitive to very

dilute concentrations of giventaste substances

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There is a high degree of specificity betweendifferent receptors and different taste qualitiesrecorded neural activity from afferent nervesinnervating taste receptors and chemical reactions

within receptors while exposed to different tastecomponents

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Some of the tongues receptorssignal pain, touch, ortemperature, rather than taste

Signals generated by thisreceptor are carried to thebrain by fibers in a branch ofthe trigeminal nerve

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Series of events for a chemical to be tasted:

1) the chemical must dissolve in saliva

2) the chemical must diffuse into the taste pore

3) the chemical must bind to and stimulate thegustatory cells

4) generation of APs in the gustatory cells

5) impulse transfer to the sensory neuron6) impulse transmission of the taste sensation to

cranial nerves in the brain

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The Taste Pathways

Taste is unique in that it sendsits neural messages over 2cranial nerves : CN 7 and CN 9

Fibers in one branch of thefacial nerve (CN 7) innervatethe front 2/3 of the tongue

Rear 1/3 innervated by fibers

comprising theglossopharyngeal nerve (CN 9)

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Axons of the brainstemneurons innervated bythese cranial nerves funneltaste-related info upward

along 2 different pathways,which mediate differentaspects of taste perception :

(1)Through a nucleus in the

thalamus(2)To amygdala and

hypothalamus

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In the thalamic nuclei that receivetaste info, taste-responsive neuronsare intermixed with other neurons,

which respond to tactile and/orthermal stimulation

All of these thalamic neuronsproject to the primary taste area in

the cerebral cortexGustatory cortex in insula

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The secondary taste area isfound in the frontal cortex,right above the orbits thathouse the eyes

Neurons signal tasteattributes that have a clear

behavioral relevance

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The Gustatory Code:How do you know what youre eating?

Pfaffmans cross-fiber theory oftaste quality : taste quality wasrepresented in the pattern ofactivity across a population ofnerve fibers that carryinformation from the tongue tothe brain

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Katz, Nicolelis, and Simon (2002) showed that thepatterns of activation among neurons in the GCchange over the first few secs following the intake offood. These changes constitute time dependent

variation in the nature of the information carried bythe neurons responses.

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Striking individual differencein taste sensitivity can betraced to the tongue (due tonumber of papillae)

Generally speaking, tongue ofsupertaster has more thantwice as many papillae than

does the tongue of the averagenontaster

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Whether you're a non-taster or a supertaster orsomewhere in-between depends on your sensitivityto a bitter chemical called 6-n-propylthiouracil(PROP). Non-tasters can't taste the bitterness ofPROP at all. Medium tasters sense the bitterness butdont mind it, while supertasters find the taste ofPROP revolting.

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Evidence suggests thatsupertasters are more sensitiveto bitter tastes and fattiness infood, and often show loweracceptance of foods that are highin these taste qualities. Tasterstend to dislike strong, bitter

foods like raw broccoli,grapefruit juice, coffee and darkchocolate.

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Smokers do not significantlydiffer from nonsmokers in tasteidentification

Females are better than males intaste identification

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Taste adaptation and modification

In general, cross-adaptation works only when theadapting substance is similar in quality to the testsubstance

The quality bitter seems to be an exception to therule : sensitivity to bitter substances can be reducedby adaptation to a different taste: sour

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Modification occurs when exposure to one substancesubsequently alter the taste of another substance

For example:

Fresh artichokes sweet taste of other food anddrinks

leaves of Gymnema sylvestre plant abolishessweet taste of sugar

toothpaste reduces sweetness of sugar

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Taste Hedonics

Craving sweet substances; aversion to bittersubstances

Effect of familiarity

Conditioned taste aversionSensory specific satiety

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Interaction between taste and smell

Paradoxical : when odor is addedto a substance that is beingtasted, people do not report thatits smell has increased instrength. They say instead thatits TASTE is enhanced.

Taste and smell blend into a

single experience, and thiscombined experience is typicallyreferred to as taste.

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Taste Disorders

The most common taste disorder is phantom tasteperception; that is, a lingering, often unpleasanttaste even though you have nothing in your mouth.

We also can experience a reduced ability to tastesweet, sour, bitter, salty, and umami, a conditioncalled hypogeusia.

Some people cannot detect any tastes, which is called

ageusia.

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Dysgeusia is a condition in which a foul,salty, rancid, or metallic taste sensationwill persist in the mouth.

Dysgeusia is sometimes accompanied by

burning mouth syndrome, a condition inwhich a person experiences a painfulburning sensation in the mouth.

Although it can affect anyone, burning

mouth syndrome is most common inmiddle-aged and older women.

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Among the causes of taste problems are:

Upper respiratory and middle ear infections

Radiation therapy for cancers of the head and neck

Exposure to certain chemicals, such as insecticides and somemedications, including some common antibiotics and antihistamines

Head injury

Some surgeries to the ear, nose, and throat (e.g., third molarwisdomtoothextraction and middle ear surgery)

Poor oral hygiene and dental problems

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ON TO QUIZ AND HOMEWORK!

End.

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Define the following terms.

Gate control theoryDiffuse fibers

Mechanoreceptors

Meissner corpuscles

Nociceptors

Merkel disks

Pacinian corpuscles

Phantom limbRapidly adapting fibers

Slowly adapting fibers