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Chapter 9
Majority of illustra3ons in this presenta3on are from Biological Psychology 4th edi3on (© Sinuer Publica3ons)
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Taste and Smell
Differences
Taste Smell
Taste is a proximal sense Smell is a distal sense
Number of chemicals producing the sensa3on of
taste are few.
Number of chemicals producing the sensa3on of
smell are large.
Parietal cortex (S2)
Temporal cortex (Entorhinal cortex)
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Taste and Smell
Similari3es
Taste Smell
1. Both are chemical senses, i.e., both senses are sensi3ve to chemicals that are delivered through fluids or air.
2. In marine animals the taste and smell sense are the same. Some rep3les (snakes) use tongues (flicking them) as an accessory smelling organ.
3. Both senses are involved in making us aware of flavor. More on flavor later in the lecture.
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Func3ons of Taste
Taste provides informa3on about foods and toxins
1. Sweet tastants are usually high calorie foods, and the individual enjoys ea3ng them.
2. Salty tastants are food elements for regula3ng internal milieu or homeostasis.
3. BiRer tastants mostly toxic (poisonous) need to be recognized early so that individuals can discharge them out. High threshold (mM) for salt, sweet, and sour tastants. Low threshold (μM) for biRer tastants.
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Ontology of Taste
Ekman and colleagues carried out experiments to observe expressions of newborn babies to different
tastes and found:
Taste Expression Water No expression Sour Pucker Sweet Smile Bitter Disgust Salt No expression
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History
“[Sensa3on of taste] lies between sweet and the biRer… on the side of the sweet, the succulent… on the side of the biRer, the saline… [and] between these
come the pungent, the harsh, the astringent, and the acid[ic]”.
Sweet Succulent BiRer Sour Salty Pungent Harsh Astringent
Aristotle outlined a one-‐dimensional arrangement of tastants.
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Henning’s Tetrahedron
Henning (1916) proposed four primary tastes on a hollow tetrahedron, to propose that any par3cular taste could lie between two primaries on an edge, or three primaries on the surface of the solid. No taste could be a combina3on of
all four primaries, thus the hollow tetrahedron.
Salty
BiRer
Sweet
Sour
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Henning’s Tetrahedron
Henning’s ideas proved not very useful either in theore3cal or prac3cal domains. Addi3on of a new primary taste like
Umami changed our basic understanding of taste.
Salty BiRer
Sweet
Sour Umami
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Supertasters & Taste Blindness
1. About 25% of people (because of two recessive alleles) have no taste for a compound propylthiouracil, (PROP) thus taste blind. Figy percent taste it as mildly biRer, and 25% as extremely biRer, having two dominant alleles.
2. Women more than men are supertasters. And supertasters tend to have more taste buds than regular tasters.
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Taste Regions: Oral Cavity
Taste regions in oral cavity comprise of tongue, palate, pharynx, epiglois, larynx and esophagus.
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Palate
Pharynx
Epiglottis
Larynx
Esophagus
Tongue
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Taste Map
Sweet
Sour
Salty
BiRer
1. Many people con3nue to think that different tastes are represented on different areas of the tongue, thus there was a dis3nct taste map on the tongue.
2. Now it is believed that different taste sensa3ons are present in all loca3ons of the tongue, however some of them are dominant in one place than the others (Bartoshuk, 1993; Collins, 1974; Yanagisawa, 1994). Sa
lty
Sweet
Sour
Bi.er
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Papillae and Taste Buds
Circumvallate
Fungiform Filiform (Non-‐gustatory)
Taste buds
Foliate
1-‐5 taste buds
1000 taste buds
1000 taste buds
10,000 taste buds total
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Taste Bud
50-‐100 Taste cells in each taste bud. Recycle in 7-‐10 days from basal
cells.
Taste buds
Pour Saliva
Taste cell
Gustatory afferent nerve
Epithelial cells
Basal cell
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Single Taste Cell Hypothesis
Single taste cell hypothesis suggested that there was only one type of cell in the taste bud, thus a bud
responded to sweet or salty or sour. However recent data suggests different taste cells in a single bud.
Cells sensi3ve to single taste
Cells sensi3ve to many tastes
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Taste Cell Physiology
Salty and sour taste use ion channels to to depolarize the taste cell causing release of neurotransmiRer (NT).
Sweet, biRer and umami ac3vate metabotropic
receptors for NT release.
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Gustatory Nerves
Facial Nerve (VII)
Glossophryngeal Nerve (IX)
Vagus Nerve (X)
2/3 of anterior tongue innervated by tympanic
nerve
1/3 of anterior tongue innervated by glossophryngeal nerve
Pharynx, Epiglois, Esophagus
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Gustatory Pathway
Nerves VII, IX and X
Nucleus of the Solitary Tract
Ventral Posterior Medial
Thalamus
Gustatory Cortex (S2)
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Olfac3on and Smell
1. Olfac3on is specific term reserved for the ac3va3on of specific class of chemoreceptors (olfactory receptors) in the nasal cavity.
2. Smell on the other hand refers to a general term describing a general perceptual experience resul3ng from ac3va3on of these chemoreceptors.
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Olfac3on & Smell: Aspects
1. Humans are less sensi3ve to odors than animals. Rats are 8-‐50 3mes and dogs are 300-‐10,000 3mes more sensi3ve than humans (Lang, Doty, & Breihpohl, 1991).
2. Recogni3on to odor declines with age due to the reduc3on of receptors cells or receptors on cilia of the olfactory epithelium.
3. Smell works as gatekeeper sense providing informa3on about irrita3ng and noxious smells, predators, prey and mates.
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Olfac3on & Smell: Aspects
4. Smell is in3mately linked with memory. Mothers can recognize their infant’s clothing by smell. And one can recognize gender by smelling breath.
5. There are 5,000 odors or so. About 20% are pleasant. Since many different smells, model for olfac3on is difficult to form. Though people have tried to develop such a model (see below).
6. Need vola3le chemicals for olfac3on.
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Flavor
1. Flavor (retronasal) is the result of sensory interac3on of smell and taste, along with other sensory cues such as texture, temperature, appearance etc of the s3mulus.
2. Sight and smell have a strong effect on flavor when tas3ng s3muli.
3. To test the significance of flavor eat something (like starburst) with eyes closed and nose plugged, and then eyes open and nose unplugged.
4. Taste is boRom-‐up while flavor is top-‐down processing.
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History
Like taste Aristotle came up with the idea of separa3ng smells. He divided all odors into four basic kinds. Essen3ally the way he described taste on a single
dimension.
Pungent Succulent Acidic Astringent
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Henning’s Pyramid
Just like his tetrahedron, Henning (1916) proposed six basic odors, where a single odor could be a combina3on of one,
two, three or four primary smells.
Putrid (Hydrogen Sulfide)
Fruity (Lemon)
Flowery (Violet)
Burnt
Spicy (Nutmeg)
Resinous (Balsam)
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Amoore’s Model
Amoore (1970) proposed seven odor receptors (later 30) based on his study of anosmia (smell blindness). He proposed seven
different kinds of molecules bound to seven receptors. However, Schiffman (1974) found no correla3on between molecular shape
and similarity of odor.
Smell Example Molecule
Camphoraceous Mothballs Football shaped
Musky Cinnamon Necklace shaped
Mint Peppermint Wedged shaped
Flowery Violets Tadpole shaped
Ethereal Ether Long-‐chain Alkanes
Putrid Peanut Large Nega3ve Charge
Pungent Coffee Large Posi3ve Charge
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Nasal Cavity & Olfactory Epithelium
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Olfactory Receptors Basal Cell
Sustentacular Cell
Olfactory Receptor
Cell
Cilia
Axon Olfactory epithelium consist of:
1. Olfactory receptor cells (Cilia, dendrites, and axons).
2. Sustentacular cells. 3. Basal cell.
Dendrite
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Olfactory Receptor Facts
1. Finite life span of receptor cells, thus con3nuously replaced.
2. Develop from basal cells.
3. The number of olfactory receptors in humans equal 25 million, with about 350 different types of receptors. To smell 5000 odors, receptors must be working in combina3on.
4. Unmyelinated axons form the olfactory nerve (cranial nerve I).
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Molecular Mechanisms
1. Odorant must be vola3le and soluble in mucus (filter and capture site).
2. Odors bind to metabotropic receptors to transduce signals and communicate them to the brain.
Cilia
Odorant
Ca3on (+)
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Olfactory Pathways
1. Mitral cells from the olfactory bulb project to Anterior Olfactory Nucleus (AON) and olfactory tubercle.
2. From here projec3ons go separately to amygdaloid complex, piriform and entorhinal cortex, and to orbitofrontal cortex.
3. From olfactory tubercle to thalamus (odor discrimina3on) and from amygdala to hypothalamus (odor emo3on).
Olfactory Receptor Neurons
Olfactory Bulb Frontal Cortex
Thalamus (Odor Discrimination)
Hypothalamus (Odor Emotions)
Hippocampus
Piriform Cortex
Olfactory Tubercle
Amygdala
Entorhinal Cortex
Olfactory Nerve
Olfactory Tract