بسم الله الرحمن الرحيم. chemical senses taste learning objectives -describe the...
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بسم الله الرحمن الرحيم
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Chemical senses
TASTE
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LEARNING OBJECTIVES
•-Describe the location of taste buds •-Name the five major taste sensations
•-How the basic taste sensations are• appreciated
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Chemical Senses
- Taste & smell:Both determine the flavour of food
Taste and smell are closely linked even though they involve different receptors
and receptive processes.
This suggests an overlap in central processing.
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Chemical Senses
Taste and smell:• Receptors are chemoreceptors
• In association with food intake, influence flow of digestive juices and affect appetite
• Stimulation of receptors induces pleasurable or objectionable sensations and signals presence of something to seek or to avoid
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Taste (Gustation)
• Chemoreceptors housed in taste buds• Present in oral cavity and throat• Taste receptors have life span of about 10 days• Taste bud consists of
– Taste pore• Opening through which fluids in mouth come into
contact with surface of receptor cells
– Taste receptor cells• Modified epithelial cells with surface folds called
microvilli• Plasma membrane of microvilli contain receptor sites
that bind selectively with chemical molecules
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Location and Structure of Taste Buds
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Taste Buds
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• Taste buds are located in:–Tongue–Epiglottis–Soft Palate–Pharynx
Sensation of Taste –
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Anatomy of Taste Buds.
• 10,000 taste buds found on tongue, soft palate & pharynx
• Taste buds consist of:– ~50 receptor cells surrounded
by supporting cells
– Basal cells develop into supporting cells then receptor cells
• Gustatory hairs project through the taste pore
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Fibres from:• Epiglottis
• Palate• Pharynx
VAGUS(X)
Nerve supply of tongue
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Sensitivity differs in different areas, but all tastes can be perceived at
most areas of the tongue
Physiology of Taste :Primary modalities of taste:
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Taste modalities over tongue
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Taste
• Tastant (taste-provoking chemical) • Binding of tastant with receptor cell alters cell’s ionic
channels to produce depolarizing receptor potential• Receptor potential initiates action potentials within
terminal endings of afferent nerve fibers with which receptor cell synapses
• Terminal afferent endings of several cranial nerves synapse with taste buds in various regions of mouth
• Signals conveyed via synaptic stops in brain stem and thalamus to cortical gustatory area
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Taste information is send to the CNS by the cranial nerves # 7, 9 and 10 the taste nucleus (n. tractus
solitarius) thalamus primary gustatory cortex insular cortex
Taste Pathway
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Dissolution in Saliva
Attachment to Receptors
Generator Potential
Action Potential
Physiology of Taste -
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Taste Perception
• Influenced by information derived from other receptors, especially odor
• Temperature and texture of food influence taste• Psychological experiences associated with past
experiences with food influence taste• How cortex accomplishes perceptual processing of
taste sensation is currently unknown
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Responses of Taste buds:• Each taste bud responds strongly to one
type of taste• But they also respond to other tastes as
well
Primary modalities of taste:
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Taste • 5 primary tastes
– Salty• Stimulated by chemical salts, especially NaCl
– Sour• Caused by acids which contain a free hydrogen ion, H+
– Sweet• Evoked by configuration of glucose
– Bitter• Brought about by more chemically diverse group of taste substances• Examples – alkaloids, toxic plant derivatives, poisonous substances
– Umami • Meaty or savory taste/ pleasant taste
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– Sour ... Acidity by {H+} – HCL
– Salt … Sodium chloride
– Sweet ..Sucrose glucose
Saccharin
– Bitter ..Strychnine hydrochloride Quinine sulphate, alkaloids
Artificial sweeteners:Aspartame, Cyclamate
Physiology of taste
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Mechanism of stimulation of taste sensations: -
Saltiness and sourness are transduced directly By sodium and hydrogen ions respectively.
The transduction process for sweetness and bitterness involve second messengers.
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Mechanism of stimulation of taste sensation:–Sour:
-Acids (H+)
-Blocks K+ channels
–Salt taste-Na+
-Depolarization
Physiology of Taste –
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Mechanism of stimulation of taste sensation:
–SweetG protein activation of
adenyl cyclase c-AMP K conductance
–Bitter (different substances have
different mechanisms) e.g.G protein Activatn. of
Phospholipase C IC-inositol(PO4)3 Ca2 release
Physiology of Taste – cont:
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Saltiness or sodium receptors allow sodium ions to cross the membrane,thereby causing depolarization.
Sourness receptors operate by closing potassium channels, which allows a positive charge to build up, thereby causing depolarization of the cell.
Ion channel
Ion channels
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A bucket brigade transmits the signal through the cell
Sour is similar
Electrical signal
Deep inside a ‘salty taste’
taste bud
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Sweet taste
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THE 4 “BASIC” TASTES ARE SALTY, SOUR, SWEET AND BITTER,Also: UMAMI (MSG)?, METALLIC? FAT? AMINO ACID?
Different ions, different receptors for different tastes
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Discrimination of intensity of taste:
Discrimination in intensity of taste:– Poor (like smell)– Requires 30% change to allow
discrimination of intensity
Sensation of Taste – cont.
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Adaptation to taste:Decreased sensation from repeated stimulus
Entirely peripheral at the receptors
Sensation of Taste – cont.
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• -Taste modifier Miraculin (a glycoprotein extracxted from miracle fruit):
– When applied to tongue makes acids taste sweet
effects in taste sensation:(Taste tricks):
Sensation of Taste .
The Miracle fruit-origin of miraculin
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Clinical considerations
• Ageusia: Absence of sense of taste• Hypogeusia: Diminshed sense of taste • Dysgeusia: Unpleasant sense of taste (metallic, salty, foul or rancid taste)
• Hypergeusia: increased sense of taste
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References
• Human physiology by Lauralee Sherwood, seventh/eighth edition
• Text book physiology by Guyton &Hall,
12th edition• Text book of physiology by Linda .s
contanzo, third edition