Download 10-Sensation of Taste lecture

Sensation of Taste
Professor A.M.A Abdel Gader
MD, PhD, FRCP (Lond., Edin), FRSH (London)
Professor of Physiology, College of Medicine &
King Khalid University Hospital
Riyadh, Saudi Arabia
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.
Anatomy of Taste Sensation
Taste Buds
Sensation of Taste –
Anatomy of Taste - cont.
Receptors:
• Located in taste buds in:
– Tongue
– Epiglottis
– Soft Palate
– Pharynx
Anatomy of Taste Buds
• 10,000 taste buds found on tongue,
soft palate & larynx
• Taste buds consist of:
– ~50 receptor cells
by supporting cells
(type 3)
surrounded
– Basal cells (type 1 &2) develop into
supporting cells then receptor cells
• Gustatory hairs project through the
taste pore
• Life span of 10 days
– cont.
Anatomy of Taste Buds
– cont.
Anatomy of Taste Buds
- cont
Papillae are found on the front, sides and back of the tongue.
(The response is not as specific as indicated on the right).
Anatomy of Taste Buds - cont
Anatomy of Taste Buds - cont
•
There are about 4500 taste
buds per average tongue
•
•
Each taste bud is innervated by 50 nerve fibers
Each nerve fiber receives input from about 5 taste
buds
Nerve supply of tongue
Fibres from:
•Epiglottis
•Palate
•Pharynx
VAGUS
Taste Pathway
Frontal Operculum
Vagus Nerve
Mouth & Larynx
Insular Cortex
Glossopharyngeal Nerve
Tongue
Thalamus
ML
Chorda Tympani Nerve
Nucleus Solitary Tract
(NST)
Taste Pathway
Taste Pathway
Taste information is
send to the CNS by
the crainial nerves #
7, 9 and 10
the
taste nucleus (n. tractus
solitarius)
thalamus
insular cortex
Physiology of Taste
Physiology of Taste - cont
Dissolution in Saliva
Attachment to Receptors
Generator Potential
Action Potential
Physiology of Taste – cont:
Primary modalities of taste:
Its not
this
simple
Sensitivity differs
in different areas,
but all tastes can
be perceived at
most areas of the
tongue
Physiology of Taste – cont:
Primary modalities of taste:
Responses of Taste buds:
• Each taste bud responds strongly to one
type of taste
• But they also respond to other tastes as
well
Physiology of Taste - cont
Primary modalities of taste:
– Sour
– Salt
– Sweet
– Bitter
– umami (deliciousness), a taste associated with glutamate & other
nucleotides has receptors located at the back of the pharynx.
Physiology of Taste – cont:
Primary modalities of taste:
– Sour
– Salt
– Sweet
– Bitter
Evidence for 4 modalities???
Physiology of Taste – cont:
Primary modalities of taste:
Sour, Salt, Sweet, Bitter
Evidence for 4 modalities:
1. Cocaine on the tongue:
Sensations disappear in the following order
Pain
sweet
sour
bitter
salt
touch
Physiology of Taste – cont:
Primary modalities of taste:
Sour, Salt, Sweet, Bitter
Evidence for 4 modalities:
2. Gymnemic acid on tongue:
• Bitter & sweet ……..disappear
• Sour & salt ……………remain
Sensation of Taste –
Physiology of Taste –
cont:
Chemical structure and taste thresholds:
Threshold concentration.
µmol/L
– Sour ... Acidity by {H+} – HCL .......... 100
– Salt … Sodium chloride ……………..… 2000
– Sweet ..Sucrose ……………………………….. 10,000
glucose ……………………………….. 80,000
Saccharin ……………………………. 23
– Bitter ..Strychnine hydrochloride …
1.6
Quinine sulphate …………………….. 8
Artificial sweeteners:
Aspartame, Cyclamate
Physiology of Taste – cont:
Mechanism of stimulation of taste sensation:
– Sour:
Acids (H+)
Blocks K+ channels
– Salt taste
Na+
Depolarization
Physiology of Taste – cont:
Mechanism of stimulation of taste sensation:
– Sweet
G protein
activation of adenyl
cyclase
c-AMP
K conductance
– Bitter
G protein
Activatn. of Phospholipase C
IC-insitol (PO4)3
Ca2 release
Deep inside a
‘salty taste’
taste bud
Sour is similar
Electrical signal
A bucket brigade transmits the signal through the cell
Ion channels
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
Sweet taste
THE 4 “BASIC” TASTES ARE SALTY, SOUR, SWEET AND BITTER,
Also: UMAMI (MSG)?, METALLIC? FAT? AMINO ACID?
Different ions, different receptors for different tastes
Mechanism of stimulation of taste sensation -
cont:
Saltiness and sourness are transduced directly
by sodium and hydrogen ions respectively.
The transduction process for sweetness and
bitterness involve second messengers.
Sensation of Taste – cont.
Discrimination of intensity of taste:
– Poor (like smell)
– Requires 30% change to allow discrimination
of intensity
Sensation of Taste – cont.
Adaptation to taste:
Decreased sensation from repeated stimulus
Entirely peripheral at
the receptors
Sensation of Taste – cont.
Ethnic variation in taste sensation:
(Genetics)
Phenylthiocarbamide (PTC):
• In dilute solution
– Taste sour in 70% of Caucasians
– Tasteless in 30% of Caucasians (Inherited
defect- autosomal recessive trait)
Sensation of Taste – cont.
After-effects in taste sensation:
(Taste tricks):
• Taste modifier Meraculin
(a glycoprotein extracxted from miracle
fruit):
– When applied to tongue makes acids taste sweet
The Miracle fruit-origin
of miraculin
Sensation of Taste – cont.
Adaptation to taste:
Decreased sensation from repeated stimulus
Entirely peripheral at
the receptors
Clinical considerations
• Ageusia: Absence of sense of taste
• Dysgeusia: Disturbed sense of taste
• Hypogeusia: Diminshed sense of taste
• Hypergeusia: increased sense of taste
Thank you
Shukran Gazeelan