Sensation of Taste

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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

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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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• Anatomy of Taste Sensation

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Taste Buds

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Sensation of Taste Anatomy of Taste - cont.

• Receptors
• Located in taste buds in
• Tongue
• Epiglottis
• Soft Palate
• Pharynx

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Anatomy of Taste Buds cont.

• 10,000 taste buds found on tongue, soft palate
  larynx
• Taste buds consist of
• 50 receptor cells (type 3) surrounded by
  supporting cells
• Basal cells (type 1 2) develop into supporting
  cells then receptor cells
• Gustatory hairs project through the taste pore
• Life span of 10 days

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Anatomy of Taste Buds cont.
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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).
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Anatomy of Taste Buds - cont
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Anatomy of Taste Buds - cont

• Each taste bud is innervated by 50 nerve fibers
• Each nerve fiber receives input from about 5
  taste buds

• There are about 4500 taste buds per average tongue

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Nerve supply of tongue

• Fibres from
• Epiglottis
• Palate
• Pharynx
• VAGUS

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Taste Pathway
Thalamus
ML
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Taste Pathway
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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
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• Physiology of Taste

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Physiology of Taste - cont

• Dissolution in Saliva
• Attachment to Receptors
• Generator Potential
• Action Potential

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Physiology of Taste contPrimary modalities of
taste
Sensitivity differs in different areas, but all
tastes can be perceived at most areas of the
tongue
Its not this simple
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Physiology of Taste contPrimary 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

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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.

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Physiology of Taste contPrimary modalities of
taste

• Sour
• Salt
• Sweet
• Bitter
• Evidence for 4 modalities???

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Physiology of Taste contPrimary modalities of
taste

• Sour, Salt, Sweet, Bitter
• Evidence for 4 modalities
• Cocaine on the tongue
• Sensations disappear in the following order
• Pain
• sweet
• sour
• bitter
• salt
• touch

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Physiology of Taste contPrimary modalities of
taste

• Sour, Salt, Sweet, Bitter
• Evidence for 4 modalities
• Gymnemic acid on tongue
• Bitter sweet ..disappear
• Sour salt remain

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Sensation of Taste Physiology of Taste
contChemical 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 .
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• Bitter ..Strychnine hydrochloride 1.6
• Quinine sulphate
  .. 8
• Artificial sweeteners
• Aspartame, Cyclamate

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Physiology of Taste cont

• Mechanism of stimulation of taste sensation
• Sour
• Acids (H)
• Blocks K channels
• Salt taste
• Na
• Depolarization

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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

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Deep inside a salty taste taste bud
Sour is similar
Electrical signal
A bucket brigade transmits the signal through the
cell
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Ion channels
Saltiness or sodium receptors allow sodium ions
to cross the membrane, thereby causing
depolarization.
Ion channel
Sourness receptors operate by closing potassium
channels, which allows a positive charge to
build up, thereby causing depolarization of the
cell.
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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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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.

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Sensation of Taste cont.

• Discrimination of intensity of taste
• Poor (like smell)
• Requires 30 change to allow discrimination of
  intensity

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Sensation of Taste cont.

• Adaptation to taste
• Decreased sensation from repeated stimulus
• Entirely peripheral at
• the receptors

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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)

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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
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Sensation of Taste cont.

• Adaptation to taste
• Decreased sensation from repeated stimulus
• Entirely peripheral at
• the receptors

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Clinical considerations

• Ageusia Absence of sense of taste
• Dysgeusia Disturbed sense of taste
• Hypogeusia Diminshed sense of taste
• Hypergeusia increased sense of taste

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Thank you Shukran Gazeelan
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