Physiology of Taste
Sensation
Taste Sense (Gustation)
• Taste sensation is mainly the function of the taste
buds present in the tongue and mouth.
Taste Buds
• Most of the 10,000 or so taste buds are found
on the tongue
• Taste buds are found in papillae of the tongue
mucosa
• Papillae come in three types: filiform,
fungiform, and circumvallate
• Fungiform and circumvallate papillae contain
taste buds
Taste Buds
Figure 15.1
Anatomy of a Taste Bud
• Each taste bud consists of three major cell
types
– Supporting cells – insulate the receptor
– Basal cells – dynamic stem cells
– Gustatory cells – taste cells
Taste Sensations
• There are five basic taste sensations
– Sweet – sugars, saccharin, alcohol, and some amino
acids
– Salt – metal ions
– Sour – hydrogen ions
– Bitter – alkaloids such as quinine and nicotine
– Umami – elicited by the amino acid glutamate
Basic Taste Modalities
• There are 5 established basic tastes:
1) Sour Taste:
• Caused by acids; eg., ascobic acid
• Tasted by edges of the tongue.
2) Salty Taste:
• Elicited by ionized salts (NaCl)
• Tasted by anterior part of the dorsum of the tongue.
3) Sweet Taste:
• Elicited by organic chemicals e.g. sugars, glycols,
amino acids, aldehydes
• Tasted by the tip of the tongue
Basic Taste Modalities
4) Bitter Taste:
• Caused by:
a. Long-chain organic substances
b. Alkaloids e.g. quinine, caffeine, strychnine, and nicotine.
• Tasted by the back of the tongue
• N.B.: Some substances that at first taste sweet have a
bitter after taste e.g. saccharin.
5) Umami Taste:
• Is the dominant taste of food containing L-glutamate
(particularly monosodium glutamate (MSG), such as meat
extracts and aging cheese.
• Tasted by all parts of the tongue.
Basic Taste Modalities
Taste buds or Receptors
Site:
• Taste buds are oval structures present in;
a. Lateral walls of ;
• Fungiform papillae → present in tip and edges of tongue
• Circumvallate papillae → present in V-shaped area in
back of tongue the
b. Mucosa of palate, epiglottis and pharynx
• Filliform papillae → present on the dorsum of the tongue
don’t contain taste buds.
No.
• Adult tongue contains 10,000 taste buds
• This number is however more in children
Taste buds or Receptors
Structure:
• Each bud is formed of 3 types of cells;
1. Supporting or sustentacular cells.
2. Gustatory receptor cells:
• Modified epithelial cells.
• Each taste bud contains 50-150 receptor cells.
• Their upper poles containing cilia that project from the
taste pore.
• Sensory nerve endings synapse with their lower poles.
• Their life span is 1-2 week (10 days)
3. Basal cells:
• They are stem cells for production of new receptor cells.
Mechanism of stimulation of Taste
Receptors
• For tastant to bind to the taste receptor, it must be in a
solution or dissolved in saliva.
• Taste binding protein (TBP) (produced by Ebner's glands)
transports the tastant and concentrates it at taste buds.
• Tastant binds to its specific receptors in the microvilli of
gustatory cells.
• Binding of the tastant to a taste receptor opens or closes
ion channels, causing potential changes in the taste
receptor cell and release of the chemical transmitter e.g.
glutamate which excite the nerve fibers to generate nerve
impulses.
• For each tastant, there is a different mechanism
Mechanism of stimulation of Taste
Receptors
Mechanism of stimulation of Taste
Receptors
Mechanism of stimulation of Taste
Receptors
Mechanism of stimulation of Taste
Receptors
Mechanism of stimulation of Taste
Receptors
Taste threshold and intensity
discrimination
• The ability of humans to discriminate differences in the
intensity of taste is limited.
• A 30% change in the intensity of the substance being
tasted is necessary before an intensity difference can
be detected.
• The threshold for taste varies widely in different
individuals.
• It also depends on the substance tasted; the next
table shows the taste threshold for some compounds
Taste Receptors Specificity
• Each taste bud usually responds to only one of the 5
primary taste modalities when the concentration of
tastant is low (adequate stimulus).
• But at high concentration, most taste buds can be
excited by 2 or more taste stimuli.
• This explains the great number of tastes that a person
can perceive.
Taste Pathway
Taste Pathway
• 3 order neurons;
1) First order neurons:
• It carries taste sensation from the taste buds to the
gustatory nucleus (nucleus solitarius) as follow;
• a) From anterior 2/3 of the tongue by facial nerve
• b) From posterior 1/3 of the tongue by glossopharyneal
nerve.
• c) From the pharynx, hard and soft palate and epiglottis by
vagus nerve.
Note:
• General sensations from the anterior 2/3 of the tongue by
trigeminal nerve and posterior 1/3 are carried by
glossophyangeal nerve.
Area 3,1,2
Sensory radiations
PVNMT
Medial
leminiscus
Facial N.
Ant. 2/3
Glossoph.
Nucleus N.
solitarius
Vagus N.
Post. 2/3
epiglottis
Taste Pathway
2) Second order neurons:
• From the gustatory nucleus (nucleus solitarius) axon of 2nd
order neuron ascend in the ipsilateral (same side) medial
lemniscus and pass directly to the ventral posteromedial
nucleus of the thalamus (VPMNT).
3) Third order neurons:
• From the thalamus (VPMNT), the axons of the 3rd order
neurons pass in the sensory radiation to the face area of
the somatosensory cortex in the ipsilateral postcentral
gyrus.
• They also pass to the anterior part of the insula which is
anterior to the face area of the postcentral gyrus
(conscious perception of taste and taste discrimination)
Taste Adaptation
• Taste sensations adapt rapidly within minutes inspite
of continuous stimulation.
• It occurs in the CNS itself.
• This is different from most other sensory systems
which adapt mainly at the receptors.
Importance of Taste Sense
• Taste sensation is important in:
1) Stimulation of GIT secretions e.g. salivary
secretion through unconditioned reflexes.
2) Prevention of ingestion of most poisons
which have strong bitter taste.
3) Determination of the flavor of food.
Taste Preference
• This means the selection of certain types of food in
preference to others.
• Usually these types are needed to the body. In
experimental animals it has been found that
adrenalectomised animals show great preference to
salty water (containing Nacl) while
parathyroidectomised animals prefer Ca++
containing water.
• Taste preference occurs in the CNS but the exact
mechanism is not known.
Taste Disturbances
1. Ageusia (taste blindness): absence of taste
sensation for certain substance.
2. Hypogeusia: diminished taste sensation
3. Gustatory hallucination: False sensation of certain
tastes
4. Dysgeusia: disturbed taste sensation.
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