SENSATION OF TASTE
TASTE BUDS
Sense organs for taste or gustatory sensation are the taste buds.
Taste buds are ovoid bodies.
Diameter of 50 μ to 70 μ.
In adults, about 10,000 taste buds are present and the number is more in children.
In old age, many taste buds degenerate and the taste sensitivity decreases.
SITUATION OF TASTE BUDS
Most of the taste buds are present on the papillae of tongue.
Taste buds are also situated in the mucosa of epiglottis, palate,
pharynx and the proximal part of esophagus.
Types of papillae located on tongue:
1. Filiform papillae – Small, conical
2. Fungiform papillae – round , near tip
3. Circumvallate papillae – large , posterior of tongue.
STRUCTURE OF TASTE BUD
Taste bud is a bundle of taste receptor cells, with supporting
cells embedded in the epithelial covering of the papillae.
Each taste bud contains about 40 cells,
Type of Cells in Taste Bud
1. Type I cells or sustentacular cells
2. Type II cells
3. Type III cells
4. Type IV cells or basal cells.
Type I cells and type IV cells are supporting cells.
Type III cells are the taste receptor cells. FIGURE 176.1: Taste bud
Type I, II and III cells have microvilli. This opening is called taste pore.
PATHWAY FOR TASTE RECEPTORS
Receptors for taste sensation are the type III
first order neuron - 1. Chorda tympani fibers
2. Glossopharyngeal nerve fibers
3. Vagal fibers
tractus solitarius
medial lemniscus ( 2nd order neuron)
terminate in posteroventral nucleus of thalamus.
parietal lobe of the cerebral cortex ( 3rd order neuron)
Center for taste sensation is in opercular insular cortex. FIGURE : Pathway for taste sensation
PRIMARY TASTE SENSATIONS
Five types: -
1. Sweet
2. Salt
3. Sour
4. Bitter
5. Umami.
Man can perceive more than 100 different tastes.
Other taste sensations are just the combination of two or more primary taste sensations.
SWEET TASTE
Sweet taste is produced mainly by organic substances like monosaccharides, polysaccharides,
glycerol, alcohol, aldehydes, ketones and chloroform.
Inorganic substances, which produce sweet taste are lead and beryllium.
Receptor for sweet taste is GPCR.
The sweet substances bind to receptor and cause depolarization via cyclic AMP.
SALT TASTE
Salt taste is produced by chlorides of sodium, potassium and ammonium, nitrates of sodium and
potassium.
Some sulfates, bromides and iodides also produce salt taste.
Receptor for salt taste is called epithelial sodium channel (ENaC).
It acts like ENaC receptors in other parts of the body.
SOUR TASTE
Sour taste is produced because of hydrogen ions in acids and acid salts.
Sour sensation also has the same ENaC receptor.
Epithelial sodium channel (ENaC), other receptors such as hyperpolarization-activated cyclic
nucleotide-gated cation channel (HCN) also are involved in sour sensation.
BITTER TASTE
Bitter taste is produced by organic substances like quinine, strychnine, morphine, glucosides, picric
acid and bile salts and inorganic substances like salts of calcium, magnesium and ammonium.
Bitterness of the salts is mainly due to cations.
Bitter receptor is a GPCR.
In bitter receptor, the sour substances activate phospholipase C through G proteins.
It causes production of inositol triphosphate (IP3), which initiates depolarization by releasing
calcium ions.
UMAMI
Umami is the recently recognized taste sensation.
Receptors of this taste sensation respond to glutamate,
particularly monosodium glutamate (MSG),
which is a common ingredient in Asian food.
However, excess MSG consumption is proved to produce Chinese restaurant syndrome in some
people taking Chinese food regularly.
Umami receptor is metabotropic glutamate receptor (mGluR4).
Glutamate causes depolarization of this receptor.
SENSATION OF SMELL
OLFACTORY RECEPTORS
Olfactory receptors are situated in olfactory mucus membrane, which is the modified mucus
membrane that lines upper part of nostril.
Olfactory mucus membrane consists of 10 to 20 millions of olfactory receptor cells supported by
the sustentacular cells.
Mucosa also contains mucus-secreting Bowman glands.
Olfactory receptor cell is a bipolar neuron.
About 10 to 12 cilia arise. Cilia are non-myelinated, with a length of 2 μ and a diameter of 0.1 μ
OLFACTORY PATHWAY –
Axons of bipolar olfactory receptors pierce the cribriform plate
olfactory bulb
axons synapse with dendrites of mitral cells.
Different groups of these synapses form olfactory glomeruli
Axons of mitral cells leave the olfactory bulb and form olfactory tract.
Olfactory tract runs backward and ends in olfactory cortex,
Through olfactory stria.
FIGURE : pathway for olfactory sensation
OLFACTORY TRANSDUCTION
Olfactory transduction is the process by which olfactory receptor converts chemical energy into action
potentials in olfactory nerve fiber.
odoriferous substance dissolves in mucus,
stimulates the olfactory receptors
bind with receptor proteins in the cilia and form substance-receptor complex.
Substance receptor complex activates adenyl cyclase
causes the formation of cyclic AMP.
opening of sodium channels, leading to
influx of sodium and generation of receptor potential.
causes generation of action potential in the axon of bipolar neuron.
CLASSIFICATION OF ODOR
Odor is classified into various types.
1. Aromatic or resinous odor: Camphor, lavender, clove and bitter almonds
2. Ambrosial odor: Musk
3. Burning odor: Burning feathers, tobacco, roasted coffee and meat
4. Ethereal odor: Fruits, ethers and beeswax
5. Fragrant or balsamic odor: Flowers and perfumes
6. Garlic odor: Garlic, onion and sulfur
7. Goat odor: Caproic acid and sweet cheese
8. Nauseating odor: Decayed vegetables and feces
9. Repulsive odor: Bed bug.
