Special senses.
Chemical Senses
• Interaction of molecules with receptor cells
• Olfaction (smell) and gustation (taste)
• Both project to cerebral cortex & limbic
system
– evokes strong emotional reactions
Taste
• RECEPTOR ORGANS(Taste Buds)
• Taste bud receptors detect gustatory
stimuli (tastants).
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Total no of taste buds-10,000
• Taste buds are located on several kinds of
papillae on the tongue and in the pharynx and
larynx.
• Taste buds contain chemoreceptor cells
arranged around a taste pore.
• Taste buds –tops of the numerous fungiform
papillae .
• Sides of less numerous circumvallate and
foliate papillae.
• Few are present on the mucosa of palate, soft
palate, epiglottis, wall of pharynx and larynx.
• Small elevated projection on dorsum of
tongue.
• Small fungiform papillae-tip and along the
edges –anterior 2/3 rd of tongue.
• Contains five taste buds at its top.
• Circumvallate and foliate are large and
contain about 100 taste buds along its sides.
• In posterior third –circumvallate papillae
located in the walls of V-shaped trough and
foliate at the edge of V.
• Dorsum of the tongue-covered by small
conical threadlike filiform papillae that does
not contain taste buds.
• No taste buds in the under surface of
tongue.
Structure of taste buds
• Oval shaped, multicellular structures.
• about 70 µm high and 50 µm in
diameter.
• 40 supporting cells, 5-18 taste cells and
basal cells.
• Gustatory Receptor cell
• Modified epithelial cell.
• Elongated and extends full length of the taste bud.
• Apical end-single, hair like microvillus projects to
the external surface through the taste pore-come in
contact with the gustatory stimuli.
• The microvilli increase the surface area of the
receptor cells.
• Receptor cells are the most numerous taste bud
cells (50-75%).
• Taste bud-50-150 taste receptor cells that are
surrounded by the supporting cells.
• Apical regions –receptor cells and supporting cells
- tight junctions-microvilli -contact with fluids in
oral cavity.
• Basal end, receptor cells form synapse - nerve
endings of primary sensory axons .
• Each taste bud-innervated by 50 afferent
fibers.
• Each afferent fibers receives input from about
five taste buds.
• Life span -10 days.
• Innervation
• Fungiform papillae-receive sensory
innervation from chorda tympani- branch of
facial nerve.
• Circumvallate and foliate papillae –
glossopharyngeal nerve.
• Taste buds in palate-facial nerve
• Other parts-vagus nerve.
Taste Pathway
• Chorda tympani branch –facial nerve and
glossopharyngeal nerve
• carry sensations -taste buds -anterior 2/3 rd
and posterior 1/3 rd of the tongue.
• Taste sensations from other areas travel in
vagus nerve.
• Afferent nerve fibers (first order neurons)medulla oblongata and terminate in nucleus
tractus solitarius.
• Second order neuron arise from NTS and
ascend in medial lemniscus to terminate in
ventral posteromedial nucleus of thalamus.
• From thalamus third order neuron- travel in
the thalamic radiation -sensory cortex
• Taste pathway
Anterior
Posterior
Pharynx
2/3 tongue 1/3 tongue
VII CN
IX CN
X CN
order neuron
1st
Nucleus tractus solitarius
Medial lemniscus
2nd order neuron
Thalamus (VPM Nucleus)
3rd order neuron
Sensory cortex (post central gyrus)
Primary taste sensations
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Sweet
Salty
Sour
Bitter
Umami
Mechanisms of gustatory transduction
• Chemoreceptors –respond to chemicals from
lingual epithelium.
• Gustatory stimulus-depolarization of receptor
cell.
• Chemicals –dissolved food particles- contact –
microvilli-activate the receptor cell
• Development of receptor potential –
glutamate- Aps in sensory axons
• Thus the process of generation of AP on the
taste buds –gustatory transduction
Salt taste
Cations and anions of ionized salts.
Ensures proper dietary electrolyte balance.
Compound
NaCl, CaCl2, KCl..
• Elicited by Na+-Sodium chloride .
• Salt sensitive receptors – epithelial Na+
channel (E NaC )in their membrane –allows
Na+ entry when ECF Na+ concentration
increases.
• ↑ICF Na + - depolarization of receptor cell
Sour taste
The stronger the acid, the more sour the
taste – proportional to pH.
Warns against potentially noxious/poisonous
chemicals.
Compound
HCl, Acetic, Citric, Carbonic
• Hydrogen ions block K + channels –apical
membrane.
• ↓ ed K + permeability produces
depolarization of receptor cell.
• Intensity of perception depends on the degree
of dissociation of the acid.
Sweet taste
• Organic compounds, sugars, glycols, amino
acids etc.
• Inorganic salts of lead and beryllium (reason
for lead poisoning).
• Identifies energy-rich nutrients.
Compound
Sucrose,Saccharin,Fructose ,Glucose
• Stimulation of G protein-coupled receptoractivates adenylyl cyclase and increases cAMP.
• Activates protein kinase-phosphorylation of
K+ channels –basolateral membrane of cell.
• Causes of closure of K+ channelsdepolarisation.
Bitter taste
Long chain organic substances and alkaloids.
• Taste threshold is lowest for bitter
substances.
• Warns against potentially noxious/poisonous
chemicals.
Compound
Nicotine, Caffeine, Cocaine, Morphine
• Bitter compound act via binding to receptors
coupled to G protein- depolarization via G
protein –mediated second messengers .
• cAMP,IP3 and DAG
Umami taste
allows the recognition of amino acids.
Compounds
• Glutamate
• Monosodium glutamate (MSG)
• Aspartate
• Glutamate –increases Na+ and Ca+depolarization of receptor cell.
• Taste thresholds.
• Lowest concentration of gustatory stimulustaste bud respond.
• Thresholds for tastes vary among the 4
primary tastes
– Thresholds of bitter compound-low and highest
for sweet.
– Women are less sensitive to sour and more
sensitive to sweet and salt compared to men.
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Factors influencing taste sensation.
Age
Gender
Concentration of tastants
Area of stimulation
Temperature of tastants
Nutrient deficiencies.
Disorders of taste:
• Ageusia :
Taste blindness/loss of taste sensation
• Gustatory agnosia
Ability to recognize stimuli is lost – due to
mouth infections, facial nerve infections,
etc.
Hunger for substances not fit for food (dirt)
• Hypogeusia:
Sense of taste is decreased.
• Dysgeusia:
Disturbed sense of taste