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Taste/Gustation
Detection of Chemicals and
Regulation of Ingestion
Chemical Sensation
• Oldest sensory system
• Bacteria detect and move toward chemical
food source
• We taste chemicals in food
• Our cells bind and respond to chemicals
within our bodies
Chemosensation
• Taste & Smell = conscious awareness of
chemicals
• Nerve endings in skin and in mucous
membranes react to irritating chemicals
• Nerve endings in digestive tract respond to
chemicals
• Receptors in aorta measure carbon dioxide
and oxygen
Organs of Taste
• Tongue, epiglottis, palate, pharynx
• Taste is due to chemicals, texture,
temperature and pain and smell
• Taste cells and somatosensory receptors
Combination of Receptors
• Complex tastes arise from activation of multiple
receptors at once
• Smell of food contributes to distinction of taste
• Texture and temperature and pain—capsaicin from
hot peppers
• Vision also participates in food selection and in
enjoyment and expectation-emotional response to
food
Cravings
• Body can detect the absence of certain
chemicals and create cravings for them
• Food Allergies: allergic to foods you crave
or “can’t live without”
• Due to abnormal flora in gut that creates
craving for energy source for that bacteria
Chemotransduction
• Detection of chemicals in the environment
(food)
• Chemicals activate chemoreceptors that
transiently alter membrane potential of taste
cell
• Called a receptor potential
• Can be depolarizing or hyperpolarizing
Papillae
• On tongue surface are protrusions (bumps)
with different shapes (ridges, pimples,
mushrooms)
• Each papilla is a collection of 100-200 buds
• Each taste bud has 50-150 taste receptor
cells arranged as orange sections
• Taste cells=1% of tongue epithelium:rest is
basal cells and gustatory afferent axons
Organization of Taste Organ
• Papillae (contain taste buds; 100s)
– Vallate (pimple)
– Fungiform (mushroom)
– Foliate (ridges)
• Taste buds (contain taste cells; 50-150)
• Taste Cells (innervated by gustatory afferent
axons of CN 7, 9, 10)
• Basal cells synapse with axons & taste cells
Taste Buds
• Normal range is 2000-5000 taste buds
• Can be as little as 500 or as many as 20,000
• 90% of taste cells respond to 2 or more
chemicals
• Allow for population coding
Taste Cells
• Do not have axons—are like hair cells that
are innervated by sensory axons which
receive excitatory input from taste receptor
cells within taste bud
Taste Cell Life Cycle
• 2 weeks—growth, death, regeneration
• Requires afferent innervation
• If axon is damaged , then taste cell
degenerates
Taste Cell Anatomy
• Apical End-membrane region near tongue
surface
• Has microvilli that project into the taste
pore
• Taste cells have synapses with endings of
gustatory afferents near bottom of taste cell
Taste Cells
• Taste bud contains 100 taste receptor cells
• Saliva has low Na+ concentration
– microvilli on apical end of taste cell detect
chemicals in the aqueous (saliva) environment
Taste Cells –Basal Cells
• Taste cells have electrical and chemical
synapse with basal cells
• Basal cells can synapse with gustatory
afferents
• Form information processing circuit within
taste bud
Modalities of Taste
• Only 4 components to taste
–
–
–
–
Salty=High sodium ions
Sour=acidic compounds=high protons
Bitter=amino acids & other organics, K+, caffeine
Sweet=sugars s.a. sucrose
• 5th Taste: Umami=japanese for “delicious”= MSG
or taste of glutamate
Bitter
Sour
Salty
Sweet
Transduction
• Tastant: taste stimuli
• Transduce the taste by
– Directly passing through ion channel (salt &
sour)
– Bind and block K ion channel (sour & bitter)
– Bind and open channel (amino acids)
– Bind receptors that activate 2nd messengers that
open or close ion channels (sweet, bitter
umami)
Saltiness
• Taste of Na+
• Na+ selective ion channel blocked by
amilioride, insensitive to voltage; always
open
• As you eat salty food the external Na+
increases and Na+ flows into cell through
channel
• Directly depolarized membrane
Sourness
• High acid foods taste sour (low pH)
• HCl generates H+ ions
• Transduced by
– H+ passing through amilioride sensitive Na
Channel, Depolarizes cell (can’t tell salt from
sour)
– H+ binds weakly & blocks K+ channels &
causes depolarization; at normal ph channel
open
Sweetness
• Sweet transduced by
– Binding specific receptors & activate 2nd
messenger cascades
– G protein triggers formation of cAMP,
activation of PKA, phosphorylation of K+
channel (not sour channel) and closes it leading
to depolarization
– Cation channels directly gated by sugars
Bitterness
• Bitter receptors detect poisons
• Transduced in many ways
– Quinine (bitter,tonic) & Ca++ bind to K+
channel and block them
– Bitter receptors that activate G proteins that
lead to increased IP3 levels & modulates NT
release without depolarizing cell—directly
causes Ca++ release from intracellular stores
Amino Acids
• Umami—glutamate, aspartate
• Glutamate transduced by
– Permeating Na/Ca ion channel, depolarizes,
opens voltage gated Ca channel that triggers
NT release
– Binds G-protein coupled, decreases cAMP
– Arginine and proline gate their own channels
Receptor Potential
• Hyperpolarization or Depolarization caused
by activation of taste cell
• Depolarization causes calcium channel
opening
• Triggers NT release at synapse with afferent
neuron (unknown NT)
• Causes AP in afferent sensory axon
Threshold Concentration
• Concentration of a basic chemical that
registers a perception of taste
• At low concentration, papilla are very
sensitive but at high concentration they
respond to all stimuli
Perception of Taste
• One afferent axon gets input from many
different taste cells each maximally
responsive to combinations of taste
• Population Coding: Groups of broadly
tuned neurons specify taste rather than
single finely tuned taste cells and neurons.
Population Coding
• Analysis of the response of population of
cells to particular food
• Some nerve cells will increase or decrease
the rate of firing
• Cortex discerns what the overall pattern of
activation is and decides you ate chocolate
CNS Pathways
Central Taste Pathways
• Taste bud- brain stem-thalamus-cerebral cx
• 3 CN carry taste
– Anterior 2/3 of tongue have afferents in CN7
facial nerve
– Posterior 1/3 of tongue have afferents in CN9,
the glossopharyngeal
– Epiglottis, pharynx, glottis have axons in CN10
vagus
Gustatory-Solitary Nucleus
• In Medulla-first synapse for taste afferents
is the gustatory nucleus that is part of
nucleus solitary
Thalamus-CNS
• From Gustatory nucleus to ventral posterior
medial (VPM) nucleus of thalamus (sensory
for head)
• To Broadman area 36 above temporal lobe
= Primary Gustatory Cortex
• To insula cortex
• Uncrossed & Crossed pathways from CN to
CX
Gustatory Projections
• Projects to nuclei in medulla involved in
swallowing, salivation gagging, vomiting,
digestion and respiration
• Hypothalamus & amygdala involved in
controlling eating
• Lesions to amygdala can cause animals to
ignore food or overeat
Somatosensory Inputs
• The tongue in also innervated by afferents
for touch temperature and pain that
contribute to recognition of foods by texture
and heat
• Travel to primary somatosensory cortex in
post central gyrus
Additional CNS Circuit
• Nucleus Solitary to Pons –Pontine Taste
Nucleus
• to Hypothalamus For feeding regulation
• To Amygdala for emotional connections
• To Thalamus for Taste perception
• Primitive Pathway