Taste/Gustation

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Taste/Gustation

• Detection of Chemicals and Regulation of Ingestion

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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

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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

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Organs of Taste

• Tongue, epiglottis, palate, pharynx
• Taste is due to chemicals, texture, temperature
  and pain and smell
• Taste cells and somatosensory receptors

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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 paincapsaicin from
  hot peppers
• Vision also participates in food selection and in
  enjoyment and expectation-emotional response to
  food

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Cravings

• Body can detect the absence of certain chemicals
  and create cravings for them
• Food Allergies allergic to foods you crave or
  cant live without
• Due to abnormal flora in gut that creates craving
  for energy source for that bacteria

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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

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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 cells1 of tongue epitheliumrest is basal
  cells and gustatory afferent axons

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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

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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

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Taste Cells

• Do not have axonsare like hair cells that are
  innervated by sensory axons which receive
  excitatory input from taste receptor cells within
  taste bud

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Taste Cell Life Cycle

• 2 weeksgrowth, death, regeneration
• Requires afferent innervation
• If axon is damaged , then taste cell degenerates

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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

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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

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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

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Modalities of Taste

• Only 4 components to taste
• SaltyHigh sodium ions
• Souracidic compoundshigh protons
• Bitteramino acids other organics, K, caffeine
• Sweetsugars s.a. sucrose
• 5th Taste Umamijapanese for delicious MSG or
  taste of glutamate

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Bitter Sour Salty Sweet
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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)

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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

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Sourness

• High acid foods taste sour (low pH)
• HCl generates H ions
• Transduced by
• H passing through amilioride sensitive Na
  Channel, Depolarizes cell (cant tell salt from
  sour)
• H binds weakly blocks K channels causes
  depolarization at normal ph channel open

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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

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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 celldirectly causes
  Ca release from intracellular stores

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Amino Acids

• Umamiglutamate, 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

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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

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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

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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.

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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

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CNS Pathways
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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

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Gustatory-Solitary Nucleus

• In Medulla-first synapse for taste afferents is
  the gustatory nucleus that is part of nucleus
  solitary

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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

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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

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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

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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