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Taste/Gustation

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.

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Taste/Gustation

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  1. Taste/Gustation Detection of Chemicals and Regulation of Ingestion

  2. 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

  3. 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

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

  5. 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

  6. 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

  7. 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

  8. 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

  9. 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

  10. 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

  11. 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

  12. Taste Cell Life Cycle • 2 weeks—growth, death, regeneration • Requires afferent innervation • If axon is damaged , then taste cell degenerates

  13. 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

  14. 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

  15. 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

  16. 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

  17. Bitter Sour Salty Sweet

  18. 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)

  19. 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

  20. 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

  21. 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

  22. 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

  23. 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

  24. 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

  25. 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

  26. 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.

  27. 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

  28. CNS Pathways

  29. 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

  30. Gustatory-Solitary Nucleus • In Medulla-first synapse for taste afferents is the gustatory nucleus that is part of nucleus solitary

  31. 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

  32. 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

  33. 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

  34. 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

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