Taste/Gustation

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

9. 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

10. 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

12. 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

13. 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

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

15. 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

17. 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

18. 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

19. 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

20. Bitter Sour Salty Sweet

21. 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)

22. 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

24. 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

26. 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

28. 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

29. 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

31. 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

33. 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

34. 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.

35. 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

37. CNS Pathways

39. 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

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

43. 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

44. 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

46. 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

47. 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