Why do we have a sense of taste?

1. Why do we have a sense of taste? • protection • palatable or poisonous? • pleasure • taste and olfaction = flavour • communication • olfaction important, taste less so

2. Sensors (taste buds) located in immediate vicinity of mouth Tongue Palate Pharynx, oesophagus, epiglottis Papillae on tongue bear taste buds From Gray’s Anatomy Usually 2000-5000 taste buds in humans

3. LM section of taste buds light, dark and basal cells From UT Houston Medical School

4. Taste buds structure cell types cell lineage cell turnover cell number Orange/onion structure EM appearance - four types of cells 3H thymidine studies for determining cell “birthdays” Very high 10-13 days most species Originally thought to be low Now thought to be 50-100 cells per taste bud

5. Cell types in taste buds Type I (dark cells) 55-75% Extend base to apex of bud Have dense granules in cytoplasm, partciularly apically Irregular nucleus Rough endoplasmic reticulum Long branched apical microvilli Type III (intermediate cells) 5-15% Extend base to apex of bud Similar to Type II, with dense cored vesicles in cytoplasm particularly at base of cell Synapses with afferent nerves Type II (light cells) 20% Extend base to apex of bud Large oval nucleus Smooth endoplasmic reticulum Short apical microvilli Type IV (dark cells) Do not reach taste pore Dense core vesicles Rough endoplasmic reticulum Precursor cell for other types? interneuron? or mechanoreceptor?

6. How do we taste? 1. Transduction of taste stimuli by taste cells. Taste buds found throughout oral cavity, oropharynx and epiglottis. Processing of taste stimuli not simple - different tastes detected by different mechanisms, and processed in the taste bud before afferent nerves are stimulated. Actually at least 5 tastes (if not more) and some are detected in more than one way.

8. Two new tastes? Or more? umami - “deliciousness” glutamate acts on a mutant glutamate receptor found in taste cells (found February 2000) first identified as a taste in 1908 Fats free fatty acids in mouth close K+ channel specific fatty acid transporters directly increase [Ca2+]I “thermal” tastes (2000)

9. How do we taste? 2. Communication of taste signals to the nervous system. Innervation of taste buds Oropharynx, epiglottis - X Posterior 1/3 - IX Anterior 2/3 - VII (in chorda tympani) Somatosensory afferents - V Efferent innervation

10. Innervation of taste buds Taste bud - 50-100 cells, 5-15 afferents  Each afferent innervates 5-10 taste cells Afferents may innervate multiple taste buds, in multiple papillae. How is the information decoded into taste? primary afferent papilla papilla Taste bud Taste cells Taste cells

11. Complex signalling in taste transduction Taste cells - don’t know specificity of taste transduction Taste buds - single buds respond to more than one taste stimulus Primary afferents - respond best to one taste but may respond to all NTS and thalamic neurons - also multiresponsive, but may be more “tuned” to one stimulus

13. Olfaction

14. Olfactory epithelium specialised epithelium above turbinate cartilage receptor cells: bipolar neurons - unmyelinated axons cilia form dense mat above cells new cells generated every 60 days from basal cells

16. Odorant detection molecules dissolve in mucus film diffuse to cilia/bind to carrier protein bind to specific receptors on olfactory neurons (cells may respond to more than one odorant) L-carvone = carraway D-carvone = spearmint specific anosmias Odorant binds to receptor = cAMP generation leading to receptor potential due to direct opening of Na+ channels Ability to  cAMP directly related to ability to generate action potentials

17. Olfaction Specific stimuli activate specfic receptors One cell = one stimulus = one response - probably not Projection to higher centres - directly to cortex, then to limbic regions and other cortical areas