1. Gustation and Olfaction

2. Why Taste? Help distinguish safe from unsafe ◦ Bitter, sour = unpleasant ◦ Salty, sweet, “meaty” (umami) = pleasant

3. Salt Serves critical role in water balance (homeostasis) ◦ Needed by kidney ◦ Allows passive re-uptake of water from urine into blood

4. Sour Mildly pleasant in small amounts Larger = more unpleasant Why? ◦ Can signal “bad” food  Over-ripe fruit  Rotten meat  Spoiled food ◦ Bacteria grow in such media

5. Bitter Almost completely unpleasant to humans Many nitrogenous organic compounds (with pharmacological effect) have bitter aftertaste ◦ Caffeine (coffee) ◦ Nicotine (cigarettes) ◦ Strychnine (pesticides) Signals possible poison to body

6. Sweet Signals presence of carbohydrates in solution Highly desirable (high calorie content due to large number of bonds) Some non-carbohydrate compounds also trigger sweet sensation ◦ Saccharin, ◦ Sucralose, ◦ Aspartame

7. Umami (Ooh-mommy) Signals presence of amino acid L- glutamate Encourages intake of peptides and proteins ◦ Used to build enzymes, ◦ proteins in body

8. Taste as Chemoreception Taste cells, contained in bundles called taste buds Contained in raised areas called papillae Found across tongue

9. Debated whether taste cells can respond to one or many “tastants” Brain may be interpreting “patterns” of larger sets of neuron responses

10. Carried to brain, interpreted Sensation carried via one of three nerves: ◦ Facial (VII) ◦ Glossopharangeal (IX) ◦ Vagus (X)

11. OLFACTION Sense of Smell:

12. Olfaction Sense of smell Specialized sensory cells in nasal cavity Detects volatile (airborne) compounds

13. Olfactory receptor neurons Express only one functional odor receptor Like a “lock and key” ◦ Each receptor binds with particular odorant

14. Several theories on how this works: Shape theory – ◦ Each receptor detects a feature of the odor molecule Weak shape theory ◦ Different receptors detect only small pieces of molecules; inputs combined to form larger perception Vibration theory ◦ Odor receptors detect the frequencies of vibrations of odor molecules in the infrared range by electron tunnelling

15. Olfactory Epithelium Proportion of olfactory to respiratory epithelium (not innervated) indicates an animal's olfactory sensitivity. ◦ Humans: 1.6 in 2 olfactory epithelium ◦ Some dogs 26 in 2.  Dog's olfactory epithelium also more densely innervated, (100 x’s more receptors/cm 2 )

16. Molecules of odorants pass through nasal concha of the nasal passages ◦ Dissolve in the mucus lining ◦ Detected by olfactory receptors on dendrites of the olfactory sensory neurons.  May occur by diffusion or by the binding of the odorant to odorant binding proteins. Mucus on the epithelium contains mucopolysaccharides, salts, enzymes, and antibodies ◦ Very important - olfactory neurons provide a direct passage for infection to pass to the brain