CHEMICAL SENSES Smell (Olfaction) and Taste

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1 CHEMICAL SENSES Smell (Olfaction) and Taste Peter Århem Department of Neuroscience SMELL 1

2 Olfactory receptor neurons in olfactory epithelium. Size of olfactory region 2

3 Number of olfactory receptor cells Receptor cells Olfactory receptor neurons synapse with mitral cells in olfactory bulb. Synapses form glomeruli. 3

4 Odoreceptor genes. Humans discriminate millions of odors?? functional odorant receptor molecules (odoreceptors) from 950 genes Transduction mechanism 4

5 Odoreceptors are G-protein coupled. Olfactant binding depolarisation and action potentials propagated to mitral cells. Summary on the transduction mechanism Olfactant binding to odoreceptor in apical end of olfactory cells activation of G protein (G olf ) activation of adenylate cyclase camp opens cyclic nucleotide gated (CNG) channels Na + and Ca 2+ inflow depolarisation and action potentials propagated to mitral cells. 5

6 Central olfactory pathways Olfactory cortex 6

7 Central olfactory pathways Olfactory fibres from n. olfactorius (n. I) bulbus olfactorius primary olfactory cortex (piriform cortex, amygdala, entorhinal cortex) thalamus neocortex (orbitofrontal cortex) hypothalamus (emotional component) hippocampus Pheromones and the accessory olfactory system 7

8 Pheromones Pheromones (species specific substances affecting social and sexual behaviour) affect both the classical olfactory system (above) and the accessory olfactory system (not functional in humans?). Figure 15.5 Hypothalamus activation in subjects exposed to a sex hormone-containing odor mix Sex hormone odours affect female and male hypothalamus differently 8

9 TASTE Taste receptor cells are organised in taste buds; located in papillae on tongue, palate and pharynx 9

10 Five submodalities (qualities): salt, sour, sweet, bitter and umami (amino acids) Taste bud 10

11 Transduction mechanisms Figure Taste transduction via ion channels and G-protein coupled receptors (Part 1) Transduction salt and sour 11

12 Figure Taste transduction via ion channels and G-protein coupled receptors (Part 2) Transduction sweet and umami Figure Taste transduction via ion channels and G-protein coupled receptors (Part 3) Transduction bitter 12

13 Summary on transduction mechanisms Salt - Na + influx in specific Na channels in apical membrane depolarisation transmitter release Sour - H + inflow through specific channels (TRP) or H + block of K channels in apical membrane depolarisation transmitter release Sweet, bitter and umami - Specific receptors, G- protein coupled IP 3 mediated effect on TRP channels depolarization transmitter release Summary of summary Salt and sour mainly direct effects on ion channels Sweet, bitter and umami mainly effects on G- protein coupled receptors and indirect effects on ion channels and transmitter release 13

14 Central taste pathways Three cranial nerves to the gustatory nucleus (nucleus of solitary tract) 14

15 Central taste pathways N.VII - n. facialis (with ganglion) N. IX - n. glossopharyngeus (with ganglion) N. X - n. vagus (with ganglion) gustatory nucleus in medulla (nucleus solitarius) thalamus (VPM) primary taste cortex, gustatory cortex (operculum and insula) Thalamus (VPM) to gustatory cortex (operculum, insula) 15

16 Questions (Smell/Taste): 1. How many odours can humans experience? 2. How many receptor molecule genes in humans? 3. Second messenger in olfactory receptor cells? 4. In most sensory systems information reaches cortex via thalamus. How is it in the olfactory system? 5. Where in neocortex does the olfactory system project? 6. How do we classify taste? 7. How are the taste cells organised on the tongue? 8. Transduction mechanism for taste of acids? 9. Transduction mechanism for taste of sugar? 10. Which thalamus nuclei process taste information? 16