Taste and Smell PDF

Taste and Smell Guy Carpenter Taste and smell Hugely important for......survival …behaviour...quality of life The plan Sensory transduction Smell Taste Some central connections Early steps in olfaction OE orthonasal retronasal Orthonasal for environmental cues (including assessing foods) Retronasal...

Taste and Smell Guy Carpenter Taste and smell Hugely important for......survival …behaviour...quality of life The plan Sensory transduction Smell Taste Some central connections Early steps in olfaction OE orthonasal retronasal Orthonasal for environmental cues (including assessing foods) Retronasal for assessing processed foods. Olfactory epithelium Nasal secretions Made by Bowman glands Controlled by nerves (autonomic) and steriods Contains mucins, secretory antibodies SigA and other proteins Also Odorant binding proteins Nerve projections in olfactory epithelium Olfactory receptors Heterochromatin 3 5 6 >1000 genes only 1 expressed per cell! Magklara et al. 2011 Cell 145:555 Clowney et al. 2012 Cell 151:724 Lyons et al. 2013 Cell 154:325 All ORs Expressed OR Olfactory transduction pathway On AC3 Ano2 Golf After odorant binding to the receptor adenlyl cyclase increases cAMP which opens the Na/Ca channel depolarising the cell (and starts the nerve impulse). The Ano 2 chloride channel, activated by the calcium influx helps to amplify the signal Olfactory transduction pathway Off Ano2 AC3 Ca2+ X NCKX4 Ca2+ X Ca2+ 3x Na+ Ca2+ PDE X Switching off is as important to prevent over-stimulation. The raised calcium levels help to deactivate the Na/Ca channel. NCKX4 helps to remove calcium along with mitochondrial reuptake. Extracellular proteases also help to remove the odorant from the receptor. Initial steps in taste Taste modalities Bitter Salty Sour Sweet Umami Fat (CD36) Water? Cell types and synapses in the taste bud. (a) Type 1 cells- glial like, ensheath type 2 and type 3 cells, have some synapse clearing properties as they some enzymes that remove potassium and ATP Type 2- receptors to sweet bitter and umami, but no obvious synapse to nerves Type 3- pre-synaptic, detects sour and salt, obvious connections to nerves Type 4- basal cells Type 5- marginal cells, might be stem cell like. Taste buds turn over fairly quickly- 8-20 days. (b) Green = type 2, red = type 3 cells (c) Green = type 2, red = Type 1, (d) Synapse between Type 2/3 cell and nerve (N;bottom) Nirupa Chaudhari, and Stephen D. Roper J Cell Biol 2010;190:285-296 © 2010 Chaudhari and Roper Taste Receptors Type 2 cells Type 3 cells Taste Receptors-Red bars indicate cells with no PKD2L1 Transduction in sweet-, umami-, and bitterresponding taste bud cells Gustducin CALHM1 (Nature 495:223) Transduction in sour- and saltencoding taste bud cells H+ X Car4 Appetitive salt only – high aversive salt detected via bitter + sour cells (Nature 494:472) Central processing of taste – brainstem NST Also input from gut Modulated by descending input from amygdala, hypothalamus, gustatory cortex Neurons have range of selectivity, can be narrowly or broadly tuned Controls licking, chewing, swallowing Central processing of smell – olfactory nerve More direct input into cortical areas, some projections into thalmus, no involvement of brain stem except in descending outputs. Further reading Textbook Olfaction and Gustation handbook, Doty (ed) 2018 Reviews The cell biology of taste, Chaudhari and Roper 2010 Journal of cell biology 190 (3) 285296 Peripheral coding of taste (2014) Neuron 81(5) 984-1000 The functions of the orbitofrontal cortex (2004) Brain and cognition 55(1) 11-29