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Olfaction 1

Olfaction 1. PSY 295 – Sensation & Perception Christopher DiMattina , PhD. The oldest sense. The simplest micro-organisms detect good chemicals (food) and avoid bad chemicals (toxins) - chemotaxis. Two chemical detection systems. Olfaction – detects molecules in air

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Olfaction 1

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  1. Olfaction 1 PSY 295 – Sensation & Perception Christopher DiMattina, PhD

  2. PSY 295 - Grinnell College - Fall 2012

  3. The oldest sense • The simplest micro-organisms detect good chemicals (food) and avoid bad chemicals (toxins) - chemotaxis PSY 295 - Grinnell College - Fall 2012

  4. Two chemical detection systems • Olfaction – detects molecules in air • Gustation – detects molecules in mouth PSY 295 - Grinnell College - Fall 2012

  5. Olfaction • Process by which we detect odorants • Odorant must be small, able to float in air, and hydrophobic PSY 295 - Grinnell College - Fall 2012

  6. Odorants • Not all small molecules are odorants • Carbon monoxide, methane • Need to have appropriate receptors to bind molecules PSY 295 - Grinnell College - Fall 2012

  7. Olfaction and other senses • Sound is organized by frequency , light by wavelength • Smell is defined in a poorly understood “chemical space” • Similar molecules can smell different, different molecules smell the same PSY 295 - Grinnell College - Fall 2012

  8. Olfactory receptor neurons PSY 295 - Grinnell College - Fall 2012

  9. Nasal cavity • Air and odorants are inhaled into the nasal cavity and pass through olfactory cleft PSY 295 - Grinnell College - Fall 2012

  10. Olfactory epithelium • At the top of the nasal cavity lies the olfactory epithelium • Contains the primary olfactory receptor neurons • “Retina” of the olfactory system PSY 295 - Grinnell College - Fall 2012

  11. Olfactory epithelium • Contains primary sensory transducers of the olfactory system PSY 295 - Grinnell College - Fall 2012

  12. Olfactory epithelium • Chemicals detected by olfactory sensory neurons (OSNs) • Basal cells are OSN precursors (OSNs die + regenerate every 28 days) PSY 295 - Grinnell College - Fall 2012

  13. OSN • Has cilia which detect odorants • Have olfactory receptors which bind odorants • Odorant binding starts a biochemical cascade which results in membrane depolarization and action potential firing PSY 295 - Grinnell College - Fall 2012

  14. OSN PSY 295 - Grinnell College - Fall 2012

  15. Combinatorial code • Different ORNs respond to different odors to varying degree • Pattern of activity can identify the odorant PSY 295 - Grinnell College - Fall 2012

  16. Olfactory receptors • Buck and Axel (1991) discovered mammalian genome contains 1000-2000 genes for olfactory receptors • Some are non-expressed (pseudogenes) depending on species – humans have about 400 functional receptors • Variability in extracelluar sequence determines odorant binding PSY 295 - Grinnell College - Fall 2012

  17. Olfactory transduction • Similar to retinal photo-transduction with a chemical second-messenger system • What are benefits? Disadvantages? PSY 295 - Grinnell College - Fall 2012

  18. Cribiform plate • OSN axons pass through cribiform plate at skull base and form the olfactory nerve, which projects to the olfactory bulb PSY 295 - Grinnell College - Fall 2012

  19. Anosmia • Absence of smell • One cause is damage to cribiform plate • ORN axons severed and scar tissue blocks regenerated axons PSY 295 - Grinnell College - Fall 2012

  20. Man’s best friend • Humans have ~20 million OSNs • Bloodhound has ~220 million OSNs • Can sense smells at concentrations 100 million times lower than humans PSY 295 - Grinnell College - Fall 2012

  21. Central processing PSY 295 - Grinnell College - Fall 2012

  22. Olfactory bulb • First stage of central processing • At base of brain PSY 295 - Grinnell College - Fall 2012

  23. Organization • Organized into regions called glomeruli PSY 295 - Grinnell College - Fall 2012

  24. Organization • Olfactory receptor neurons (ORNs) expressing same receptor project to the same glomerulus PSY 295 - Grinnell College - Fall 2012

  25. Output neurons • Glomeruli formed by dendrites of mitral and tufted cells • These cell types are the output cells of the olfactory bulb PSY 295 - Grinnell College - Fall 2012

  26. Inhibitory interneurons • Lateral inhibition between mitral cells can sharpen differences between olfactory patterns • Helps discriminate odors PSY 295 - Grinnell College - Fall 2012

  27. Organization • Similar chemical substances can sometimes map to nearby glomeruli PSY 295 - Grinnell College - Fall 2012

  28. Olfactory bulb projections PSY 295 - Grinnell College - Fall 2012

  29. Olfactory system • Ipsilateral organization • Projects directly to pyriform cortex, bypassing thalamus • Projects to many limbic system structures important for memory and emotion (i.e. amygdala and hippocampus related structures) PSY 295 - Grinnell College - Fall 2012

  30. The feel of scents • Nociceptors in nose and can be stimulated by ordorants • Ammonia burns, menthol is cool • Trigeminal nerve information (V) is fused with olfactory nerve information (I) PSY 295 - Grinnell College - Fall 2012

  31. From odorants to perception PSY 295 - Grinnell College - Fall 2012

  32. Shape pattern theory • Odorants and receptors are like locks and keys • Different odorants bind to varying degree with different receptors PSY 295 - Grinnell College - Fall 2012

  33. Pattern theory PSY 295 - Grinnell College - Fall 2012

  34. Specific anosias • Some individuals lack ability to smell specific scents (androsteone, found in sweat and pork) • This variability is due to genetic differences in OR expression in different individuals PSY 295 - Grinnell College - Fall 2012

  35. Stereoisomers • Pattern theory explains why two isomers can smell different • Different shapes bind different sets of receptors PSY 295 - Grinnell College - Fall 2012

  36. Patterns • Humans can discriminate 100,000 odors • Only 400 different receptor types • How might this work? PSY 295 - Grinnell College - Fall 2012

  37. Population coding PSY 295 - Grinnell College - Fall 2012

  38. Population coding • Assume only two states for a receptor (on/off) • 2400 different possible odors! PSY 295 - Grinnell College - Fall 2012

  39. Analogy with vision • Relative activities of three photoreceptor types permit us to represent millions of colors PSY 295 - Grinnell College - Fall 2012

  40. Limitation • Different molecules with similar receptor activation profiles will be in-distinguishible PSY 295 - Grinnell College - Fall 2012

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