2007 Sensory Physiology

Olfactory pathways

  • individual primary afferent neurons express a single type of receptor
  • each neuron with expressing a particular receptor projects to a specific glomerulus within the main olfactory bulb (MOB), establishing an organizational map at the level of the second order neurons
    • i.e., dark and light green ORNs are scattered randomly throughout the main olfactory epithelium, but their axons segregate into different glomeruli
  • GC-D neurons (pink) project to a completely separate set of necklace glomeruli, that are found on the periphery of the main olfactory bulb
Olfactory bulb glomeruli
Olfactory receptor codes
  • this segregation sets up a mechanism whereby odourants can be coded by the pattern of activity in the olfactory bulb
  • odourants can be coded based on which the combination of receptors (14 are shown here) are encoded
  • Richard Axel and Linda Buck won the 2004 Nobel prize in Medicine for unraveling the coding scheme for odourants

Odourtopic maps

  • systematic mapping of the responses to >300 odourants (reviewed in Johnson and Leon, 2007) confirms the spatial organization of the olfactory bulb: an odourtopic map that has a similar organization to the stimulus-specific maps found in other sensory systems
  • these studies have also shown that lateral inhibition narrows the range of chemicals that indivdual olfactory bulb neurons respond to


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