Olfaction (Zoltán Nusser)

L3. Olfaction (Zoltán Nusser)



Olfactory epithelium:

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  Cilium and microvilli on the surface, Browman gland → mucous surface
  
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  olfactory sensory neuron/OSN (10-20 million in human, 200 in dogs), supporting cells
  
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  basal cell, which produce ̴60 new olfactory sensory neuron daily
  

The Jacobson organ (vomeronasal organ) is a sensory organ specialized for detecting pheromones → presence in human is questioned
Superfamily of the olfactory genes:

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  More than 1000 genes responsible for olfaction (3% of the total number of genes in human)
  
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  OSNs that express a given olfactory receptor/OR gene show widespread distribution in the epithelium, but send their axons in two well defined glomeruli
  
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  ORs are seven transmembrane, G protein () coupled receptors that activate adenylyl cyclase and increase the intracellular concentration of cAMP → cAMP activates cyclic nucleotide-gated ion channel → ions activate either nonselective cation or channels
  
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  One OSN expresses only a single type of OR gene. However, one OR binds and gets activated by many different odors/chemical molecules
  
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  Each odor evokes a specific spatio-temporal activity of the OSN population
  
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  The electric responses of OSNs show adaptation.
  
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  The concentration of an odor is encoded by the latency and frequency of the AP
  

The Main Olfactory Bulb (MOB):

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  Glomerulus: OSN + mitral/tufted dendrit + PGC axon/dendrit
  

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  Mitrall cells (excitatory, glutamergic): principal cells of the MOB, providing the main output of the bulb. The primary dendrite arborizes in a single glomerulus, many secondary dendrites are in the external plexiform layer, where they receive dendro-dendritic inhibition from granule cells
  
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  Tufted cells (excitatory, glutamergic): principal cells of the MOB. Provide an extensive local collateral system in the internal plexiform layer.
  
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  Granule cells (inhibitory, GABAergic): They receive excitatory inputs form mitral/tufted cells and inhibitory from dSACs (deep short-axon cells = GABAerg (dis)inhibitory) through their dendrites. Axonless!
  
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  Periglomerular cells (inhibitory, GABAergic): small cell bodies located around the glomeruli. Excitatory input from OSN/mitral/tufted cell dendrites, GABAergic, inhibitory outputs to mitral/tufted and other PGCs
  

Oscillations and informatic coding:

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  Local filed potentials (LFP) are the consequences of the spatio-temporal summation of synaptic ligand/voltage-gated currents → reflects the population activity in the brain
  
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  Oscillation: rhythmic changes in the LFP. It requires periodic and synchronous neuronal activity. The LFP oscillation on its own does not carry any information, but indicates that the activity of a population of nerve cells in a given brain region is periodic and synchronous
  
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  Odors evoke stimulus-specific, temporally changing responses
  
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  The same odor evokes different activity pattern in distinct neurons, but different odors evoke different activity patterns in one neuron → the odor evoked activity pattern in the brain is stimulus and cell-specific
  
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  The identity of the participating cells, the temporal structure of their activity and the synchronicity among them are important → the brain uses “population”, “identity” and “temporal” code for olfactory information