Olfaction (Zoltán Nusser)

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L3. Olfaction (Zoltán Nusser)

Olfactory epithelium:

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

  • More than 1000 genes responsible for olfaction (3% of the total number of genes in human)

  • OSNs that express a given olfactory receptor/OR gene show widespread distribution in the epithelium, but send their axons in two well defined glomeruli

  • 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

  • One OSN expresses only a single type of OR gene. However, one OR binds and gets activated by many different odors/chemical molecules

  • Each odor evokes a specific spatio-temporal activity of the OSN population

  • The electric responses of OSNs show adaptation.

  • The concentration of an odor is encoded by the latency and frequency of the AP

The Main Olfactory Bulb (MOB):

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

  • 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

  • Tufted cells (excitatory, glutamergic): principal cells of the MOB. Provide an extensive local collateral system in the internal plexiform layer.

  • 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!

  • 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:

  • 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

  • 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

  • Odors evoke stimulus-specific, temporally changing responses

  • 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

  • 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

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