Smell

• The olfactory epithelium develops from a placode (specialised thickening of ectoderm).
• Where are primary olfactory sensory neurons?

  In olfactory epithelium

  Roof of nasal cavity, beneath cribiform plate of ethmoid bone

  Axons penetrate the plate
• Where do primary olfactory sensory neurons project to?

  Olfactory bulb
• What are odourants detected by?

  Olfactory receptors/proteins -> 7 transmembrane G-protein coupled receptors
• Each olfactory sensory neuron expresses a unique olfactory receptor gene. Each receptor has a unique response to differently to a range of odourant molecules.

  
• AP generation is due to a result of the summation of receptor potentials in the dendritic tree.
• What is the function of a convergent circuit?

  Tends to amplify signal -> increase detection capacity at the expense of acuity
• What is the function of periglomerular & granule cells?

  Lateral inhibition

  They are inhibitory & GABAergic

  If you smell something bad, you can ‘mask’ it with a better smell -> competition between perceptions of smell 
• Neurobiology of Smell

  Odorant molecule binds to GPCR -> Golf dissociates (G alpha protein) & binds to adenyl cyclase -> increases levels of cGMP -> opens cGMP-gated Ca+2 channels -> opens Ca+2-gated Cl- & Na+ channels -> depolarisation (mainly from Cl- moving out of cells) -> generates pattern of Aps -> sensory neurons travel through cribriform plate -> converge onto 2nd order neurons (in glomerulus, within the olfactory bulb) -> projects to multiple targets in the cortex (olfactory cortex & limbic areas (amygdala & entorhinal cortex)) 

  DOES NOT go the thalamus 
• There is no evidence of a cortical map of olfaction.