Sense Memory

• Olfaction is ipsilateral

  The right olfactory bulb gets information from the right nostril
• Olfactory sensory neuron axons

  • They are among the thinnest and slowest in the body
  • The time it takes to process the sensation of odors is long and the time it takes to react to a scent doubles perception time
• Olfactory bulb

  • First relay for olfactory sensory neurons in the brain
  • Axons of the mitral cells and other neuron types in each bulb combine to form the olfactory tract
• Olfactory cortex (piriform cortex)

  A set of cortical areas that receives highly divergent input from neurons in the olfactory bulb (involved in odor discrimination)
• Primary olfactory cortex

  Made up of a number of areas (e.g. amygdala, parahippocampal gyrus, and other interconnected areas) and interacts closely with the entorhinal cortex
• Limbic system


  A group of neural structures that is involved in many aspects of emotion and memory
• Connections from the olfactory bulb to the olfactory cortex

  • They are non-topographic and highly divergent
  • Each mitral cell gets input from a single glomerulus and sends its axon on to the olfactory cortex
• Pyrimidal neuron in the olfactory cortex

  Receives converging inputs from many different mitral cells associated with many different glomeruli
• epitope

  A feature of an odorant to which an olfactory receptor may bind selectively
• Olfactory sensory neurons


  • Each expresses specific olfactory receptor molecules, which binds a specific odorant
• code is in the Glomeruli

  Odor identity is encoded in the pattern of activated glomeruli (Population coding!)
• Odorant receptor genes

  • Humans have approximately 350-400 different functional odorant receptor genes
  • ~425 "pseudogenes" that are present on the chromosomes but the proteins coded for by the genes do not get made
• Huge diversity in the repertoire of functional odorant receptor genes among different people
• Differences in odor perception

  Can reflect which genes are expressed and how many copies of a specific receptor a person has
• The more copies of a receptor an individual has

  The more sensitive they will be to certain odorants
• Whether you have a pseudogene or a functional gene for a given odor


  Can alter odor perception
• Genes associated with olfactory receptor expression

  Determines our sensitivity to four other food relevant odors (e.g. banana, beer, blue cheese, violets)
• Having only a few receptors of a given type

  Can lead us to perceive a scent only weakly
• Heavy alcohol consumption (~3 drinks per hour)

  Impairs olfactory sensitivity
• Having just one drink

  Improves olfactory sensitivity
• Somatosensory system

  Most odorants stimulate the somatosensory system to some degree through free nerve endings within the nose
• Trigeminal nerve (nV)


  Mediates these somatosensory sensations
• It is often impossible to distinguish between sensations traveling up cranial nerve I and those traveling up cranial nerve V
• Shape pattern theory


  Different scents activate different arrays of olfactory receptors in the olfactory epithelium as a function of odorant-shape to OR-shape fit
• Odorant molecules have different shapes and an odorant will be detected by a specific OR to the extent that the odorant's molecules fit into that OR
• Chemicals of specific shapes

  Fit receptors with shapes that best accommodate them
• Combinatorial code

  • One odorant may bind to several different receptors
  • One receptor may bind several different odorants to varying degrees
• 
  

  Differences in pattern of glomeruli activation provide the basis for the array of odors that we perceive
• The perception of a rose starts with a chemical analysis of the molecules that make up the rose scent
• Specific ORs recognizing specific chemical features (shape-pattern theory)
• Information is transmitted to the olfactory bulb, creating a unique pattern of spatio-temporal activity in the glomeruli to the mitral cells
• The inhibitory network within the olfactory bulbs

  Can be modulated according to the physiological state of the individual (e.g. hunger, past experience)
• Olfactory receptors

  Produce specific patterns of activity in the olfactory bulb
• Perception of a rose

  1. Chemical analysis of the molecules that make up the rose scent

  2. Specific ORs recognizing specific chemical features (shape-pattern theory)

  3. Information is transmitted to the olfactory bulb, creating a unique pattern of spatio-temporal activity in the glomeruli to the mitral cells
• "Rose" = 1000 different chemicals
• The inhibitory network within the olfactory bulbs enhance the contrast between various spatio-temporal patterns that different odors produce
• Physiological state


  Can modulate olfactory perception (e.g. hunger, past experience)
• Olfactory bulb

  Granule cells receive descending projections from the olfactory cortex
• Food smells more appealing when we're hungry
• Odor memory

  Our ability to detect, remember and odor, and recall an odor can be manipulated by experience