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Sensation Perception (PowerPoint download)


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									Sensation & Perception

 Lecture 18: Chemical Senses
         Andy Clark
      December 1, 2004
             Taste & Smell
• Chemical Senses vital for survival
• Allows for approach to appetitive stimuli /
  Retreat from noxious stimuli
  – macrosmatic: highly developed sense of smell
    / reliance on olfactory system
  – microsmatic: weakly developed sense of smell
    / less reliance on olfactory system
• Humans less sensitive to odorants than a #
  of other organisms
  – Individual receptors capable of reliably
    signaling one molecule of a compound
  – Analogous to exquisite sensitive of rod
    receptors in vertebrate retina
• Disparity in sensitivity due to differences in
  absolute # of receptors
• Humans capable of detecting small
  differences in odor intensity
  – Previously thought that Weber fraction for
    smell was worst of any sense
  – Now know that previous reports of poor
    sensitivity were simply artifacts of poor
    experimental control
            Olfactory System

Olfactory      Olfactory       Central

Mucosa         Bulb            Projections

                               •Pyriform Cortex
Olfactory System
Olfactory Mucosa
   Olfactory Receptor Neurons
• Signal Transducers
  – 1000 different types
  – 10,000 of each type
  – Each type found in only 1 zone of mucosa

  – Vision:
  – 3 cone types, 1 type of rod
  – 6 million cones, 120 million rods
         Olfactory Receptors
• Membrane bound proteins
  – Located in cilia on tips of ORN’s
• Cause change in membrane potential of
  ORN when bound by ligand
• 1000 different types of receptor
  – Only 1 type per ORN
        Olfactory Mucosa-Organization


Zones               3

Olfactory Bulb
       • Organized into 4
       • Glomerulus
          – Primary structure w/in
            bulb-receives input
            from 5,000-10,000
            ORN (input
            predominately from 1
            type of ORN)
          – 1000-2000 glomeruli
            Neural Coding
• Currently know little about how odor
  perception relates to physical & chemical
  properties of molecules (I.e. structure,
  electrical charge, etc.)
• Similar molecular structure  different
• Disparate molecular structure  similar
Neural Coding-Specificity
             • Most Olfactory
               Receptor Neurons
               exhibit significant
               response to a wide
               range of odorants
             • Little regard to
               molecular structure
Neural Coding-Distributed
             • Dispute as to the
               sparseness of odorant
               tuning w/in individual
               ORN and glomeruli
             • Accepted that different
               odorants produce different
               patterns of activation
               across the population
             • Odorants producing
               similar patterns produce
               similar perceptions
         Central Processing
• Many neurons w/in cortical areas in the
  olfactory system respond to a variety of
• Many multi-modal (respond to pairing of
  taste with smell)
• Responses affected by behavioral/emotional
   Accessory Olfactory System
• Many organisms possess an accessory
  olfactory system
• Involved in the processing of chemosignals
  from conspecifics (pheromones)
  – Vomeronasal organ
• Influence mating, paternal, group behavior
  in insects, rodents, elephants, etc.
• McClintock (’71)
  – Synchronization of menstrual cycles amongst
    women living together
• Russel et al (’80)
  – Sweat from donor periodically applied to upper
    lip of subjects increased correlation between
    donor and subject’s menstrual cycle
• Major Histocompatibility Complex
  – Influences immune recognition, susceptibility
    to infectious disease, mating preference etc.
• Found that human females prefer odors
  from males with allelic matches to
  paternally inherited MHC genes
• Distinction made between taste & flavor
• Taste signaled by action of gustatory system
• Flavor signaled by joint action of olfactory
  and gustatory systems
•    Organ-tongue
•    Covered with structures termed papillae
•    4 types
    1.   Filiform (conical, entire surface)
    2.   Fungiform (mushroom, tip & sides)
    3.   Foliate (folds along sides)
    4.   Circumvallate (flat mounds, back)
Taste Buds
     • Only Filiform papillae
       don’t contain taste buds
     • 10,000 total
     • Outer taste pore contains 4
       types of receptor sites
     • When bound with ligand
       cause change in ion flux
       (either directly or
       indirectly) which lead to
       change in membrane
       potential of nerve cell
Taste Receptors
        •   4 basic receptor types
            on taste cells
        •   Each associated with
            a particular taste
            1.   Salty
            2.   Sweet
            3.   Sour
            4.   Bitter
                        Gustatory System
                       Vagus Nerve                             Frontal Operculum

Mouth & Larynx
                                                                                   Insular Cortex

                                     Glossopharyngeal Nerve



                                      Nucleus Solitary Tract
Chorda Tympani Nerve                  (NST)
Gustatory System
• Genetic differences affect individual’s taste
  – Phenlythiocarbamide (PTC)
     • ‘Tasters & Non-tasters’
  – 6-n-propylthiouracil (PROP)
     • Tasters have higher density of taste buds than non-
Neural Coding-Specificity
             • Neurons w/in cranial
               nerves and NST show
               significant response to a
               number of substances
             • Some tuned sharply for a
               particular class
                – Application of receptor
                  antagonist blocks inhibits
                  subsequent responses
Neural Coding-Distributed
             • Different substances
               produce different patterns
               of firing in the cranial
               nerves carrying signals
               from taste receptors
             • Substances producing
               similar activity patterns
               judged more similar
           Flavor Perception
• Requires combination of oral and nasal
• Subjects have a hard time identifying flavor
  if nostrils are clamped shut
• Strength of taste qualities also perturbed
  when nasal stimulation is limited
         Flavor-Central Basis
• Physiological processes operating outside
  the sensory pathways also have an influence
  on perceived flavor
  – alliesthesia: decrease in pleasantness as
    consumption increases
     • Central time to develop / doesn’t need to be
    Flavor-Sensory Adaptation
• Sensory specific adaptation can also
  influence an individual’s perception of
  – Subject’s ratings of the pleasantness of an odor
    decrease after consuming that substance
  – Similar effect occurs regardless of whether
    food is: swallowed or just chewed
• Neuron’s within the Orbitofrontal Cortex
  receive input from both olfactory and
  gustatory cortices (as well as visual and
• Respond to similar qualities across
  modalities (I.e. smell and taste of similar
  substances, etc.)
        • Hunger also influences
          responses of primate
          Orbitofrontal cortex
        • With additional
          consumption neurons fire
          less in response to
        • Animal’s behavior
          correlates well with
          observed firing pattern

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