Sensation

1
Sensation Perception

• Lecture 18 Chemical Senses
• Andy Clark
• December 1, 2004

2
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

3
Olfaction

• 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

4
Olfaction

• 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

5
Olfactory System

• Pyriform Cortex
• Orbitofrontal Cortex
• Amygdala

6
Olfactory System
7
Olfactory Mucosa
8
Olfactory Receptor Neurons (ORN)

• 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

9
Olfactory Receptors

• Membrane bound proteins
• Located in cilia on tips of ORNs
• Cause change in membrane potential of ORN when
  bound by ligand
• 1000 different types of receptor
• Only 1 type per ORN

10
Olfactory Mucosa-Organization
ORNs
1 2 3 4

4 Zones
11
Olfactory Bulb

• Organized into 4 zones
• Glomerulus
• Primary structure w/in bulb-receives input from
  5,000-10,000 ORN (input predominately from 1 type
  of ORN)
• 1000-2000 glomeruli

12
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 smells
• Disparate molecular structure ? similar smells

13
Neural Coding-Specificity

• Most Olfactory Receptor Neurons exhibit
  significant response to a wide range of odorants
• Little regard to molecular structure

14
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

15
Central Processing

• Many neurons w/in cortical areas in the olfactory
  system respond to a variety of odorants
• Many multi-modal (respond to pairing of taste
  with smell)
• Responses affected by behavioral/emotional context

16
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.

17
Pheromones-Humans

• 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 subjects menstrual cycle

18
Pheromones-Humans

• Major Histocompatibility Complex (MHC)
• 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

19
Taste

• Distinction made between taste flavor
• Taste signaled by action of gustatory system
• Flavor signaled by joint action of olfactory and
  gustatory systems

20
Tongue

• Organ-tongue
• Covered with structures termed papillae
• 4 types
• Filiform (conical, entire surface)
• Fungiform (mushroom, tip sides)
• Foliate (folds along sides)
• Circumvallate (flat mounds, back)

21
Tongue
22
Taste Buds

• Only Filiform papillae dont 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

23
Taste Receptors

• 4 basic receptor types on taste cells
• Each associated with a particular taste quality
• Salty
• Sweet
• Sour
• Bitter

24
Gustatory System
Thalamus
25
Gustatory System
26
Taste-Genetics

• Genetic differences affect individuals taste
  experience
• Phenlythiocarbamide (PTC)
• Tasters Non-tasters
• 6-n-propylthiouracil (PROP)
• Tasters have higher density of taste buds than
  non-tasters

27
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

28
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 (psychophysically)

29
Flavor Perception

• Requires combination of oral and nasal
  stimulation
• Subjects have a hard time identifying flavor if
  nostrils are clamped shut
• Strength of taste qualities also perturbed when
  nasal stimulation is limited

30
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 / doesnt need to be
  ingested

31
Flavor-Sensory Adaptation

• Sensory specific adaptation can also influence an
  individuals perception of flavor
• Subjects ratings of the pleasantness of an odor
  decrease after consuming that substance
  (specific)
• Similar effect occurs regardless of whether food
  is swallowed or just chewed

32
Flavor-Encoding

• Neurons within the Orbitofrontal Cortex receive
  input from both olfactory and gustatory cortices
  (as well as visual and somatosensory)
• Respond to similar qualities across modalities
  (I.e. smell and taste of similar substances, etc.)

33
Flavor-Encoding

• Hunger also influences responses of primate
  Orbitofrontal cortex neurons
• With additional consumption neurons fire less in
  response to stimulus
• Animals behavior correlates well with observed
  firing pattern