Download Slides from Lecture 12/01/2004 (Andy Clark)

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
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
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
Olfactory System
Olfactory
Olfactory
Central
Mucosa
Bulb
Projections
•Pyriform Cortex
•Orbitofrontal
Cortex
•Amygdala
Olfactory System
Olfactory Mucosa
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
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
ORN’s
1
…
2
4
Zones
3
4
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
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
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
odorants
• Many multi-modal (respond to pairing of
taste with smell)
• Responses affected by behavioral/emotional
context
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.
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 subject’s menstrual cycle
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
Taste
• Distinction made between taste & flavor
• Taste signaled by action of gustatory system
• Flavor signaled by joint action of olfactory
and gustatory systems
Tongue
•
•
•
Organ-tongue
Covered with structures termed papillae
4 types
1.
2.
3.
4.
Filiform (conical, entire surface)
Fungiform (mushroom, tip & sides)
Foliate (folds along sides)
Circumvallate (flat mounds, back)
Tongue
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
quality
1.
2.
3.
4.
Salty
Sweet
Sour
Bitter
Gustatory System
Frontal Operculum
Vagus Nerve
Mouth & Larynx
Insular Cortex
Glossopharyngeal Nerve
Tongue
Thalamus
Chorda Tympani Nerve
Nucleus Solitary Tract
(NST)
Gustatory System
Taste-Genetics
• Genetic differences affect individual’s taste
experience
– Phenlythiocarbamide (PTC)
• ‘Tasters & Non-tasters’
– 6-n-propylthiouracil (PROP)
• Tasters have higher density of taste buds than nontasters
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
(psychophysically)
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
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
ingested
Flavor-Sensory Adaptation
• Sensory specific adaptation can also
influence an individual’s perception of
flavor
– Subject’s 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
Flavor-Encoding
• Neuron’s 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.)
Flavor-Encoding
• Hunger also influences
responses of primate
Orbitofrontal cortex
neurons
• With additional
consumption neurons fire
less in response to
stimulus
• Animal’s behavior
correlates well with
observed firing pattern