Transcript Chapter 08: The Chemical Senses

Smell
Olfaction brings both good news and bad news
Pheromones
Smell— a mode of communication as well as
of detecting environment
Important signals
Reproductive behavior
Territorial boundaries
Identification
Aggression
Role of human pheromones ; not as prominent
as in other animals but still play roles
unwittingly!
Slide 1
Smell
The Organs of Smell
Olfactory epithelium
Olfactory receptor cells : 4-8 wks life cycle
Supporting cells : like glia, help producing mucus
Basal cells : olfactory neurons source (stem cell)
(Cranial nerve I)
Slide 2
Smell
The Organs of Smell
Mucus layer :
water base with mucopolysaccharides
antibodies - critical defense en route to brain
various enzymes
odorant binding proteins - concentrate odorants
Olfactory acuity :
The size of olfactory epithelium and the number of
olfactory receptors
e.g. dog vs human : 170 cm2 vs 10 cm2, 100 times more
receptors per sqarecentimeters
Vomeronasal organ :
pheromone sensing organ
project to accessory olfactory bulb
Slide 3
Smell
Olfactory Receptor Neurons
Olfactory Transduction
• Cl- channel opening
mediate outflow of chloride
ions :
Unusually high intracellular Clconc. enhance depolarization
(in contrast to what happens in
other neurons)
• Termination of olfactory
sensation :
 Odorants diffusion
 Degradation (scavenger
enzymes)
 2ndary signalling
pathway to end the
transduction – adaptation
(even when odorants still
be around)
Slide 4
Smell
Olfactory Receptor Neurons
Olfactory Transduction
• Cl- channel opening
mediate outflow of chloride
ions :
Unusually high intracellular Clconc. enhance depolarization
(in contrast to what happens in
other neurons)
• Termination of olfactory
sensation :
 Odorants diffusion
 Degradation (scavenger
enzymes)
 2ndary signalling
pathway to end the
transduction – adaptation
(even when odorants still
be around)
Slide 5
Smell
Olfactory Transduction
Odorant receptors
• More than 1000 genes in rodents
(Linda Buck and Richard Axel)
— The largest gene family in
mammals
— ~350 in human
• Genes are scattered (in clusters)
about on the genome
• Each Olfactory neurons seem to
express single odorant receptor
gene : choice mechanism unknown!!.
• Zonal and random (within a zone)
expression
— each zone express a different,
nonoverlapping subset of
receptor genes
Slide 6
Smell
Olfactory Transduction
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VNO has its own receptors :
— ~300 in mice, 5 in humans
— Structurally different from
odorant receptors
— Ligands are largely unknown
Unusual cAMP gated channels are
evolutionarily conserved : Used in the
visual transduction as well
Recent finding suggests both
olfactory system and vomeronasal
system regulate behavioral outcome
such as reproduction
Population coding : both odorant type
(which cells) and strength (how
active) are used to classify the odors
Slide 7
Smell
Central Olfactory Pathways
• Each Ob has about 2000 glomeruli
• Each glomerulus contains about
25,000 primary olfactory axons and
dendrites from about 100 secondorder olfactory neurons (Mitral cells)
Slide 8
Smell
Central Olfactory Pathways
Slide 9
Smell
Central Olfactory Pathways
 Precise mapping :
• All axons from neurons that express
one particular receptor gene converge
onto glomeruli on each bulb
• Each glomerulus receives input from
only receptor cells of one particular
type
• Symmetrical positions
• Consistent positions of glomeruli
from one mouse to another
 Intricate circuitry :
• Excitatory and inhibitory connections
-Within glomeruli
-Among glomeruli
-Between obs
• Broad categorization of odorant
signals might be done at this level
Slide 10
Smell
Central Olfactory Pathways
Direct projections to cortex (OB-OC):unique to olfactory system
Paleocortex VS. Neocortex
Multiple projections (from OB) to many brain structures
Direct and widespread influence on odor discrimination, emotion,
motivation, memory..
• OB-OT-Medial dorsal nucleus of thalamus pathway may be
responsible for the conscious perception of smell, while
connections to amygdala and entorhinal cortex may be involved in
the affected (associated) components of olfaction such as
aversiveness
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Slide 11
Slide 12
Smell
Spatial and Temporal Representations of Olfactory
Information
Olfactory Population Coding
• Combinations increase resolutions
Olfactory Spatial Maps
• A sensory map is an orderly arrangement of neurons that correlates
with certain feature of the environment
• Reproducible spatial pattern in the system that no where information
is needed- WHY?
- Discrimination among different chemicals?
- Just for the efficiency? - Easier communications with neighbors
- Systemic relationships between chemicals and map positions?
Slide 13
Smell
Spatial and Temporal Representations of
Olfactory Information
Chemotopy
Slide 14
Smell
Spatial and Temporal Representations of
Olfactory Information
ORN receptive fields and acuity
Slide 15
Smell
Spatial and Temporal Representations of Olfactory
Information
Temporal coding
Odors are inherently slow stimuli - The timing of odors is not
so important but the quality of odors is
Oscillation between ob and cortex when odors are present -
imply the presence of temporal odor code
ORNs responsive to a particular odor display a wide range of
temporal spiking pattern
In honeybee expts, ablating the synchrony of firing brought
about a loss of bee’s ability to distinguish between similar
odors ( not between broad category of odors)
Slide 16
Smell
Spatial and Temporal Representations of Olfactory
Information
Temporal coding hypothesis
Slide 17