Transcript PY262.13.Chemical.olfaction

Olfactory Perception – Smell
“…We have five senses, but only
two that go beyond the boundaries
of ourselves. When you look at
someone, it’s just bouncing light,
or when you hear them, it’s just
sound waves, vibrating air, or
touch is just nerve endings
tingling. Know what smell is?... It’s
made up of the molecules of what
you’re smelling.”
Angels in America, p. 17
Olfactory Perception – Smell
• What is smell?
– The substance that you’re “smelling” has been emitted from that
source as gas molecules, which in turn you take into your body
– Different from light, sound & cutaneous stimulation, which
involves transformations of receptors
– Molecule detectors
Olfactory Perception – Smell
• We have not yet defined the physical
parameters the way we have in other
senses (i.e., frequency, spatial contrast)
• We have not yet defined the perceptual
experiences like in other senses such as
vision (brightness, color) or hearing
(loudness, pitch)
• Very difficult to manipulate the stimulusmedium (i.e., how do you control the
concentration of airborne molecules?)
• We know something about the receptors and neuroanatomy, but
without understanding the stimuli we remain quite limited in what
we know.
• Net result: smell is still largely not understood
Significance of Smell?
• General alarm system (smoke,
disease)
• Component of flavor (quality of
food)
• Part of communication
(pheromones)
• Discrimination of social context
(family, gender, attractiveness…)
• General
rules:
smells good  “safe”
smells bad  “dangerous”
How can we classify odors?
• Already stated, we do not have
the yardsticks of frequency or
spatial measurement
• Largely done by description of
examples
– “Soap smells like lavender”
– Some wines have a “woody” aroma
Olfactory system fun facts: human and dog
receptors are the same
Human receptors work as good as dogs even
though the dog’s ability to smell is 300 to 10,000
times more sensitive then humans
Difference? Number of receptors.
Humans = 10 million
Dogs = 1 billion
Olfactory system fun facts: human recognize their own
odor and can distinguish between male v. female odors
• Russell (1976) had subjects where a shirt for 24-hours
consecutively, then sealed the shirts in plastic bags
• Subjects could recognize there own shirt (from a set of 3) 75%
of the time (chance: 33%)
• Subjects could recognize the strangers (male and female) also
75% of the time
• McKlintock (1998) women who live together for sufficiently
long periods of time synchronize menstrual cycles…
due to chemical signals in their sweat
• Savic et al. study: PET scans reveal differential responses in
the Hypothalamus in response to opposite gender odors
Sekuler &
Blake, 2002
• Six primary
(independent) odor
qualities: fragrant,
etheral, resinous,
spicy, burned, putrid
• They form a basis to
describe any smell
sensation as
combination of these
6 components
(similar to Trichromatic
color vision)
• This corresponds to
6 types of receptors,
which are reflected by
specific anosmias
('smell blindness') –
though the actual
number of receptors
are still disputed
•
Can we localize smell? Comparisons to other
senses, we are very poor at this
Experimentally, this
problem is less easy to
approach than in vision
or hearing?!
Some researchers
invented some ingenious
sniffo-meters…
NOTE: other animals
seem to perform much
better (dogs)
Where are the
receptors for
olfaction?
Olfactory system from the top down
• Odors pass through
the nose and mouth
into the sinuses and
across the Olfactory
Mucosa (why don’t we
smell much through
our mouth?)
• Olfactory bulbs are
above the Mucosa &
bone
• Olfactory Receptor
Neurons (ORN) run
through the Mucosa
• Cilia
of ORN are imbedded in base of the Mucosa
(look like roots growing down into Mucosa)
Receptor pathway of smell:
From bottom up:
• Olfactory Receptor
(on the end of the
cilia) 
• Olfactory Receptor
Neuron 
• Olfactory Bulbs
(which are the
beginning of the
Central Nervous
System (CNS))
• Olfactory Tract (to
higher (CNS) sites)
The Olfactory Receptor
• The olfactory receptor at
the end of the cilia are little
proteins (like in the visual
system)
• The OR is the actual site
of transduction from
chemical reactions to
electrical impulses
• 1000 different protein
patterns (each of which
respond to the same
group of odorants)
• All OR’s are the same
on each ORN (1000
different types)
Organization of olfactory mucosa and olfactory bulb: zones
10 Million Olfactory Receptor Neurons (ORN) synapse
on 1-to-2,000 Glomeruli
Translation: 5,000-to-10,000 ORN’s per Glomerulus
 lots of transduction at Olfactory Bulb
What do we know about neural coding
or central (Brain) processing for odor?
• Almost nothing.
• Only thing clear? Different olfactory receptor
neurons (ORN’s) fire in a coordinated pattern to
an odorant.
• Piriform cortex is the primary receiving area, but
we don’t know what it is doing.
• Orbital Frontal cortex is secondary receiving
area where there is some evidence of differential
responding to different combinations of odors.
Where is the Orbitofrontal
Cortex