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“What is real? How do you define real? If you're talking
about what you can feel, what you can smell, what you
can taste and see, then real is simply electrical signals
interpreted by your brain."
- Morpheus from "The Matrix"
- Complete Study Guide 1 for Quiz Section on Friday
- Exam 1 coming up on Monday, October 19. This exam will cover
introductory material, spot check questions on neuroscience,
psychophysics, and chemical senses.
- submitted by Jawad Husain
Odors reach the olfactory epithelium through the nose
or through the mouth
The olfactory epithelium
The olfactory epithelium is located on the roof of the nasal cavity. It
contains the olfactory receptor cells, supporting cells, and axons that
lead to the olfactory bulb.
Olfactory receptor cells
The olfactory epithelium
The olfactory epithelium lies at the top of the nasal cavity. The axons
of receptor cells enter the skull through tiny perforations in the bone.
Olfactory receptor cells, unlike
other sensory receptors, are true
neurons.
They are also the only neurons
that die (life span 5-8 weeks)
and are replaced.
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Mucus performs a useful function!
Olfactory receptors are true neurons
Odorant molecules are picked up by special protein molecules in
the mucus and transported to receptor sites on the cilia.
The axons of olfactory
receptors project directly
to the olfactory bulb
The dendrites of olfactory
receptors have cilia that
stick out into a layer of
mucus
Olfactory receptors are true neurons
The axons of olfactory
receptors project directly
to the olfactory bulb
The dendrites of olfactory
receptors have cilia that
stick out into a layer of
mucus
Olfactory capabilities vary among species
•
Mice have more than 1400 different types of olfactory receptor proteins,
humans have about 350 different types, and fish about 60 different types.
•
Humans have about 10-12 million olfactory neurons, but dogs have about
4 billion!
Olfactory neuron responses to odorants
Binding of odorants
causes a receptor
potential at the
dendrites, an
integration signal at
the cell body, and
action potentials
in the axon.
The olfactory system of the mouse contains about 1400 different
receptor proteins. Each receptor neuron expresses just one
protein.
Humans have about
350 different receptor
proteins
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Olfactory pathways in the CNS
The olfactory epithelium
Has no clear topography of
receptor types, but all
receptors of the same type
project to the same
glomerulus. This pattern of
projections creates a map
that is conserved up to the
level of the cortex.
Olfactory nerve to
olfactory bulb
Olfactory bulb to
olfactory cortex
Olfactory cortex to
olfactory region of
thalamus
Thalamus to olfactory
cortex
Buck & Axel, 1991
Olfactory pathways in the CNS: Another view
Orbitofrontal cortex is a site for integration across
multiple sensory modalities
The olfactory system is the
only sensory system that
synapses in the cortex
before the thalamus.
How is information about odor represented in the nervous
system?
One idea is that a different population of neurons is active for each odor
(or class of odor).
However, it cannot be this simple because each olfactory receptor, nerve
fiber and central olfactory neuron responds to multiple odorants.
How is information about odor represented in the
nervous system?
One idea is that a different population of neurons
is active for each odor (or class of odor).
However, each olfactory nerve fiber and central
olfactory neuron responds to multiple odorants.
The spatial distribution of activity in the
salamander olfactory bulb is the same for three
very different odors (although absolute levels of
activity are different).
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A single olfactory receptor or nerve fiber responds
to many different odorants
As in the taste system,
the temporal pattern of
response is different for
different odorants.
Interaction between taste and smell
How is information about odor represented in the nervous
system?
Another idea is that
each odor elicits a
unique pattern of
activity across time
as well as space.
Olfactory preferences are affected by many factors
including what has recently been eaten
White bars = taste only
Black bars = taste + smell
Age differences in olfactory performance
Sex differences in olfactory performance
It!s not as bad as the graph makes it look! The 99-year olds got
over half of the odors right.
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