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Chemical Senses
Gustation (taste) + Olfaction (smell) = Flavor
Smell
Olfactory
Epithelium
(~5 cm2)
Turbinates
Olfactory
neurons live
only a few
weeks
(the ONLY
neurons that are
directly exposed
to the external
environment)
basal cell
olfactory neuron
support cell, or
sustentacular
cell
Amoore’s Stereospecific Theory of Smell
Camphoraceous
Musky
Floral
Minty
Ethereal
Pungent
Putrid
Now thought to have ~1,000 receptor types
Odorant-Binding
Proteins (OBPs)
and
PhermoneBinding Proteins
(PBPs)
receptor recognizes complex,
but not ligand alone
ligand transferred to receptor
only with assistance of OBP
ligand spontaneously dissociates
from the complex and binds to
the receptor
Receptors for odorants
resemble receptors for
neurotransmitters
Figure 32-4
Four zones in olfactory
epithelium defined
functionally
Neurons with the same
receptors are restricted
to a single zone, but
Neurons with different
receptors are found within
each zone
The 3-D model shown on the right (from Couto &
Dickson, 2005) is that of a fly antennal lobe (AL),
the equivalent of the vertebrate olfactory bulb. As
you can see, the AL is composed of spheroidal
structures, the glomeruli. While vertebrate
olfactory bulbs may contain thousands of
glomeruli, the fly AL contains only ~43 glomeruli.
Furthermore, each glomerulus is uniquely
identifiable, based on its size, shape and position
in the AL neuropil. The glomeruli are so invariant
in these features that they have been given
specific names. Each glomerulus receives
innervation from olfactory receptor neurons
(ORNs) expressing a single odorant receptor.
The VA1v glomerulus for example, receives input
from ORNs expressing the OR47b receptor. A
glomerulus may thus be thought of as
representing a single unit of smell. The
constellation of unique glomeruli forms the
Olfactory Map of the animal.
Two Parallel Pathways
• Lateral
– Direct connections between olfactory bulb
and entorhinal cortex, pyriform cortex
– Connections to amygdala and other limbic
structures
• Medial
– Olfactory tubercle
– Medial dorsal nucleus (thalamus)
– Orbitofrontal cortex
Taste
Primary Tastes
Salt
Na+ cation, small anion
Sour
Low pH
Sweet
Bitter
Large, non-dissociating
organic molecules
Alkaloids
Umami
Amino acids, esp. glutamate