The Chemical Senses
Gustation and Olfaction
The peripheral taste system
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Primary receptors: about 4000
taste buds in tongue and oral
cavity
Each taste bud contains 30-100
receptor cells (modified epithelial
cells) together with supporting
cells and stem cells.
Classically, 4 primary flavor
submodalities can be identified in
humans: sweet, salty, sour and
bitter. We presently believe that
for the sweet and bitter
submodalities, there is a small
number of receptor proteins which
have very wide ability to recognize
stimulant molecules.
This map of submodality specific tastebuds
on the tongue in many textbooks…
…is BS, or at least a
misrepresentation of how the
system works. Unfortunately,
a great many texts still
include it.
Actually, all taste buds are
more or less sensitive to all of
the classic submodalities, so
taste receptors are not
labeled lines.
There are additional submodalities
• In humans, a fifth submodality, called
umami (Japanese for “yummy”) is
stimulated by amino acids – this is why
monosodium glutamate is an effective
flavor enhancer
• Recently, a fat submodality has been
proposed for humans.
• At least some animals (rats, for example),
have a water submodality.
Central pathways for gustatory information
The key points here:
Taste info passes from primary
afferents in the tongue and pharynx
along cranial nerves VII, IX and X
to the gustatory nuclei of the
brainstem. Individual axons in
these pathways typically show
some responsiveness to all 4
classic submodalities, but typically
respond best to one of the 4.
From the brainstem nuclei the
information passes to the thalamus
and then to the tongue-mouth area
of the primary somatosensory
cortical map. Note that this is a
non-decussating pathway.
The Olfactory System
Olfactory Transduction
• 1o olfactory receptors are neurons
• In most mammals there are two locations
for primary receptors – the main olfactory
area, which serves for odors not related to
social interactions, and the vomeronasal
organ, which specializes in pheromones.
• Some humans have a vomeronasal organ
or two, but it seems largely non-functional.
From the olfactory epithelium to the olfactory bulb
The number of olfactory receptor proteins
coded in the mammalian genome is
typically very large (about 30,000 genes
or about 10% of the total genome of the
rat, for example).
There are several million primary receptor
cells in a typical mammal, and it seems
probable that each receptor cell
expresses one or only a few types of
receptor protein.
Glomeruli are sites of synaptic connection
between receptor cells and 2nd order
(mitral) cells. Each glomerulus contains
dendrites of about 25 mitral cells and
receives input from about 25,000 receptor
cells. The axons of mitral cells and tufted
cells enter the olfactory nerve.
( periglomerular cells and granule cells
are local circuit neurons and do not
project to the rest of the brain.)
The basic circuit between the olfactory receptors
and the olfactory bulb
How is odor information sorted out by the N.S?
• The concept of primary odors (i.e. a small set of odor
submodalities) is not useful – there are too many odors,
and almost all natural odor stimuli are chemical mixtures.
Discriminating such mixtures is apparently of selective
advantage. For example, a trained dog can distinguish
between an apparently unlimited number of individual
humans.
• Receptor neurons that express particular odorant
molecules send their axons to distinct subsets of
glomeruli, so when the animal is presented with a
particular odor, a corresponding set of glomeruli will
respond – if the brain sees the pattern of glomerulus
activation as a “picture” of a particular odor, a large
number of different odors could be represented.
The olfactory bulb is a map of the olfactory
epithelium
The olfactory bulb carries out a computational analysis of
odor stimuli
This olfactory bulb of a living mouse
has been stained with a dye that
reveals all of the superficial glomeruli.
If the animal were stimulated with a
sample of its home cage air,
approximately 6 glomeruli would
become active. The odor of camphor
lights up about the same number of
glomeruli, but they are at different
locations from the ones reactive to
cage air. However, as far as we know
now, there is no cortical map of
glomerular locations.
Olfactory cortical cells gain specificity for particular
molecules by integrating input from glomeruli that
recognize different features
Olfactory pathways go to both the cortex and the
limbic system
Main
olfactory
epithelium
Vomeronasal
organ
Olfactory bulb
Accessory
olfactory
bulb
Thalamus
Orbitofrontal
cortex
Limbic system
In humans, the main olfactory pathway branches
to go to the cortex and the limbic system
To cortex
To limbic
system
Impacts of odor information on behavior and
physiology in humans
• Exposure to the odor of males can make
female menstrual cycles more regular
• Pheromonal communication between
women living in close contact can cause
menstrual cycles to become synchronized
• A pheromone apparently produced by all
individuals promotes friendliness and
social behavior.
The vomeronasal organ is part of a separate path for
olfactory information to the limbic system in non-human
mammals
Pheromonal Communication in non-human
animals
• Female estrus pheromones elicit male sexual
behavior; male pheromones facilitate female
receptiveness.
• Urine from an unfamiliar male mouse causes
pregnant female mice to abort their pregnancies
and initiate ovulation
• Individual-specific odors are important in mate
choice in rodents – because they carry
information about how closely related the choice
animal is to the chooser.