The Chemical Senses
Chemoreceptors
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Chemically sensitive cells located throughout
the body to monitor:
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Irritating chemicals on skin or in mucus
Ingested substances in digestive organs
Levels of carbon dioxide and oxygen in blood
Acidity in muscles indicating oxygen debt
following exertion
Gustation (taste) and olfaction (smell) –
processed in parallel and merged in cortex.
Basic Tastes
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Sweet – things that are good for us tend to
taste sweet.
Bitter – things that are toxic (poisonous) tend
to taste bitter.
Salt
Sour
Savory (umami) – associated with proteins
and found in meat (MSG – monosodium
glutamate).
How Taste Works
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Taste buds (taste receptor neurons) line
papillae found in different areas of the tongue.
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Each papillae responds to one taste (sweet, sour)
when the stimulus is weak but multiple tastes
when the stimulus is strong.
Identification of tastes occurs in the brain.
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Population coding – responses of a large number
of broadly tuned neurons specifies the taste.
Transduction
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Transduction – the process by which an
environmental stimulus causes an electrical
response in a sensory receptor cell.
Tastants (tastes) use multiple mechanisms:
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Pass through ion channel directly (salt).
Bind to and block ion channels (sour & bitter).
Bind to and open ion channels (amino acids).
Activate second messengers in complex ways
(sweet, bitter, umami).
Taste Pathways
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Taste buds send information to primary gustatory
axons.
Axons go into the brain stem to the ipsilateral
thalamus (VPM) and then to the primary gustatory
cortex.
Conscious taste is mediated by the cortex.
Control of feeding (swallowing, saltivation,
vomiting, digestion) is controlled by medulla.
Motivation to eat is controlled by hypothalamus.
Supertasters
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Supertasters tend to dislike things other
people like to eat: brussels sprouts, brocoli,
spinach.
Excess sensory receptors for bitter flavors in
the taste buds.
Non-tasters have fewer sensory cells.
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In danger when children because they will eat or
drink anything.
Smell (Olfaction)
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Combines with taste to help us identify food
and increases enjoyment of flavors.
Warns of potentially harmful substances or
places.
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Only 20% of smells are pleasant.
Pheromones released by the body are signals
for reproductive behaviors, identify
individuals, mark territory and dominance.
Olfactory Epithelium
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We smell with a thin sheet of cells located
high in the nasal cavity.
Three cell types:
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Olfactory receptors – tranduction of smell to
neural activity.
Supporting cells – produce mucus, like glia.
Basal cells – source of new olfactory receptors.
Receptors die and are replaced every 4-8 wks.
How Smell Works
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Sniffing brings air through the nasal passages
to the olfactory epithelium.
Odorants (chemical stimuli in the air) dissolve
in the mucus layer before reaching receptors.
Odorants then bind with cilia of the receptor
cells causing G-protein activation resulting in
an action potential.
Olfactory Pathways
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Axons from the olfactory receptors form the
olfactory nerve.
The axons penetrate a thin layer of bone
called the cribiform plate, then enter the
olfactory bulb.
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Axons map onto glomeruli in the bulb.
Anosmia – inability to smell due to severing
the olfactory axons at the cribiform plate.
How Smells are Identified
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Initial processing at the glomeruli separate
smells into broad categories.
Information passes from the bulbs into
olfactory tracts (bundles of axons) projecting
to primitive regions of cortex, then to the
thalamus, and finally to the cortex.
Parallel pathways process smell in many areas
of the cortex.
How is Smell Coded?
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Three ways of telling smells apart:
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Population coding – combinations of responses
form patterns related to specific smells.
Sensory map – activation of different areas of the
glomeruli correspond to specific odors. The form
of a map for each odor may be distinct.
Temporal coding – the timing of action potentials
along the axons may differentiate smells.
Number, temporal pattern, synchronicity, rhythm.