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Taste and Smell
How many things can we
taste, how many things do
we smell?
4 tastes
Research has demonstrated that
there are only four basic taste
sensations: salty, sour, bitter, and
sweet. Further, each of these
sensations is associated with a
different area of the tongue.
To demonstrate some or all of
the four basic taste sensations
• Take some toothpicks, and dip them in sugar water,
lemon juice, Angostura bitters and salt water
• Then touch each toothpick to the various parts of
your tongue identified in the diagram
– sugar water will taste the sweetest on the tip of
your tongue
– lemon juice should taste the sourest on the sides
of your tongue
– salt water should taste the saltiest just in back of
the tip of your tongue
– Angostura bitters you can test the area at the
back of your tongue for its ability to detect bitter
tastes
What is “Bitter?”
•
Some people are much more able to taste
bitter sensations. Those who dislike diet soft
drinks usually say they do not like them
because they leave a bitter aftertaste.
• Some people are not able to taste that bitter
sensation.
• Also, most poisons are bitter. So, it may make
evolutionary sense for people to dislike bitter
tastes.
• In fact, growers have developed strains of
celery that are not bitter for that reason.
Senses of Smell and Taste
• taste and smell represent phylogenetically
old sensory system
• they are the sensory systems that let us
detect and discriminate the molecules in
our environment
• these senses help to link the external
environment with internal needs, e.g.
hunger, thirst, etc.
• are vivid emotionally and perceptually
• important nutritionally for regulating food
intake
Taste (gustatory) -- transduction of
soluble chemicals
Taste is sensed by gustatory
receptor cells (taste buds)
on the tongue, palate, and
pharynx, with some on the
epiglottis and upper
esophagus
Different regions of the
tongue are more sensitive
to either bitter, sour, salty,
or sweet.
Where’s the taste?
• sweet - associated with organic
molecules sweetest compounds are
the proteins, thaumatin and monellin
can be detected at concentration of
10-8 M
• bitter - often associated with harmful
stimuli not know which chemical
structures elicit the taste
• sourness - acids elicit sour taste
• saltiness - a molecule that breaks into
its ions, the ions pass through a
channel in the cell membrane
drawing of taste bud
Microscopic view of taste
bud
• each taste bud
contains 50-150
receptors (S and N),
basal cells (B), and
support cells
• the taste bud
contacts substances
through the taste
pore (arrow at top of
image)
taste buds (TB) are found on papillae
How are the different tastes
encoded?
• different receptors respond preferentially to 1 of the 4 basic
tastes
• taste receptors are differentially distributed across the tongue
• specific pathway theory -- nerve fiber responds to only one taste
(labeled-line)
• across-fiber pattern coding -- central neuron compares pattern
of input from a group of fibers, each fiber is more sensitive to a
particular taste but also responds to other taste stimuli
• probably both theories are correct
• taste coding involves comparison of activity across groups of
fibers responding preferentially to different tastes (contrast)
Smell
Smell
•some at concentrations of only a few parts per trillion
•odors enter the nasal cavity
•olfactory binding protein attaches to the odorant molecule
•odorants cause olfactory neurons to depolarize
•Chemically, in order for a molecule to possess a scent, it
must have at least the following two qualities:
•It must be somewhat volatile so that it can reach the
nose
•It must interact with the protein receptors located in
the nerve endings.
Smell
The process of smell
When an animal inhales odorous molecules, these
molecules bind to specialized proteins, known as
receptor proteins, that are found in the nerve endings
in the olfactory epithelium.
Binding of odors to these receptors initiates an electrical
signal that travels along the axons to the olfactory
bulb, which is located in the front of the brain, right
behind the nose.
The olfactory bulb serves as the first relay station for
processing olfactory information in the brain; the bulb
connects the nose with the olfactory cortex, which
then projects to higher sensory centers in the cerebral
cortex, the area of the brain the controls thoughts and
behaviors.
Smelly facts…
• fruity, floral, and herbaceous most potent
scents
• putrid stimuli are much weaker scents
• individual olfactory neurons respond to
multiple odorants
• seems likely that individual neurons
respond to only a few distinct odorant
molecules
• the sensation of smell rapidly adapts
• repeated presentation of an odorant
results in reduced sensitivity
• sensitivity to other odors can be effected
• smell is more sensitive than taste
• Humans can discriminate 5% change in the
intensity of an odorant
• Taste Preferences -- Learned and Innate
– Chocolate Lover or Broccoli Hater? Both genes
and experience effect which tastes we prefer.
– intake of food greatly effected by the smell and
taste of the food
• We have developed extensive nomenclature
to describe foods (e.g. coffee tasting
terminology or wine tasting terminology)
that derives from the smells, flavors, and
other sensations associated with the food
• When deprived of salt we will seek out foods
high in salt to compensate
Some of what we have
learned…
• obvious evolutionary advantage to not
eat a particular food that makes you
sick
• taste and smell allow us to perceive
chemicals in our environment
• taste and smell sensations seem to be
coded following the same general rules
used by other sensory systems
Straight shot to the cerebral
cortex
Note that the olfactory system is
the only sense system that does
not synapse through the thalamus.
Rather it is connected directly to
the cortex.
Your House Stinks!
• When you return to your house after a
long absence, it always seems to smell
worse than usual. Well, it always smells
that way.
• It is just the phenomenon of sensory
fatigue. As you live in your house, you
no longer can perceive the smells
around you. But, while on a trip, your
nose can recover. Then when you
return, you smell your house like
everyone else smells it.