Taste and Smell

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Taste and Smell

• How many things can we taste, how many things do
  we smell?

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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.

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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

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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.

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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  

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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.

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Wheres 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

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drawing of taste bud
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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)

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• taste buds (TB) are found on papillae

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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)

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Smell
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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.

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Smell

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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.
  

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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

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• 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

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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

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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.

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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.