NROC64H3 Lecture Notes - Microvillus, Potassium Channel, Quinine

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17 Mar 2012
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Chapter 8 The Chemical Senses 285 to 308
Introduction
- Life evolve in a sea of chemicals
- The most familiar of chemical senses: taste/gestation and smell
- Many types of chemically sensitive cells chemoreceptors
distributed throughout body
- Gustation and olfaction serve to detect environmental chemicals
Taste
- Humans evolved as omnivores (Latin: omnis-all, vorare-to eat), eating
plants and animals
The Basic Tastes
- We only recognize basic taste 5
- Saltiness, sourness (acids H+),
sweetness (sucrose), bitterness (simple
ions K+ and Mg+ for complex organic
molecules like caffeine), umami (MSG)
- Each food activates different combination
of basic tastes and distinctive flavour as
result of taste and smell
Organs of Taste
- Tongue, palate, pharynx and epiglottis
- Odors from food can also pas from
pharynx to nasal cavity
- Papillae (Latin: bumps): Shaped like
ridges (foliate papillae), pimples (vallate
papillae) or mushrooms (fungiform
papillae)
- Taste Buds, contains 50-150 taste
receptor cells/taste cells w/I bud
epithelium, also has basal cells that
surround taste cells
- Usually 2000-5000 taste buds
Taste Receptor Cells
- Chemically sensitive part of taste receptor
cell is small membrane regions called apical
end, near surface of tongue have thin
extensions called microvilli
- When taste receptor cell is activated by chemical membrane potential
changes (depolarizing) receptor potential (if depolarizing and large enough,
most taste receptor cells cause v-gated Ca channel to open Ca enters cytoplasm
to trigger release of transmitter molecules
Mechanisms of Taste Transduction
- Transduction (Latin- transducere “to lead across’): Stimulus causes an
electrical response in a sensory receptor cell
- Taste transduction involves several different processes; taste stimuli may 1.
Directly pass through ion channels (salt and sour) 2) bind to and block ion
channels (sour), or 3) bind to G-protein-coupled receptors in membrane that
activate 2nd messenger systems that open ion channels (bitter sweet, and umami)
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Saltiness
- Prototypical salty chemical and table salt (NaCl) which is major component of blood, ocean and chicken coup
- Cation Na_ and concentration must be high to taste it
- The amiloride-sensitive Na channel is quite different form voltage-gated channel that generates AP: the taste
channel is insensitive to voltage and stays open all the time
- The anions of salt affect the taste of the cation the larger an anion is, the more it inhibits the salt taste of the
cation
Sourness
- Food taste cour because of thei high acidity (low pH)
- Acids, such as HCl, dissolve in water and generate H-ions (protons of H+)
- H+ permtes amiloride-sensitive Na channel inward H+ and
depolarization
- H+ ions bind and block K+ selevtive channel depolarization
- Changes in pH affect all cellular processes
Bitterness
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