Copyright ©Mark Nelson, 2002. All rights reserved.
Chapter 13: Chemical Senses
What you need to know
(exam questions will be a drawn from this subset of material)
What are three broad categories of chemical senses? Why are they considered
separate senses? (p. 309-310)
1) gustation (taste)
2) olfaction (smell)
3) internal chemoreception (pH, O2,
CO2)
they are considered separate senses,
because the inputs are processed in different regions of the CNS
Where do vertebrate gustatory receptors project in the CNS? (p. 310-311)
first to the medulla (solitary
nucleus), then to the thalamus (diencephalon), then to the cortex (telencephalon)
Where do vertebrate olfactory receptors project in the CNS? (p. 311-312)
first to the olfactory
bulb (telencephalon) then to olfactory cortex (telencephalon)
What is unique abou the pathway from olfactory recepetors to the telencephalon?
(p. 311-312)
olfactory inputs "bypass"
the thalamus; they have a more direct route to the cortex and limbic
system;
(however, there is a side-loop that goes to the thalamus
and back to cortex, so they don't bypass it completely)
olfactory memories often seem to carry more emotional
content than other sensory memories
Whats similar about the microenvironment around gustatory and olfactory
receptors? (p.312-314)
the both have ciliary
processes that are protected by a liquid or mucus microenvironment
air-borne odorants are dissolved in the mucus layer of
the olfactory epithelium before they are transduced
What is the "typical" way that chemoreceptors transduce chemical stimuli?
(p. 312-321)
most commonly chemosensory
transduction involvesa G-protein mediated second messenger cascade
What special mechanisms are involved in transduction of sour (acidic) tastes?
(p. 317, Fig. 13-5A)
pH-gated ion channels
What special mechanisms are involved in transduction of salty tastes? (p. 317, Fig. 13-5B)
direct entry of the salt
cation (Na+, for example) through open ion channels
What type of sensory coding is used in the gustatory system? (p. 321-322)
it's a mix between labeled-line
and across-fiber coding, "partly labeled line"
there are a few distinct labeled categories (salty, sweet,
sour, bitter)
but the pattern of responses across many fibers is important
for identifying individual flavors
What type of sensory coding is used in the olfactory system? (p. 322-324)
certain odorants, such
as pheromones, are coded using a labeled line system
more general odorants are coded using an across-fiber
code
How many kinds of vertebrate olfactory receptor proteins are there? How many
kinds does a single receptor express (p. 322-324)
approx 1000 different
receptor proteins; an individual receptor cell only expresses one, or at
most a few, of these
How do signals get from the vertebrate olfactory receptors to the brain?
(p. 324-325; Fig. 13-10)
the receptor cells send
projections through the cribiform plate to glomeruli in the olfactory bulb
What are olfactory glomeruli? (p. 325-326)
small spherical clusters of neurons
in the olfactory bulb
each glomerulus tends to receive inputs from receptors
that respond to similar odors
so different groups of glomeruli respond to different
types of odors
Where are you likely to find insect chemoreceptors? (p. 326-327)
on the antennae, feet,
body; insects don't have a "nose"
What's similar about olfactory receptors of insects and vertebrates? (p.326-328)
receptors have ciliary
prcesses surrounded by fluid
transduction mechansims are similar (G-protein mediated
second messenger systems)
What's similar about the initial stages of olfactory processing in the CNS
of insects and vertebrates? (p.329-332)
they both have glomeruli
that are specialized for processing certain types of odors