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