Transcript II Sensory - Washington State University

II Sensory
Chemoreceptors: A diverse and
evolutionarily ancient class of
receptors
Olfactory Receptor Neurons and the Olfactory
Epithelium
The olfactory system is designed to detect
volatile chemicals present in the air
• Airborne chemicals reach the olfactory
epithelium from the external air (or from
volatile components released as food is
consumed).
• The pathway followed by olfactory neuron
axons crosses the cribiform plate and
enters the brain: this is one path into a
highly protected zone…
Primary olfactory neurons can
regenerate
• the neurons turn over with a half-life of 1-2
months.
• Exposure to chemicals can lead to olfactory
neuron death – for example, zinc salts can be
used to eliminate them experimentally.
• Accidents that accelerate or decelerate the brain
relative to the skull can sever the axons of
olfactory neurons as they exit the cribriform
plate, leading to a temporary anosmia.
Olfactory
neurons
and
support
cells and
the
neuron’s
membrane
features
that allow
integration
and action
potential
generation.
Illustration of how
the graded
receptor potential
is transformed into
spikes where the
decrementally
conducted signal
exceeded
threshold and
active channels
are present. In the
axon, there are
only spikes seen
traveling to the
CNS.
Airborne or aerial chemical
detection requires special features:
• The mucus (illustrated 2 slides back) is both
watery and viscous. Mucus protects the
sensitive cilia of the receptor neurons from
drying, but it also does something much
more crucial in the transduction process…
• The chemicals that we can smell are
typically hydrophobic – so the problem of
dissolving in the watery mucus has to be
solved. The next slide has information
relevant to this process.
In the secreted
mucus there
are odorantbinding
proteins:
beta barrels -->
• Odorant-binding proteins
are secreted (by
Bowman’s glands) in the
olfactory mucus of all land
vertebrates. The basic
structure of these diverse
proteins (there may be
over 2,000) is that of a
beta barrel; two barrels
unite to form the
functional dimer, which
holds an odorant molecule
inside.
Electrical
responses
to
chemicals
indicate
that the
receptors
for
odorants
are
present
on the
cilia
Nature of the olfactory receptors
Note: these are distinct from the odorant binding molecules
• 1. 7 transmembrane-spanning regions coupled
to G proteins.
• 2. Large gene family devoted to these receptor
proteins – 1000 genes in dogs and mice, 400 in
the human genome.
• Each receptor neuron expresses only one of the
receptor genes; all the cells with the same
odorant sensitivity project to the same
postsynaptic cells in the olfactory bulb. This is
the first step in neural coding of odorants.
The “labeled-line”
organization of
olfactory receptors: first
revealed by exposing a
rat to the pure odor of
green bell pepper! (All
the receptors could be
shown to have the
receptor for that odor
and they all could be
traced to the same
glomerulus in the
olfactory bulb.)
The olfactory transduction cascades – Yes,
there are 2…
• 1. Adenyl cyclase pathway: G protein activation
leads to activation of adenyl cyclase. Cyclic
AMP opens cation channels admitting Na+ and
Ca++. The Ca++ then opens a Cl- channel that
further depolarizes the cell.
• 2. IP3 pathway: The activated G protein activates
phospholipase C which generates IP3; the IP3
opens cation (Na+ and Ca++) channels and Ca++
opens Cl- channels.
The adenyl cyclase pathway: part 1
Adenyl cyclase, second stage…Golf stands for the
olfactory form of the G protein in olfactory cells
The IP3 Pathway
Second messenger families: some
examples of odors that evoke them
Switching Modalities: Taste
Taste = Gustation
Taste cells: modified epithelial cells
• Cells regularly wear out and are replaced
by division of stem cells (basal cells).
• There are at least the following tastes:
sweet, sour, bitter, salty, umami, metallic
(?) -- and therefore taste cells possess
multiple transduction mechanisms.
• More than 90% of the cells respond to two
“tastants”, and many respond to all…
• There is a lot of diversity in taste
competence among the vertebrates
Ordinary epithelial channels (ENaC) are used by both salt and
hydrogen ions, and H+ can block K+ channels
Sweet receptors can be tricked by a wide variety of molecules
that are not similar to glucose…
Bitter has multiple transduction mechanisms
PDE is
phosphodiesterase;
gustducin
got its name
before we
realized that
it is just a gprotein.
Umami reception is for amino acids, such as “Accent”
(monosodium glutamate)
Taste transduction has mainly been studied
in animals like catfish and rats…
• Many differences exist in the taste capabilities
and mechanisms of different vertebrates – for
instance, rats can taste pure water – whether or
not the information is relevant to human
gustation is unclear.
Gustation issues relevant to human
biology and health
• The chemosensory senses decline with age,
posing problems for elderly people with regard
to the palatability of food and the ability to detect
food spoilage.
• There are significant genetic differences
between individual human subjects in ability to
taste bitter molecules – due to the fact that there
are 3 families of receptor proteins. These
differences significantly affect food choice.