Windsurfing has been my obsession for 25 years, but Im now'

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Windsurfing has been my obsession for 25 years,
but Im now.
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Kiteboarding, which is possible here in winter on
the snow and ice!
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Lecture 21 - Smell and taste in animals Reading -
Concept 49.3, page 1054
Olfaction in mammals. olfactory sensory neurons
express one of many odorant receptor proteins
they send axons to glomeruli in the olfactory
bulb of the brain olfaction is a combinatorial
code mammals differ in the numbers of odorant
receptors general and specific
anosmias Vomeronasal organ. specialized for
pheromones specialized subset of receptors
Taste in mammals. taste involves at least 6
modalities and additional sets of 7TM receptors
Olfaction in insects. olfaction works in a
similar fashion to mammals except olfactory
sensory neurons are split up into
sensilla Chemoreception in nematodes. worms have
many more chemoreceptor proteins than insects
4
Smell is an crucial sense for mammals.
5
Smell (one aspect of chemoperception, along with
taste) is the oldest sense, going back all the
way to bacteria, and is prominent in most
animals, sometimes being their primary
sense. Many mammals are more capable than us,
either in terms of sensitivity of smell, e.g.
dogs, or range of odors detected, e.g.
rats. Most other mammals use sense for social
communication too, such as territorial
advertisements, and for sexual signaling, e.g.
detection of oestrus via chemicals in female
urine. Some use scents as weapons, e.g. skunks
male ring-tailed lemurs fight by smearing scent
from their wrist glands on their long tails, and
then flick their tails at each other. On Meerkat
Manor you see them furiously scent-marking. Some
mammals have lost their sense of smell
completely, e.g. dolphins and whales. We are
able to differentiate 10,000 odors, but how?
6
Overview of mammalian olfaction This was all
figured out by neurophysiologists
The odorant receptors are in tiny cilia on the
ends of olfactory sensory neurons in our nasal
epithelium. These neurons send axons to glomeruli
(collections of axon termini and dendrites of
mitral cells) in the olfactory bulb of the brain,
from where the signals are relayed to the rest of
the brain by the mitral cells.
7
In 1991 Linda Buck and Richard Axel at Columbia
University in New York published the odorant
receptors in rats. In 2004 they received the
Nobel Prize in Physiology or Medicine for this
and subsequent work. Although they are the same
age, Buck was a postdoc associate in Axels lab.
She then obtained her own lab, first at Harvard,
and now in Seattle.
8
2D structure of an odorant receptor protein with
seven transmembrane domains in a cell
membrane They are about 300 amino acids long.
Outside where the odorants are passing by
Transmembrane domain
Individual amino acid
Phospholipid bilayer comprising cell membrane
Inside the cilium of an olfactory sensory neuron
9
No odorant receptor has been crystallized, so we
can only model their 3D structure in the neuron
cell membrane.
membrane
membrane
A condensed 2D view of 7TM receptor
A 3D model of 7TM receptor
10
Most mammals have 1000-2000 genes encoding
odorant receptors, which is 3-6 of their gene
count of 20-30,000 genes.
These genes occur in clusters of various sizes on
most of the chromosomes.
11
Each odorant receptor protein is slightly
different in the amino acid sequence encoded by
its gene. Therefore each one can detect a
different suite of odorants. A phylogenetic tree
of this huge vertebrate OR gene family shows that
fish (red) have many subfamilies, while two
subfamilies have hugely expanded by gene
duplication in frogs (green), birds (blue), and
mammals (purple). These are thus often paralogous
genes.
12
A range of organic chemicals with different side
groups that were tested for binding to a subset
of the ORs.
13
Each odorant receptor can detect and be activated
by a different range of organic chemicals, thus
there is a combinatorial code.
14
Each olfactory sensory neuron expresses only one
odorant receptor protein out of 1000-2000 genes.
15
All the olfactory sensory neurons that express a
particular odorant receptor protein send their
axons to one tiny region of the olfactory bulb in
the brain, called a glomerulus. So there are
1000-2000 glomeruli.
Olfactory bulb in brain
axons
neurons
Nasal epithelium
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What we interpret as a smell is a subset of
glomeruli being differentially activated by the
differential binding of odorants to several
different olfactory receptor proteins. This
overall picture of glomerular activity is
conveyed to the rest of the brain by interneurons
called mitral cells.
Mitral cells send this combinatorial code to the
rest of the brain.
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So the odorant receptors (ORs) are truly
remarkable 1. It turns out that expression of
one OR in an olfactory sensory neuron inhibits
expression of any others, hence the rule of one
receptor per neuron, and it is the OR protein
itself that somehow does this inhibition. 2. ORs
recognize overlapping sets of chemicals, allowing
a combinatorial code. 3. ORs are also present at
the ends of the sensory neuron axons and
facilitate the convergence of all neuron axons
expressing a particular OR to a single
glomerulus, allowing mitral cells to communicate
the combinatorial code to the rest of the
brain. We dont fully understand the details of
how they do each of these things, let alone all
three, so there is lots of room for more research.
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Mammals differ enormously in the size of their
nasal olfactory epithelium, with humans among the
smallest this accounts for differences in
sensitivity with dogs among the best. We have
also lost many of the odorant receptor genes we
once shared with other mammals, down to 300
functional genes, yet we can differentiate
10,000 smells. Rats have over 2000 functional OR
genes and probably can differentiate even more
smells.
19
Anosmias or loss of smell General anosmias
involve general loss of smell, and can result
from a variety of causes, including genetic
(congenital), trauma (accidents), viral
infections (rarely), and even a deficit or excess
of zinc, which somehow is needed for receptor
function. They cause significant problems,
including loss of much of the pleasure of taste,
but not taste itself. Partial or specific
anosmias involve loss of ability to smell
particular molecules, and generally involve
mutational loss of particular odorant receptors.
For example, there is considerable variation in
the ability of people to smell androstadienone,
which is a candidate for a human pheromone.
20
Vertebrates also have a second olfactory organ
called the vomeronasal organ (VNO) with its own
quite different set of 7TM vomeronasal receptors
in neurons sending axons to an accessory
olfactory bulb in the brain.
21
In mammals the VNO is primarily responsible for
detecting pheromones (social and mating signals),
which we humans dont do much of, so our
vomeronasal organ is vestigial and appears to
have no function. In other vertebrates like
snakes the VNO is involved in general olfaction.
(and infra-red or heat-sensing pits)
22
The flehmen response by some male mammals on
smelling female urine is a way to transport the
pheromonal odorants therein to the VNO.
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Taste
Mammals taste chemicals using taste sensory
neurons with taste receptor proteins in the taste
buds on our tongues. There are at least six
separate modalities, sweet, sour, salty, bitter,
umami (e.g. MSG/meaty), and fatty. Whole sets of
different receptors mediate these modalities, and
each taste bud has sensory neurons for most of
them.
www.unmc.edu/Physiology/Mann/mann10.html
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We study all four of these kinds of insects.
25
Insect antennae (and mouthparts, legs, and
elsewhere) have many long sensilla that contain
the olfactory sensory neurons, which in this case
go to glomeruli in the antennal lobe of the brain.
sensillum
26
The insect chemoreceptors were published from
Drosophila in 1999. Again they are 7TM proteins,
but have very different amino acid sequences from
the mammalian ORs or VRs or Taste receptors.
There are about 100-200 of them.
27
And like mammals, each olfactory sensory neuron
expresses just one OR, and all those ORNs send
their axons to a single glomerulus. So once again
insect olfaction involves a combinatorial code.
This can be visualized in the antennal lobe of
the bee brain where different subsets of their
160 glomeruli are active in response to
different chemicals.
Octanol
Clove oil
Glomeruli
28
Nematode worms like Caenorhabditis elegans are
almost completely dependent on their sense of
smell/taste or chemoperception. In 1995 their
own distinctive set of 7TM chemoreceptors was
discovered, and I spent the next decade building
this gene family to a total 1700 genes, which is
8 of their gene complement, but we dont know
what most do.
Olfactory sensory neurons
A nematodes nose
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This research occurs at the intersection of
multiple scientific fields, and integrates
evolution, ecology, ethology, neurobiology,
genetics, molecular biology, and chemistry. Thank
you and I hope you will join us in IB as majors!
Evolution, ecology, ethology
Physiology behavior
Genetics and molecular biology