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We
have been talking about taste sense for many weeks. I
remember
a 1975 movie called, A Boy And His Dog, starring a
very
young Don Johnson. It was a post-apocalyptic story of a
guy,
his dog, and cannibalism. The best line of the movie? “Well,
she
might not have had good taste, but she sure tasted good.”
Of
course, this isn’t the kind of tastes we have been talking about.
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We have even learned that in rare cases, the cold receptors
can be heat sensors, like in chickens and insects where TRPA1 sense hot instead
of cold. And this leads us to today’s exception. It’s time to talk about how
these relatives of taste receptors help animals to become better hunters and to
better sense their environment. Today let’s focus on snakes.
Snakes have a number of ways to catch prey (see this post).
Some lie in wait, blending in with the jungle or background until a moving
potential dinner catches their eye and moves across their path. Vision is their
primary way of finding dinner. As a consequence, most sight-hunting snakes are diurnal (active in daylight).
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Here
is the southern black racer. You can see it has big eyes with
round
pupils so lots of light can enter – it’s a sight hunter. Many
grow
to be 5 ft. (1.5 m) long, so they can look intimidating. But
they
are not venomous and will usually exit the seen if disturbed.
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Other snakes use the combination of scent and taste that we
talked about a while back. The Jacobson
organ (more scientifically called the vomeronasal
organ, VNO) in their mouth can sense the molecules that the tongue pulls in
from the air. Like it or not, every organism has molecules floating off of them
continuously. Snakes' VNO can pick these up. See this post for more on the VNO.
Some snakes “hear” their prey coming. True, snakes don’t
have an outer ear opening or the small bones that convert sound waves into
mechanical waves in our middle ear (see this post for an explanation). But they
do have a cochlea, the organ for
sensing the vibrations and converting them to a nerve signal. Many snakes can
sense the vibrations that their prey generate when they move through the
environment using this cochlea and their lower jaw.
Similar to something called bone conduction hearing in animals with ears like ours, vibrations that travel
through the bone can also cause movement in the hairs of the cochlea. As we
discussed previously, the bending of the sensory hairs of the cochlea are
transduced to chemico-electrical signals that travel to the hearing centers of
the brain.
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This
is from a scientific paper showing the bone hearing of a python.
The
red is the lower jawbone. The bark blue is the quadrate bone
and
the green is the equivalent to our stapes bone of the middle
ear.
The light blue is the inner ear space and the purple is where
the
cochlea is housed. Vibrations go from red, to blue, to green to
light
blue, to purple. You can see how sound waves would find it
tough
to get to the cochlea.
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This was followed by a 2012 study that showed pythons have
very sensitive vibratory hearing, but poor sound pressure hearing. Almost all
their hearing input comes from the vibrations they sense in the ground or tree,
or whatever they happen to be lying on. So be on tip toes, that snake may hear
you coming.
But how does any of this relate to a receptor for painful
cold and controls mammalian breathing rate? Well, another way some snakes find
their prey is by sensing the heat they give off – even from a few meters away.
Pit vipers are a subfamily of the Viperdae family, called Crotalinae.
There are two types of vipers; all of them have hinged fangs, the ones that are
folded up into the upper jaw when the mouth is closed, but protrude for striking
as the mouth is opened. Pit vipers differ from true vipers in that they have
pits (duh!); more about these below. True vipers live exclusively in Africa and
tropical Europe and Asia.
In America, where I live, there are a lot of pit vipers.
Cottonmouths, rattlesnakes (all 30 species), water moccasins, copperheads –
these are all pit vipers. From southern Canada to Argentina, and from Eastern
Europe to parts of Asia, pit vipers are not rare. Eyelash vipers (Bothriechis schlegelii) of
South America are arboreal (live in the trees). They have bright
coloring, but sit still and wait for their prey to happen by. They strike from
above, so they scare the heck out of jungle hikers.
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On
the left is the eyelash viper. You can see it doesn’t mean business
because
its hinged fangs aren’t extended. In the middle is the two-striped
forest
pit viper. It is protecting it’s young, so the fangs are extended. On
the
right is a sidewinder rattlesnake. Sidewinders are amazing and will
get
their own post soon.
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But it’s specific part of the pit viper that we are
interested in today – namely the pit. The pit organ is located between the eye
and the nostril, on each side of the snake’s head. It is a hollow pit, so the
actual business end of the pit organ is inside the snake’s skull.
The pit is lined with epithelium, but it also has a membrane
that is stretched across the base. As a consequence of the location membrane,
there are air pockets on each side of the membrane. The trigeminal nerve
innervates the membrane and there are thermosensors in the cells of the
membrane.
So, the pit organ is a thermosensor that helps them locate
prey animals (or predators). But wait you say. Sure, pit vipers may use a
thermosensitive ion channel to sense the heat given off by passing prey
animals. But we just said they use a COLD sensing ion channel, TRPA1. What
gives?
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The
pit on a pit viper is located between the nostril and the eye.
It
would be easy to mistake the pit for the nostril. The cartoon
shows
the pit anatomy. The air chamber helps cool the air
quickly
and stops the TRPA1 receptors from firing again. This
is
so the snake won’t get a residual image of something warm,
when
the target may have moved in the interim period.
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So maybe it’s not so terribly bizarre that pit vipers use
TRPA1 to sense their prey. But before
they touch it??? We eat chili peppers and we react to the capsaicin in our
mouths and noses. We go out on a summer day, and the heat activates our TRPV
receptors in skin and other tissues. We eat something cold (or menthol) and we
feel the cold sensations it touches or tissues. But snakes feel the heat of
their prey before they eat, from a distance away! There must be more at work.
And there is. The TRPA1 ion channels in the pit organs of pit
vipers have a mutated version of TRPA1. Here’s how things work according to a 2010 study that identified TRPA1 as the heat sensor. The pit is a hole with a
membrane stretched toward the back. Consequently, there is an air chamber on
both sides of the membrane. The
membrane is highly vasculature and has the sensitive nerve endings with the
TRPA1 channels.
The TRPA1 receptors are always firing, but at a low rate.
Neutrally warm objects don’t change the firing rate, but warmer objects (as
little as 0.001 ˚C warmer than background) will increase the firing rate. The
receptor is mutated according to a 2011 study, with 11 amino acids of the pit
TRPA1 divergent on only pit-containing snakes. These changes make the receptor
so sensitive that it can react to infrared light signals (heat) from several
feet away. That would be like our mouth burning over a chili pepper that we
walked past in the supermarket.
Since the sensors are spread across the entire membrane, the
effect on locating the source is sort of like vision or a pinhole camera. Light
passes through the pupil and diverges before it hits the retina. This provides
for a larger spread of the “image” across the membrane and allows for precise
two-dimensional map of the target. The difference in heat between the target
and the background gives a “picture” of the object that is warm.
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The
Taylor’s Cantil viper will play dead and then strike, but this
brings
up an important point. DON’T get near a pit viper, even if
you
are sure it’s dead. The pit is wired directly to the brain and
muscles.
A dead snake, even one with a severed head, can still
strike
as long as there is any residual neural electrical flow. People
die
every year from snake bites from dead snakes.
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The picture generated is also a little like hearing, since
the heat will reach one pit earlier or more strongly. By comparing the timing
and the strength of the signals from each pit, the distance and direction to
the target can be detected by the brain (see this post for localization of
sound waves).
Because the heat “picture” pit vipers pick up is based on
the difference between the temperature of target and background, most pit
vipers hunt when coolest, so temperature gradient between environment and prey
is greatest. Prey will stick out the most.
Snakes can also use the pit more conventionally, as a
thermosensor for its whole body. The basal rate of firing will tell the snake
when to move to shade if it’s too warm or move to sun if it’s too cold. This is
how it regulates its body temperature.
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Pythons
and boas can also have heat-sensing pits, but they are
5-10
times less sensitive because of their differing anatomy.
The
amazing thing is that they evolved the same special power
independently
from pit vipers, although they both use mutated
versions
of TRPA1. The nostril has a black arrow and the pits
have
red arrows.
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Boas and pythons with pits have 3-4 simple pits in their upper
lips. They don’t have the suspended membrane for sensing temperature, the TRPA1
sensors are housed within the epidermal cells at the back of the pit.
Next week – vampire bats and mosquitoes get into the mutated
thermosensor act as well.
For
more information or classroom activities, see:
Pit
vipers –
Bone
conduction hearing –
VNO
(Jacobson organ) -
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