TRPV1 capsaicin receptors sense pain and heat. We have also learned that TRPV1 capsaicin receptors have cousins that sense cold - TRPM8 and TRPA1. They may generate pain, and they certainly help to warm us when we are cold.
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).
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.
A 2008 study showed that many snakes rest their jaw bones against the ground. The vibrations caused by moving animals are transferred from the ground to the bone, and from the bone to the buried cochlea. The sensation in the brain is a lot like muffled knocks, not unlike the bass that is turned up too loud in peoples’ cars.
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.
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?
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.
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.
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) -