Sensory Systems/Snakes

Introduction
When seeing or sometimes even thinking of snakes, many people feel uncomfortable or even scared. There is a reason that they are considered being mythical. Snakes are different when compared to other animals: they do not have legs, they are long and move elegantly and without a noise, some of them are venomous and they steadily use their forked tongue to smell. Some of them are fast and effective killers even by night. Something that definitely makes them special is their „sixth sense“: the ability to detect infrared radiation. Similar to night viewers, snakes are capable of detecting heat changes in their surroundings and thus obtaining a detailed picture of it. There are at least two different groups of snakes which have separately developed this ability: in the first are the pit vipers, and in the second boas and pythons (those two are often classified into one group called “boids”). However, snakes are not the only species which have evolved this sense: vampire bats and some groups of insects have also developed it. Even at night pit vipers, boas and pythons can make out rodents due to the heat they emit. It can be detected by a sensory system that allows them to „see“ electromagnetic radiation with long wavelengths ranging from 750 nm to 1 mm. The organs which make that possible are called “pit organs”, and are located under their eyes, inside two hollows of the maxilla bone. They are immensely sensitive as they can even detect changes in temperature of as little as 0.003K.

Anatomy of Sensing Organ
The infrared-sensing organs of vipers and boids are similar in their physiological structure but differ in their number, location and morphology. The anatomy is quite simple and will be explained in the example of the crotalus, a venomous pit viper found only in the Americas from southern Canada to northern Argentina. It consists of a hollow space that is separated into two air-filled chambers by a thin membrane of the thickness of 0.01 mm. It is filled with sensory cells of the trigeminal nerve (TNM). Roughly 7000 in number, they transduce the heat through heat sensitive ion channels, and increase their firing rate when a positive change in temperature occurs and decrease in the opposite case. They are very sensitive due to the spatial proximity of these thermoreceptors to the outside and also because of the air-filled chamber that lies underneath. This air-filled chamber works as an insulator in separating tissues that would otherwise quickly exchange heat energy. Thus, the absorbed thermal energy is used exclusively by the sensory system and is not lost to lower-lying tissues. This simple but sophisticated anatomy is the reason for the unique sensitivity of the pit organs. The pit organs’s physique allows even the detection of the radiation’s direction. The external opening is roughly half as large as the membrane. Thus, the whole organ works according to the optics of a pinhole camera: the position of the irradiated spot provides information about the object’s location. The heat itself is detected by the activation of heat sensitive ion channels called TRPA1. In other animals these channels also exist but have other functions like detecting chemical irritants or cold. Pit vipers and boids seem to have evolved the infrared-sensing independently. Since the heat sensitive ion channels have different thermal thresholds in different snakes, the temperature sensitivity differs among the snakes. Crotalus have the most sensitive channels. Snakes that are not able to detect infrared radiation also possess those channels, but their thermal threshold is too high to detect infrared radiation.

Brain’s Anatomy
Every sensory organ has a dedicated brain region to process the collected information. Snakes evaluate infrared sensory input from the pit organs in the nucleus of the lateral descending trigeminal tract (“LTTD”), a unique region in their metencephalon which has not been found in other animals. The LTTD is linked to the tectum opticum via the reticularis caloris (“RC”). Its function is still unknown. In the tectum opticum visual and infrared stimuli are connected, in order to provide a detailed idea of the animal’s surrounding.

Physiology
Experiments have shown that the detection of heat targets must be quite accurate as snakes hit thermal sources with a low error even without the help of vision. Measurements have determined that the opening angle of an infrared beam falling onto the pit organ is 45 to 60 degrees. Depending on where the heat source is relatively to the snake, the beam hits the pit’s membrane on a different spot. The receptive field of the infrared sensing system on the tectum opticum is similarly represented as the visual receptive field. The front-end of the tectum opticum receives its input from the back part of the pit membrane and the retina, and thus processes stimuli from the front part of the visual field. Similarly, the back and the sides of the visual field are represented in the back part of the tectum opticum and the front part of the pit membrane and the retina. The receptive fields of the visual and infrared sensory systems overlap almost perfectly within the tectum opticum, such that the neurons there receive and process sensory information from two senses, from more or less the same direction. While crotalus only have two pit organs, the anatomy of the temperature sensors is much more complicated in boas and pythons. They possess 13 pit organs on each side of the head. Every one of those also works like a pinhole camera that reverses the picture. The information of the front part of the visual field is again processed in the front part of the tectum opticum but now, the receptive field of every pit organ is projected onto a different part of it. The front pit organs are represented in the front part of the tectum opticum and the back parts in the back. In addition, the receptive fields of the different pit organs overlap, and thus provide a more or less continuous projective field that matches the visual one. It is curious that the front part of every pit organ is projected to the back part of the receptive field in the tectum opticum, an organization that is quite complicated and unique. The tectum opticum contains six different kinds of neurons which fire for infrared and/or visual stimuli. Some cell types respond only if there is a visual and an infrared stimulus, while others respond for any kind of stimulus. There are cells that respond for one of the sensory input if it comes alone, but increases its firing rate for simultaneous input from both systems. The last group of cells works the other way around. Some of them respond strongly for visual stimuli and stop firing when stimuli from the pit organs also arrive or vice versa. What do snakes with pit organs need these different kinds of neurons for? The processing in their brain has to help the snakes with different tasks: first of all, the snake should be able to detect and locate stimuli. Second, they have to be identified and reacted to appropriately. The cells that respond to both visual and infrared stimuli independently from each other could be responsible for the first task. Cells that only respond if they get both stimuli at the same time could work as detectors for living, moving objects. Moreover, cells that stop firing as soon as the visual stimuli is completed with an infrared signal could be especially important for detecting the cool surrounding like leaves or trees. The interaction between the different types of cells are important for correctly identifying the stimuli. They are not only used for identifying warm-blooded prey, but also for identifying predators and the snake’s thermoregulation.



Infrared Sensing in Vampire Bats
Vampire bats are the only mammals that are able to detect infrared radiation. To do so they have three hollows in their nose which contain the sensing organs. While they also use ion channels to detect heat, it is a different type of ion channels than in snakes. In other mammals and even everywhere in their own body except for the nose this type of molecule is responsible for sensing pain and burning. However, in the nose the threshold is much lower. The channel already detects changes in temperature from 29 °C on. This allows vampire bats to locate heat sources at a distance of 20 cm and helps them to find blood-rich spots on their prey.