Histology 3500 Blog 1 Tissue Choice: Pit Organs in Snakes
Date Due: October 22nd 2018
Blog by Alex Byrne
When we are young, we are told that there are five main senses; hearing, taste, touch, scent and sight. However, many organisms often omit some senses (such as sight in the case of fossorial organisms which have poorer sight), while others have additional senses that extend beyond the traditional five (such as electroreception in sharks). All animals are able to sense temperature and heat to some extent, but snakes are among the animals that have a specialised system that allow for a greater use than simply distinguishing "hot" and "cold". Some groups of snakes have specialised thermal receptive organs called pit organs. These specialised thermal receptors function as part of the snakes already impressive sensory system, specifically through their detection of infrared radiation (IR), which assists in the snakes ability to detect and capture prey.
Blog by Alex Byrne
Importance of Thermal Detection
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| What a tasty meal may look like to a snake hunting a mouse using its pit organs to detect infrared (IR) radiation. https://www.nature.com/news/2010/100314/images/news.2010.122.mouse.jpg |
Pit Organ Overview
Pit Organs are found within two main groups of snake species.
The first group consists of snakes within the sub-family Crotalinae (commonly called pit vipers) within the family Viperideae (vipers as a whole). Some snake genera included within this group are Crotalus (rattlesnakes), Agkistrodon (moccasins) and Bothrops (lanceheads). This group of snakes is distinguishable by the presence of a single large pit organ that is found approximately midway between the eye and nostril, and is about 1 - 5 mm in diameter. This single pit organ is referred to as a loreal pit.
The second group of snakes that have pit organs of some capacity are those within the families Boidae (Boas) and Pythonidae (Pythons). It should be noted that while much of the literature cites Boidae as the only family, recent taxonomic changes have made Pythonidae their own family, rather than as part of the Boidae group. One potential reason for this confusion, aside from superficial resemblance between the two is the method of thermal detection used. Unlike the loreal pits of Crotaline snakes, these families have a series of smaller pit organs located along the labial scales of the head (called labial pits), and the organs themselves are not as specialised as the loreal scales of Crotaline snakes.
Both groups use a similar mechanism of action to function. The floors of the pit organs are stimulated by the presence of infrared (IR) radiation. The floors of the pit organs are covered in sensory nerves which elicit responses in single units, leading to terminal nerve masses (TNM). These TNMs are connected to peripheral nerves, which in turn are connected to higher order neurons before being processed by the brain (in a part of the brain called the nucleus of the lateral descending trigeminal tract (LDTT) in both types before more specific processing occurs based on the type of snake).
Within the loreal pit is a thin membrane that is suspended within the pit, separating an inner and outer region of the pit. The outer chamber opens out into the space between the eye and nostril, and the inner cavity is exposed to external air through a pore (sphincter controlled) that is located in front of the eye. The presence of external air on both sides of the membrane limits the amount of heat that is lost due to conduction to other tissues, allowing for the maximum heating of the membrane using IR radiation. Additionally, the membrane contains high number of mitochondria, is very well vascularised and is well innervated by nerves from the trigeminal portion of the somatosensory system (associated with signals of touch/position and pain/temperature in the facial, maxillary and mandibular regions that are conducted to the brain). These trigeminal nerves have three branches that are involved with the pit membrane, one ramus ophthalmicus branch and two ramus maxillaries, and these branches connect on the same side of the body in the lateral descending nucleus of the medulla oblongata. This system is kept separately from the common trigeminal system, and from the ophthalmic and maxillomandibular ganglia
These mitochondria rich terminal nerve masses (TNM) are key to what makes the temperature sensing in Crotalinae snakes so keen, with the vasculature acting to constantly supply oxygen to these nerves, while also acting as a coolant. A major receptor that plays a role in sensing thermal thresholds was found to be the receptor TRPA1. This receptor is of particular interest due to its unique evolutionary position. In many vertebrate organisms, including humans, the orthologues of TRPA1 are used to detect chemical irritants (given the nickname of "wasabi receptor". In contrast, invertebrate TRP channels can contribute to temperature detection using either the sensing of heat as an activating mechanism (as in Drosophila melanogaster) or through the use of photo-detection to activate the channel (as in Caenorhabditis elegans).
Sensory systems tend to have specialised cells for detection, but in the somatosensory system this is not the case. Instead, bare nerve ends act as the detectors of thermal instance in this case, with terminal ganglia on the surface being more significant than dorsal root ganglia. TRPA1, as the thermal receptor, has a higher expression on the terminal ganglia than on the dorsal root ganglia, and even when compared to a member of the same family, TRPV1.
The pit organs in snakes of the families Boidae and Pythonidae may not have the same pit organs as described in Crolatine snakes. Instead of a single large pit containing a membrane with numerous IR receptors, the pit organs in these snakes are found as shallow pits that are located in between the labial scales, with some variation on exact shape depending on their location. For instance the pits present in the upper labial scales can be seen as triangular, sickle shaped or ovular while those in the lower labial scales are round or square, but not as elongated as the upper pits. While these pits lack the membrane of loreal pit organs, they are still highly innervated, with a network of capillaries present underneath the epidermis of the pit. TNM's are present in labial pits as well, but due to their presence within the epithelium instead of on a separate membrane, the TNMs are shed along with the epithelium. Myelinated nerve fibres are located underneath the floor of the labial pit and branch out to form identical TNMs to replace those lost. These nerves originate from the maxillary portion of the trigeminal nerve for the upper labial pits and the mandibular portion of the trigeminal nerve for the lower labial pits, though some species have some more complicated systems (such as the species Python reticulatus). Much like in Crolatine snakes, TRPA1 is also used as the channel to detect heat, but is not present in as high amounts as in the loreal pits.
While both kinds of pits organs serve their functions well, the Crolatine loreal pits have been shown to be more sensitive than those on the loreal scales, with the TRPA1 sensors also showing a similar pattern. This makes sense since it has been shown that Boas and Pythons generally have a lower sensitivity to infrared radiation than those of Pit Vipers. Both groups of snakes using their IR sensing mechanisms, along with other senses such as sight and chemo-reception, can allow for the easy detection of prey.
The detection of IR radiation using this sensing system is also different from the more traditional "neural firing" in the sense that it gradually adjusts to the stimulus involved. Typically, neurons are activated upon reaching a threshold, where an action potential is fired. This firing usually continues until the stimuli has stopped, also halting the signal and respective response. The pit receptors for snakes however are "phasic". This means that when an IR stimulus is first received it activates as normal, but over time the frequency of neural responses decrease, eventually returning the response levels to a normal setting despite the continual stimulus that is being applied. This allows these receptors to be very responsive to minute changes within the environment (picking up subtle IR radiation given off by prey organisms), while slowly adapting to stronger stimuli (potentially allowing the snake to s constant "background" radiation.
Of the many animals being under study, snakes are one group that should be further examined due to their exceptional use of senses that humans simply cannot compare to. While it could be said that snake chemo-reception and thermoreception are exaggerated versions of senses such as smell, taste and touch, the abilities of all of these senses together allow for sensory perception unmatched by anything that many mammals have to offer. The little amount of investigation into these sensory structure (partially due to the difficulty of working with snakes and the little amount of genomic data gathered when compared to other organisms of interest such as laboratory mice or agriculturally important organisms) means that there is still much to discover about these sensory systems. This includes any potential diseases involved with these sensory systems. As of the date of this blog, no pathological illnesses solely involving the pit organ system could be found, though some are bound to exist. This opens up a wide range of potential research opportunities that may be explored by scientists in future endeavours.
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Bothriechis lateralis, a pit viper species. The loreal pit organ
is clearly visible between the eye and nostrilhttps://upload.wikimedia.org/wikipedia/commons/9/99/Groengele-groefkopadder-5.jpg |
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| Labial pits of the ball python, Python regius, observed on the upper lip. https://ball-pythons.net/forums/cache2.php?img=http://img.photo bucket.com/albums/v93/aznw1f/DSC_0200_zps1f032bf2.jpg |
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| Outline of snake brain showing connections form TNMs to LTTD. Figure from Chen et al., 2012. |
Loreal Pit Information
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| Diagram of a Loreal Pit from Moon, 2011. The two air pockets on either side of the highly innervated membrane can be easily observed. |
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| Tissue layout of the pit organs if taken in a cross section. OEC=Outer Epithelia and Connective Tissue. MF=Myelinated Nerve Fibre. UMF=Unmyelinated Nerve Fibre. IEC=inner epithelium and connective tissue. TNM=terminal nerve mass. BV=Blood Vessel. Figure from Moon, 2011. |
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| Figure from Gracheva et al, 2010. Histological slides showing In situ hybridization for expression of TRPA1 and TRPV1. TRPA1 expression is highly upregulated in the terminal ganglia while regulation of TRP genes in other locations remains consistent. |
Labial Pits Information.
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| Diagram of a Labial Pit from Moon, 2007. The left figure shows receptors present on the surface of the labial scales while the one on the right shows receptors within the fundus of pit organs. |
Sensory Importance of Pit Organs.
While both kinds of pits organs serve their functions well, the Crolatine loreal pits have been shown to be more sensitive than those on the loreal scales, with the TRPA1 sensors also showing a similar pattern. This makes sense since it has been shown that Boas and Pythons generally have a lower sensitivity to infrared radiation than those of Pit Vipers. Both groups of snakes using their IR sensing mechanisms, along with other senses such as sight and chemo-reception, can allow for the easy detection of prey.The detection of IR radiation using this sensing system is also different from the more traditional "neural firing" in the sense that it gradually adjusts to the stimulus involved. Typically, neurons are activated upon reaching a threshold, where an action potential is fired. This firing usually continues until the stimuli has stopped, also halting the signal and respective response. The pit receptors for snakes however are "phasic". This means that when an IR stimulus is first received it activates as normal, but over time the frequency of neural responses decrease, eventually returning the response levels to a normal setting despite the continual stimulus that is being applied. This allows these receptors to be very responsive to minute changes within the environment (picking up subtle IR radiation given off by prey organisms), while slowly adapting to stronger stimuli (potentially allowing the snake to s constant "background" radiation.
Concluding Remarks
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| Figure showing the wide range of detection granted by the pit organs of a snake. http://www.robinsonlibrary.com/science/zoology/reptiles /squamata/serpentes/graphics/pitorgans.gif |
References Used
Amemiya, F., Goris, R. C., Masuda,
Y., Kishida, R., Atobe, Y., Ishii, N., & Kusunoki, T. (1995). THE SURFACE
ARCHITECTURE OF SNAKE INFRARED RECEPTOR ORGANS. Biomedical Research, 16(6),
411-421.
Campbell, A. L., Naik, R. R., Sowards, L.,
& Stone, M. O. (2002). Biological infrared imaging and sensing. Micron,
33, 211-225.
Chen, Q., Deng, H., Brauth, S. E., Ding,
L., & Tang, Y. (2012). Reduced Performance of Prey Targeting in Pit Vipers
with Contralaterally Occluded Infrared and Visual Senses. PLoS ONE, 7(5),
1-8. doi:10.1371/journal.pone.0034989
Gracheva, E. O., Ingolia, N. T., Kelly, Y.
M., Cordero-Morales, J. F., Hollopeter, G., Chesler, A. T., . . . Julius, D.
(2010). Molecular basis of infrared detection by snakes. Nature, 464,
1006-1011. doi:10.1038/nature08943
Moon, C. (2011). Infrared-sensitive pit
organ and trigeminal ganglion in the crotaline snake. Anatomy and Cell
Biology, 8-13. doi:10.5115/acb.2011.44.1.8
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