THERMORECEPTORS: IN HUMANS, IN ANIMALS, IN PLANTS - BIOLOGY - 2025

The thermoreceptors are those receptors that possess many living organisms perceiving stimuli terms around. They are not only typical of animals, because plants also need to register the environmental conditions that surround them.

The detection or perception of temperature is one of the most important sensory functions and is often essential for the survival of species, since it allows them to respond to the thermal changes that are typical of the environment where they develop.

Crotalus willardi, with one of the two distinctive cranial pits (thermoreceptors) visible between the nose and the eye. Robert S. Simmons.

His study includes an important part of sensory physiology and, in animals, it began around the year 1882, thanks to experiments that were able to associate thermal sensations with localized stimulation of sensitive sites on human skin.

In humans there are thermoreceptors that are quite specific with respect to thermal stimuli, but there are also others that respond to both “cold” and “hot” stimuli, as well as to some chemicals such as capsaicin and menthol (which produce similar stimuli). to hot and cold sensations).

In many animals, thermoreceptors also respond to mechanical stimuli and some species use these to get their food.

For plants, the presence of proteins known as phytochromes is essential for thermal perception and the growth responses associated with it.

Thermoreceptors in humans

Human beings, like other mammalian animals, possess a series of receptors that allow them to better relate to the environment through what has been called the “special senses”.

These "receptors" are nothing more than the final portions of dendrites in charge of perceiving the different environmental stimuli and transmitting such sensory information to the central nervous system ("free" portions of sensory nerves).

4 Models for the structure of the sensory system in humans (Source: Shigeru23 via Wikimedia Commons)

These receptors are classified, depending on the source of the stimulus, as exteroceptors, proprioceptors and interoceptors.

The exteroceptors are closer to the surface of the body and "sense" the surrounding environment. There are several types: those that perceive temperature, touch, pressure, pain, light and sound, taste and smell, for example.

Proprioceptors are specialized in the transmission of stimuli related to space and movement towards the central nervous system, meanwhile the interoceptors are in charge of sending sensory signals that are generated inside the body organs.

Exteroceptors

In this group there are three types of special receptors known as mechanoreceptors, thermoreceptors and nociceptors, capable of reacting to touch, temperature and pain, respectively.

In humans, thermoreceptors have the ability to respond to temperature differences of 2 ° C and are subclassified into heat receptors, cold receptors, and temperature-sensitive nociceptors.

- Heat receptors have not been properly identified, but they are thought to correspond to “naked” nerve fiber endings (not myelinated) capable of responding to increased temperature.

- Cold thermoreceptors arise from myelinated nerve endings that branch and are found mainly in the epidermis.

- Nociceptors are specialized in responding to pain due to mechanical, thermal and chemical stress; These are myelinated nerve fiber endings that are branched in the epidermis.

Thermoreceptors in animals

Animals, as well as humans, also depend on different types of receptors to perceive the environment around them. The difference between thermoreceptors in humans compared to those of some animals is that animals often have receptors that respond to both thermal and mechanical stimuli.

Such is the case of some receptors in the skin of fish and amphibians, of some felines and monkeys, which are capable of responding to mechanical and thermal stimulation alike (due to high or low temperatures).

In invertebrate animals, the possible existence of thermal receptors has also been experimentally demonstrated, however, separating a simple physiological response to a thermal effect from the response generated by a specific receptor is not always easy.

Specifically, the "evidence" indicates that many insects and some crustaceans perceive thermal variations in their environment. Leeches also have special mechanisms to detect the presence of warm-blooded hosts and are the only non-arthropod invertebrates where this has been demonstrated.

Likewise, various authors point out the possibility that some ectoparasites of warm-blooded animals can detect the presence of their hosts in the vicinity, although this has not been much studied.

In vertebrates such as some species of snakes and certain blood-sucking bats (which feed on blood), there are infrared receptors capable of responding to the “infrared” thermal stimuli emitted by their warm-blooded prey.

Photograph of a blood-sucking ("vampire") bat (Source: Ltshears via Wikimedia Commons)

The "vampire" bats have them on their faces and help them determine the presence of the ungulates that serve as food, meanwhile the "primitive" boas and some species of poisonous crotalines have them on their skin and these are free nerve endings that they branch out.

How do they work?

Thermoreceptors work in more or less the same way in all animals and they do it essentially to tell the organism of which they are a part what the surrounding temperature is.

As discussed, these receptors are actually nerve terminals (the ends of neurons connected to the nervous system). The electrical signals generated in these last only a few milliseconds and their frequency is highly dependent on the ambient temperature and exposure to sudden changes in temperature.

Under constant temperature conditions, the skin's thermoreceptors are constantly active, sending signals to the brain to generate the necessary physiological responses. When a new stimulus is received, a new signal is generated, which may or may not last, depending on its duration.

Heat sensitive ion channels

Thermal perception begins with the activation of thermoreceptors in the nerve endings of peripheral nerves in the skin of mammals. The thermal stimulus activates temperature-dependent ion channels in the axon terminals, which is essential for the perception and transmission of the stimulus.

These ion channels are proteins that belong to a family of channels known as "heat-sensitive ion channels" and their discovery has allowed the mechanism of thermal perception to be elucidated in greater depth.

Molecular identity of nerves that respond to cold or heat depending on the expression of heat-sensitive ion channels (Source: David D. McKemy via Wikimedia Commons)

Its job is to regulate the flow of ions such as calcium, sodium, and potassium, to and from thermal receptors, leading to the formation of an action potential that results in a nerve impulse to the brain.

Thermoreceptors in plants

For plants it is also essential to be able to detect any thermal change that occurs in the environment and issue a response.

Some research on thermal perception in plants has revealed that it often depends on proteins called phytochromes, which also participate in the control of multiple physiological processes in higher plants, among which are the germination and development of seedlings, flowering, etc.

Phytochromes play an important role in determining the type of radiation plants are subjected to and are capable of acting as molecular "switches" that turn on under direct light (with a high proportion of red and blue light), or that turn off in the shade (high proportion of “far red” radiation).

Schematic representation of an active (Pr) and an inactive (Pfr) phytochrome (Source: Bengt A. Lüers - BiGBeN_87_de via Wikimedia Commons)

The activation of some phytochromes promotes “compact” growth and inhibits elongation by acting as transcription factors for the genes involved in these processes.

However, it has been proven that, in some cases, the activation or inactivation of phytochromes can be independent of radiation (red or far red light), which is known as the “dark reversion reaction”, whose speed apparently depends on temperature.

High temperatures promote the rapid inactivation of some phytochromes, causing them to stop working as transcription factors, promoting growth by elongation.

References

1. Brusca, RC, & Brusca, GJ (2003). Invertebrates (No. QL 362. B78 2003). Basingstoke.
2. Feher, JJ (2017). Quantitative human physiology: an introduction. Academic press.
3. Hensel, H. (1974). Thermoreceptors. Annual review of physiology, 36 (1), 233-249.
4. Kardong, KV (2002). Vertebrates: comparative anatomy, function, evolution. New York: McGraw-Hill.
5. M. Legris, C. Klose, ES Burgie, CCR Rojas, M. Neme, A. Hiltbrunner, PA Wigge, E. Schafer, RD Vierstra, JJ Casal. Phytochrome B integrates light and temperature signals in Arabidopsis. Science, 2016; 354 (6314): 897
6. Rogers, K., Craig, A., & Hensel, H. (2018). Encyclopaedia Britannica. Retrieved December 4, 2019, at www.britannica.com/science/thermoreception/Properties-of-thermoreceptors
7. Zhang, X. (2015). Molecular sensors and modulators of thermoreception. Channels, 9 (2), 73-81.