As thermal reception or Thermozeption is temperature sense referred beings. The corresponding receptors are nerve cell endings ( receptor cells ) in the skin or in the mucous membranes of the intestines . They generate action potentials (nerve impulses) whose frequency and temporal pattern change depending on the temperature. These impulses are passed on via the nerve fibers and then via several neuronal switching points to the brain. Thermal receptors form the basis of temperature perception. In addition, they act - in combination with heat-sensitive nerve cells in the brain, especially in the thermoregulatory centers in the hypothalamus - on the thermoregulation with. In some living things, thermal receptors are bundled into special sensory organs that are used for orientation (navigation) or to find prey ( see thermal reception in animals ). Physiologists Ernst Heinrich Weber and Max von Frey were among the pioneers of scientific research into the sense of temperature .
Peripheral cold and warm receptors
Thermoreceptors are able to perceive the temperature or its change. The corresponding sensors in the skin and mucous membranes register the current temperature of the tissue; a temperature change can take place, for example, through contact of the tissue with a (temperature-controlled) object or with a medium (air) and through the action of radiation (especially infrared radiation ).
The peripheral thermoreceptors can be clearly characterized in electrophysiological experiments based on the stimulus dependence of their impulse activity and differentiated from other skin receptors. A distinction can be made between cold and warm receptors . This corresponds to the different qualities of feeling cold and warm in humans (eg “cold” is not just “less warm”).
The endings of nerve cells, whose cell bodies are concentrated in the spinal ganglia or in the ganglia of the cranial nerves (such as the trigeminal nerve ), serve as thermoreceptors . These "pseudounipolar" neurons send axons into the skin, where they run out as free nerve endings near the surface, and to the dorsal horn of the spinal cord , where the incoming " afferent" signals are switched to further nerve cells. The axons are unmyelinated , slowly conducting C fibers (warm and cold receptors) or thinly myelinated, faster conducting Aδ fibers (cold receptors).
One to five cold spots per square centimeter are found in the palms of a person, but only 0.4 warm spots. These points correspond to the receptive field of a single thermal receptor. The density (frequency) of the sensors varies greatly depending on the body region: There are up to 20 cold receptors per cm² on the human lips. The receptive fields of cold and warm receptors do not overlap.
|Body region||Cold spots per cm²||Hot spots per cm²|
|Upper arm (flexor surface)||5.7||0.3|
|back of Hand||7.4||0.54|
How the receptors work
Warm and cold receptors generate a certain, constant number of action potentials (impulses), the so-called spontaneous frequency, at a constant temperature. The receptors respond to a sudden change in temperature with a sudden, “excessive” change in the pulse frequency (dynamic frequency) up to a maximum or minimum value; then the frequency settles to a new (higher or lower) value. Cold receptors react to a decrease in temperature with a sudden increase in frequency, to an increase, on the other hand, with a similar decrease (see figure on the right); Warm receptors respond exactly the other way around. Temperature jumps of up to a few tenths of a degree are answered in each case. The greater the temperature jump and the faster it occurs, the stronger the dynamic response.
Warm receptors are active in the range between just under 30 to just over 40 ° C. With them, the spontaneous frequency increases parallel to the increase in temperature, but then drops again steeply above a maximum. Cold receptors are sensitive between around 5 and almost 40 ° C. Dynamic sensitivity and spontaneous activity reach a maximum at a certain average temperature, which is usually around 20 to 25 ° C. Both parameters decrease towards higher or lower temperatures (Fig. Left). Some cold receptors can become active again if the skin is quickly heated to over 45 ° C. This phenomenon is possibly responsible for the so-called paradoxical sensation of cold .
So-called heat receptors are to be distinguished from the cold and warm receptors . These only generate action potentials at temperatures that are perceived as painful, i.e. from around 43 ° C. They are usually also activated by other painful or tissue-damaging (noxious) stimuli such as strong pressure and are therefore classified as “polymodal nociceptors ”.
The activity of most mechanoreceptors is also influenced by temperature. Spontaneously active mechanoreceptors have a temperature dependency that is very similar to that of cold receptors. The main difference to specific cold receptors is that they cannot be activated by moderate, non-painful pressure stimuli.
Electrophysiological studies have shown that the nerve impulses (action potentials) in peripheral thermoreceptors have variable patterns; this includes, for example, typical groups of pulses, so-called bursts . A large number of research work has led to the assumption that oscillating (cyclically repeating) processes take place in the cell envelope ( membrane ) of these receptors due to the interaction of different ion channels , the frequency and strength ( amplitude ) of which changes depending on the temperature, which means that explain the different impulse patterns.
The molecular processes that determine the specific properties of different thermoreceptors have been researched very intensively for several years, including direct measurements on thermosensitive ion channels. Since this has so far not been possible on the difficult to access receptive nerve endings in the skin, such experiments are carried out on their cell bodies in the trigeminal or spinal ganglia.
In such investigations, it was found that so-called TRP channels ( Transient Receptor Potential ) are located in the nerve cell membrane, which - together with other ion channels - clearly play an essential role in thermal reception. Such channels change in response to a temperature stimulus in such a way that ions (primarily charged sodium or calcium ions ) flow through the membrane inside the channel, which changes the electrical potential of the cell.
So far, a number of different thermosensitive TRP channels have been identified and characterized, which could be responsible in particular for the dynamic response to rapid temperature changes that distinguish heat- and cold-sensitive receptors. So far, a number of TRP channels have been detected that are active in the range between zero and more than 50 ° C in a specific, limited temperature range. Research has shown that the so-called melastatin or cold menthol receptor (TRPM8, CMR) plays a decisive role in cold receptors; these receptors are active between approximately eight and 28 degrees. At lower temperatures the so-called ankyrin receptor (TRPA1) is active, while at higher temperatures (up to the range of heat pain) different channel types of the vanilloid receptor group ( TRPV1 to 4) and the melastatin group (TRPM4 and 5) are corresponding Take over functions. A large number of studies indicate that the cold or heat reception in different animal species or in different tissues are each controlled by a finely balanced system of different ion channels. Despite all the research advances made in recent years, the detailed molecular mechanisms of thermal reception are still far from being fully and conclusively explained.
The sensation for a certain temperature gradually decreases, even if it remains objectively constant (sensory adaptation ). Although the receptors continue to transmit the “current temperature”, an adaptation to the stimulus takes place in the central nervous system .
In the medium temperature range (between 20 and 40 ° C), cooling or warming only temporarily leads to a warm or cold sensation, after which the sensation is neutral (complete adaptation). This can easily be checked with a warm bath.
Thermal reception in animals
Many animals have temperature sensors similar to humans, but some species have a far more effective sense of heat. These include, for example, the Australian large foot fowl : the thermometer chicken ( Leipoa ocellata ) uses receptors in its beak to measure the heat for its eggs to hatch. The bird does not incubate its eggs itself, but uses the fermentation processes in a heap of organic material to breed. The chickens are able to keep the temperature at exactly 33 ° C. By ventilating the hill or increasing or decreasing it, you can regulate the temperature sensitively.
In snakes, the ability to perceive infrared radiation ( thermal radiation ) has evolved independently of one another in three different families - pit otters (Crotalidae), pythons (Pythonidae) and boas (Boidae). The warmth sense organ of the pit vipers (subfamily Crotalinae), to which the rattlesnakes belong, is based on the so-called bolometer principle and enables the reptiles to localize prey even in complete darkness and to take targeted bites in certain, easily vulnerable body regions. The labial organs of the Boiden and Pythoniden consist of pits along the upper and lower lip. There is now evidence that pit vipers also use their sense of warmth for thermoregulation and for locating potential predators. In the receptors and ganglia of crotalids, one predominantly finds TRP channels of the ankyrin type (TRPA1).
The vampire bat ( Desmodus rotundus ) also has a sensitive organ of warmth. Vampire bats (subfamily Desmodontinae), of which three species occur exclusively in the neotropics , are the only mammals that feed exclusively on blood. Numerous cold and warm receptors (free nerve endings, see above), which are innervated by the trigeminal nerve, are concentrated in the central part of the nasal attachment (the "nasal blade") of Desmodus , especially on its thin edge and the middle ridge . The properties of the receptors differ from those found in other mammals (cats, hamsters, mice, primates including humans); for example, the range of activity of the warm receptors is shifted to lower temperatures, that of the cold receptors to higher temperatures. The thermal sensory organ enables the bats to perceive the thermal radiation emanating from warm-blooded prey (especially cattle and horses, but also wild mammals) at distances of up to 13 centimeters. It is believed that the bats also use this sense of warmth to locate parts of their prey that are particularly well supplied with blood. Recently research has shown that in the sensory cells of the nose attachment of Desmodus TRP molecules are active that are so far unique in the animal kingdom: Here there is an accumulated molecule (TRPV1-S) that has a different structure compared to the "normal" TRPV1 and has modified properties: While TRPV1 is active from around 43 degrees and thus represents a heat pain sensor, the short version already reacts to temperatures from around 30 degrees. This value agrees perfectly with the thresholds that had already been found in physiological studies on the warm receptors of these bats. The central facial region of Desmodus forms a real "warmth sense organ".
Examples of efficient heat sensory organs can also be found in insects . A number of butterfly species , for example from the genera Troides , Vanessa or Pachliopta, have thermosensors in their antennae or in the veins of the wings. These help the insects to regulate their body temperature and prevent overheating. To do this, the animals change the angle of attack of their wings in relation to the solar radiation, for example, if they are exposed to too much heat.
In the case of the blood-sucking bug Triatoma infestans, there is strong evidence that the animals can perceive infrared radiation from a distance. Apparently the corresponding IR receptors are on the antennas.
The black pine jewel beetle Melanophila acuminata performs extraordinary sensory performances . These insects can detect large forest fires from a distance of up to one hundred kilometers. Melanophila only lays its eggs in fire-damaged trees. The hatched larvae feed on the burned wood, undisturbed by feeding competition. In addition to numerous chemical sensors, the beetles also have IR sensory organs, which are arranged in pits on both sides of the chest ( thorax ) near the middle pair of legs. It is now known that the IR radiation is converted into a mechanical stimulus in the sensory organs, which is then picked up by the corresponding sensory hair. IR receptors have also been discovered in the Australian beetle species Acanthocnemus nigricans (which is also fire-seeking); However, these are based on the bolometer principle and thus more closely resemble the sensory organs of pit vipers.
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