Thermoception
As thermoreception or Thermozeption the temperature sense of living things is known. As corresponding receptors nerve cell endings are in the skin or in the mucous membranes of the intestines. Generate action potentials ( nerve impulses ) to change the frequency and temporal pattern as a function of temperature. These pulses are passed through the nerve fibers and then over several neural switch points to the brain. Thermal receptors form the basis of temperature perception. In addition, they act - in interaction with thermosensitive neurons in the brain, especially in the thermoregulatory centers of the hypothalamus - the thermoregulation with. Some creatures thermal receptors are bundled into specialized sensory organs that are used for orientation (navigation), or prey -discovery (see thermoreception in animals ).
As the perception of pain is part of the sense of temperature to the surface sensitivity. For an overview see sensibility (medicine).
Peripheral cold and warm receptors
Thermal receptors are able to perceive the temperature or the change. The respective sensors in the skin and mucous membranes to register the current temperature of the tissue; a temperature change can occur, for example, by contacting the tissue with a ( temperature-controlled ) or object with a medium ( air ) and by the action of radiation ( in particular infra-red radiation ).
The peripheral thermal receptors can be characterized unambiguously by means of the stimulus pulse, depending on their activity and differ from other skin receptors in electrophysiological experiments. One can distinguish between hot and cold receptors. This corresponds to the different qualities of sensation hot and cold in humans ( "cold" for example, is not only " less warm ").
As thermal receptors serve endings of nerve cells whose cell bodies in the dorsal root ganglia or in the ganglia of the cranial nerves ( the trigeminal nerve about ) are concentrated. Send this " pseudounipolar " neurons on the one hand processes ( axons) in the skin where they expire as free nerve endings near the surface, on the other hand, the posterior horn of the spinal cord, where the incoming " afferent " signals to further nerve cells are switched. When the axons are unmyelinated, slowly conducting C- fibers (warm receptors and cold receptors ) or by thin myelinated, fast -conducting Aδ - fibers (cold receptors).
In the palms of a man is found one to five cold spots per square centimeter, but only 0.4 hot points. These points correspond to the receptive field of a single thermal receptor. Depending on the localization, the density (frequency ) of the sensors varies widely: To be found on the lips of the people up to 20 cold receptors per square centimeter. The receptive fields of cold and warm receptors do not overlap.
Functioning of the receptors
Warm and cold receptors produce a certain constant number of action potentials ( impulses ), the so-called spontaneous frequency at constant temperature. A sudden change in temperature answer the receptors with an abrupt, " excessive " changing the pulse frequency ( Dynamic Frequency ) up to a maximum or minimum value; then the frequency levels off to a new ( higher or lower ) value. Cold receptors respond to a decrease in the temperature with a sudden increase in the frequency, on the other hand, an increase of just such a waste ( pictured right ); Warm receptors respond exactly the opposite. In each case, temperature variations of up to a few tenths of a degree to be answered. The greater the temperature jump and the faster he runs, the stronger the dynamic response.
Warm receptors are active in the range between just under 30 to just over 40 ° C. For them, the spontaneous frequency increased simultaneously with the increase in temperature, but then falls above a maximum again steeply. Cold receptors are sensitive from about 5 to nearly 40 ° C. Dynamic sensitivity and spontaneous activity reached a maximum, which is generally at about 20 to 25 ° C at a determined average temperature. At higher or lower temperatures take both parameters (Fig. left). Some cold receptors can become active again with rapid heating of the skin to over 45 ° C. This phenomenon may be responsible for the so-called paradoxical cold sensation.
To be distinguished from the cold or warm receptors are so-called heat receptors. These generate action potentials only at temperatures that are perceived as painful, that is from about 43 ° C. They are usually activated by other painful or tissue-damaging ( noxious ) stimuli such as heavy pressure, and therefore attributed to the " polymodal nociceptors ".
Also the activity of most mechanoreceptors is influenced by the temperature. Spontaneously active mechanoreceptors have a temperature dependence which is very similar to that of the cold receptors. The difference with specific cold receptors essentially consists in that these are by moderate, non- painful pressure stimuli can not be activated.
Molecular Mechanisms
Electrophysiological investigations have shown that the nerve pulses ( action potentials ) which in peripheral thermoreceptors variable pattern; This information can include, for example, typical groups of pulses, called " bursts ". A variety of research has led to the assumption that in the cell membrane (membrane) of these receptors oscillating through the interaction of various ion channels ( cyclically repeating ) are processes whose frequency and intensity ( amplitude) changes depending on the temperature, bringing can explain the different pulse patterns.
The molecular processes that determine the specific properties of different thermal receptors are being studied very hard in recent years by direct measurements on heat-sensitive ion channels. Since this is the hard to reach receptive nerve endings in the skin has not been able, such experiments are made to their cell bodies in the trigeminal and dorsal root ganglia.
In such studies it has been found that in the nerve cell membrane called TRP channels ( Transient Receptor Potential) are localized, which - together with other ion channels - obviously play an essential role in the thermal reception. Such channels change in response to a temperature stimulus such that ions ( especially sodium or calcium charged molecules) flow in the interior of the channel through the membrane, thereby changing the electrical potential of the cell.
Been a number of different thermosensitive TRP channels have been detected and characterized, which might be responsible in particular for the dynamic response to rapid changes in temperature, which is distinguished by heat and kältsesensitive receptors. It has been quite a number of TRP channels detected, each active in a certain, limited temperature range between zero and more than 50 ° C. Thus, research has shown that so-called cold receptors of the Melastatin or cold and menthol receptor ( TRPM8, CMR) plays a key role; these receptors are active between about eight and 28 degrees. At lower temperatures, the so-called ankyrin receptor ( TRPA1 ) is active, while at higher temperatures ( up to the area of the heat pain into ) various channel types of vanilloid receptor group ( TRPV1 to 4 ) and the Melastatin group ( TRPM4 and 5) corresponding take over functions. A variety of studies indicates that the cold - or heat reception are controlled in various animal species and in different tissues in each case by a finely balanced system of different ion channels. Despite all the research advances in recent years, the detailed molecular mechanisms of thermal reception are not yet fully elucidated and, finally, by far.
Central adaptation
The sensation of a certain temperature decreases gradually, even if the lens is constant ( sensory adaptation). Although the receptors continue to transmit the " current temperature ", the adaptation takes place to the stimulus in the central nervous system.
In the intermediate temperature range ( 20-40 ° C), cooling or heating leads only to a temporary hot or cold sensation, then the sensation is neutral (full adaptation). This can be easily verified by a warm bath.
Thermoreception in animals
Many animals have similar temperature sensors as man, however, some species have a much more effective sense of warmth. These include the Australian Megapodes: The thermometer chicken ( Leipoa ocellata ) measures with receptors in its beak the breeding heat for its eggs. The bird incubates its eggs itself, but uses the fermentation processes in a pile of organic material for breeding. The chickens are able to keep the exact temperature at 33 ° C. By aerate or increase or decrease the hills they are able to regulate the temperature sensitive.
In snakes, the ability has infrared radiation (heat radiation) perceive in evolution independently in three different families - pit vipers ( Crotalidae ), pythons ( Pythonidae ) and Boas ( Boidae ) - trained. The heat sensory organ of pit vipers ( subfamily Crotalinae ), which include rattlesnakes is based on the bolometer so-called principle and allows the reptiles to locate prey even in complete darkness and execute targeted bites in particular, easily vulnerable areas of the body. The Labialorgane the boids and Pythoniden consist of pits along the upper or lower lip. Meanwhile, there are indications that pit vipers use their sense of heat for thermoregulation and for the detection of potential predators. In the receptors and ganglia crotalid There are mainly TRP channels of ankyrin type ( TRPA1 ).
The vampire bat ( Desmodus rotundus ) has a heat sensitive sense organ. Vampire bats ( subfamily Desmodontinae ) which occur exclusively in the Neotropics, with three species are the only mammals that feed exclusively on blood. In the central part of the nose attachment ( the " nose leaf " ) of Desmodus, in particular on its thin edge and the central ridge, numerous hot and cold receptors ( free nerve endings so ) concentrates, which are innervated by the trigeminal nerve. The receptors differ in their properties from those that are found in other mammals (cats, hamsters, mice, primates) or in humans; as that of the cold receptors at higher temperatures, for example, the activity area of the hot receptors at lower moved. The heat sensory organ allows the bats of the warm-blooded prey ( mainly cattle and horses, but also wild mammals ) outgoing thermal radiation at distances perceive up to 13 centimeters. It is believed that the bats also use this sense of warmth to locate their prey on particularly strong blood of the body. Recently, research has demonstrated that TRP molecules are active in the sensory cells of the nasal attachment of Desmodus, which so far are unique in the animal kingdom: a place to be heaped a molecule ( TRPV1 - S) compared to the "normal" TRPV1 an altered structure and modified properties: While TRPV1 from about 43 degrees actively and thus represents a heat pain sensor, the short version already responds to temperatures from about 30 degrees. This value is in perfect agreement with the thresholds that had been found already at physiological studies on the receptors of these hot bats. The central area of the face of Desmodus thus forms a real " heat sensory organ ".
Examples of high-performance heat sensory organs are also found in insects. A number of butterfly species, such as from the genera Troides, Vanessa or Pachliopta have heat sensors in their antennae and in the veins of the wings. These help the insects to regulate their body temperature and to prevent overheating. For this purpose, the animals change if too much heat exposure, for example, the angle of attack of their wings in relation to the sunlight.
In the bug Triatoma infestans bloodsucking there are strong indications that the animals can perceive infrared radiation from a distance. Apparently, the corresponding IR receptors are located on the antennae.
Unusual sensory abilities accomplishes the jewel beetle Melanophila acuminata. These insects can locate large forest fires from distances of up to one hundred kilometers. Melanophila lays its eggs exclusively in damaged by fire pines. The hatched larvae feed on, undisturbed by eating competition, of the burnt wood. The beetles have in addition to numerous chemical sense probes and IR-sensing organs (thorax) are located near the middle pair of legs in pits on both sides of the chest. We now know that the IR radiation in the sense organs in a mechanical stimulus is converted, which is then received by corresponding sensory hairs. IR receptors are also in the ( also fire -seeking ) Australian beetle Acanthocnemus nigricans discovered; However, these are based on the bolometer principle and are thus similar to rather the sense organs of pit vipers.