THERMORECEPTION

process in which different levels of heat energy (temperatures) are detected by living things. Temperature has a profound influence upon living organisms. Active life among animals is feasible only within a narrow range of body temperatures, the extremes being about 0 C and 45 C. On the Fahrenheit scale the same range is 32 F and 113 F. Limitations depend on the freezing of tissues at the lower temperature and on the chemical alteration of body proteins at the higher end of the range. Within these limits the metabolic rate of the animal tends to increase and decrease in parallel with its body temperature. Body temperature and metabolism among more highly evolved animals (e.g., birds and mammals) are relatively independent of direct thermal influences from the environment. Such animals can maintain considerable inner physiological stability under changing environmental conditions and are adaptable to substantial geographic and seasonal temperature fluctuations. A polar bear, for example, can function both in a zoo during summer heat and on an ice floe in frigid Arctic waters. This kind of flexibility is supported by the function of specific sensory structures called thermoreceptors (or thermosensors), which enable the animal to detect thermal changes and to adjust accordingly. Temperature of the body directly reflects that of the environment among cold-blooded (poikilothermic) animals, such as insects, snakes, and lizards. These creatures maintain safe body temperatures mainly by moving into locations of favourable temperature (e.g., in the shade of a desert rock). Warm-blooded (homoiothermic) organisms, such as the polar bear, normally keep practically constant body temperature, independent of environment. Homoiothermic animals, including man, are able to control their body temperature not only by moving into favourable environments but also through the internal regulatory (autonomic) effects of the nervous system on heat production and loss. Such autonomic adjustments depend on lower brain centres; the behavioral (movement) responses require the function of the brain's outer layers (the cerebral cortex). A variety of behavioral responses is elicited through stimulation of thermoreceptors, including changes in body posture that help regulate heat loss and the huddling together of a group of animals in cold weather. In some species, thermoreceptors are also involved in food location and sexual activities. Bloodsucking insects, such as mosquitoes, are attracted by thermal (infrared) radiations of warm-blooded hosts; such snakes as pit vipers can locate warm prey at considerable distance by means of extremely sensitive infrared receptors. Man has achieved the widest range of adaptability to extremes in temperature, since his technology allows him to protect himself under a considerable variety of thermal conditions on earth and even in outer space. Perceptual aspects of thermoreception are found in evidence that man and other animals have conscious temperature sensations and emotional experiences of thermal comfort and discomfort. The effects of temperature on productive efficiency and behaviour (e.g., on one's ability to think) have led to the installation of heat-regulating equipment in homes, public buildings, factories, and similar shelters for people, livestock, and other animals. Thermoreception can be studied in different ways: (1) on the basis of reports of temperature sensations and thermal comfort by human subjects; (2) through observations of behavioral responses to variations in temperature by all kinds of animals; (3) by the measurement of compensatory autonomic responses (e.g., sweating or panting) to thermal disturbances in the environment; and (4) by recording electrical impulses generated in the nerve fibres of thermoreceptors in laboratory animals and human subjects. the sensory capacity an animal uses to determine the temperature of its immediate environment. Thermoreception is important not only because it helps the animal to respond to dangerous situations (e.g., being exposed to fire) but because it also aids in the regulation of the animal's body temperature. Life processes can continue only when the temperature of the body stays inside certain limits, between about 0 C (32 F) and 45 C (113 F), depending on the kind of animal. Outside these limits, the tissue structures and chemical components vital to life suffer damage. Warm-blooded animals (homoiotherms), such as birds and mammals, have metabolisms that maintain their body temperatures at stable levels. Thermoreception, then, helps a warm-blooded animal keep its body temperature fixed by regulating autonomic responses to temperature changes in its environment. By contrast, the body temperature of cold-blooded animals (poikilotherms), e.g., insects and reptiles, changes with the temperature of the environment. Thus, thermoreception stimulates a cold-blooded animal to find an environment that is within its required temperature range. The study of thermoreceptors began when it was found that sensations of temperature are generated by minute areas of the skin that are sensitive to heat and cold. The application of electrodes to particular areas of the body further allowed researchers to trace the electrical impulses of specific nerve cells and, thus, to examine involuntary responses to temperature that are not consciously registered. The nerve cells responsible for thermoreception function in basically the same way in all animals. If the temperature is raised, the frequency of impulses from warm-receptors speeds up and the frequency of impulses from cold-receptors slows down, in both cases quite suddenly. If the temperature is lowered, the responses of the two kinds of receptors are reversed. In any event, if the temperature stabilizes at the new level, the frequency of impulses soon returns to close to its initial level, where it remains until the temperature changes again. It should be noted that, in many cases, thermoreceptor cells respond also to chemical, mechanical, or electrical stimuli. Although the mechanism of thermoreception is similar in all animals, the distribution of thermoreceptor cells varies widely from species to species. A fascinating thermoreceptive organ is found in certain snakes called pit vipers, including rattlesnakes. These snakes have small pits beneath their eyes that contain a delicate membrane, laced with thermoreceptive cells. Even without the use of its eyes, nose, and tongue, a rattlesnake can use these pits to locate an animal 40 cm (16 inches) away that is 10 C (18 F) warmer than the general surroundings. The pit organs serve to detect temperature differentials or changes, not the level of temperature itself. Additional reading J. Bligh and H. Hensel, Modern Theories on Location and Function of the Thermoregulatory Centers in Mammals Including Man, in Advances in Biometerology, vol. 1 (1973), covers thermosensors in the central nervous system; H. Hensel, Physiologie der Thermoreception, Ergebn. Physiol., 47:166368 (1952), a comprehensive review with references, Allgemeine Sinnesphysiologie: Hautsinne, Geschmack, Geruch (1966), discusses skin receptors, with comprehensive references, and Cutaneous Thermoreceptors, in Handbook of Sensory Physiology, vol. 2 (1973), describes thermoreceptors in the skin; R.W. Murray, Temperature Receptors, Advances Com. Physiol. Biochem., 1:117175 (1962), covers the comparative physiology of thermoreceptors; H. Precht, J. Christophersen, and H. Hensel, Temperatur und Leben (1955), a comprehensive review on temperature and life, covering microorganisms, plants, humans, and other animals; Y. Zotterman, Thermal Sensations, in John Field (ed.), Handbook of Physiology, section 1, Neurophysiology, 1:431458 (1959), and Specific Action Potentials in the Lingual Nerve of Cat, Skand. Arch. Physiol., 75:105120 (1936), a classic showing first electrical records from specific thermosensitive nerve fibres; M. Blix, Experimentela bidrag till lsning av frgan om hudnervernas specifika energi, Uppsala LkFr. Frh., 18:87102 (188283), reports on the discovery of cutaneous hot and cold spots; E.A. Brearley and D.R. Kenshalo, Electrophysiological Measurements of the Sensitivity of Cat's Upper Lip to Warm and Cool Stimuli, J. Comp. Physiol. Psychol., 70:514 (1970); T.H. Bullock and F.P.J. Diecke, Properties of an Infrared Receptor, J. Physiol., 134:4787 (1956); C.B. De Witt, Precision of Thermoregulation and Its Relation to Environmental Factors in the Desert Iguana, Dipsosaurus Dorsalis, Physiol. Zol., 40:4966 (1967); H. Hensel and K.K. Boman, Afferent Impulses in Cutaneous Sensory Nerves in Human Subjects, J. Neurophysiol., 23:564578 (1960), contains first records of neural impulses from human thermoreceptors; H. Hensel and D.R. Kenshalo, Warm Receptors in the Nasal Region of Cats, J. Physiol., 204:99112 (1969); Ainsley Iggo, Cutaneous Thermoreceptors in Primates and Subprimates, J. Physiol., 200:403430 (1969); S. Landgren, Convergence of Tactile, Thermal, and Gustatory Impulses on Single Cortical Cells, Acta Physiol. Scand., 40:210221 (1957); R. Loftus, The Response of the Antennal Cold Receptor of Periplaneta Americana to Rapid Temperature Changes and to Steady Temperature, Z. Vergl. Physiol., 59:413455 (1968); T. Nakayama et al., Thermal Stimulation of Electrical Activity of Single Units of the Preoptic Region, Am. J. Physiol., 204:11221126 (1963), first records of impulses from thermosensors in the cat hypothalamus; D.A. Poulos and R.M. Benjamin, Response of Thalamic Neurons to Thermal Stimulation of the Tongue, J. Neurophysiol., 31:2843 (1968). See also Giorgio Macchi, Aldo Rustioni, and Roberto Spreafico (eds.), Somatosensory Integration in the Thalamus (1983). Herbert Hensel