Neural processes in long-term thermal adaptation.

Short-term habituation of cats to repeated cold stimuli includes a peripheral component owing to the dynamic response of cutaneous cold receptors. Long-term cold adaptation of up to 4 years causes significant changes in the static and dynamic cold fiber afferents from the cat's nose. These changes, however, do not seem to be essential for the adaptive changes of the thermoregulatory system. Fur growth of cats living for 2 years at 5 C ambient temperature was 30-40% higher than that of controls living at 30 C, and the metabolic response curve was shifted to lower ambient temperatures in the cold-adapted animals. These adaptations are maintained by thermal afferents from the periphery rather than from central thermosensors. Repeated local hypothalamic or spinal cooling in unanesthetized rats for 130 h increased nonshivering thermogenesis after noradrenaline and operant work for warmth in cold environments but did not change general cold resistance. Local cold thresholds of the hand in human subjects were practically independent of the general thermoregulatory state at 10 and 50 C ambient temperature, respectively, whereas thermal comfort elicited by cooling or warming the hand changed considerably with thermoregulation. It is thus assumed that temperature sensation and thermal comfort are mediated by different neural pathways. Repeated exposure to warm or cold environments for 5 days led to a nonspecific increase of affective responses to heating as well as to cooling the hand.