Differential control of efferent sympathetic activity revisited.

This article reviews 40 years of research (1970-2010) into the capability of the efferent sympathetic nervous system to display differential responsiveness. Discovered first were antagonistic changes of activity in sympathetic filaments innervating functionally different sections of the cardiovascular system in response to thermal stimulation. During the subsequent four decades of investigation, a multitude of differential sympathetic efferent response patterns were identified, ranging from opposing activity changes at the level of multi-fiber filaments innervating different organs to the level of single fibers controlling functionally different structures in the same organ. Differential sympathetic responsiveness was shown to be displayed in response to exogenous or artificial stimulation of afferent sensory fibers transmitting particular exogenous stimuli, especially those activating peripheral nociceptors. Moreover, sympathetic differentiation was found to be characteristic of autonomic responses to environmental changes by which homeostasis in the broadest sense would be challenged. Heat or cold loads or their experimental equivalents, altered composition of inspired air or changes in blood gas composition, imbalances of body fluid control, and exposure to agents challenging the immune system were shown to elicit differential efferent sympathetic response patterns which often displayed a high degree of specificity. In summary, autonomic adjustments to changes of biometeorological parameters may be considered as representative of the capability of the sympathetic nervous system to exert highly specific efferent control of organ functions by which bodily homeostasis is maintained.

Heat disorder in Yamanashi Prefecture during the summer from 1995 to 2004.

The incidence of heat disorders in July and August during 10 yr (1995-2004) reported for the population of Yamanashi prefecture was analyzed, with special consideration of an aging society, in relation to levels and patterns of phases with high daily maximal temperatures. There was an increasing tendency for years with hot summers in comparison to preceding decades. Two climatic characteristics associated with increased incidence of heat disorders have become apparent: first, sustained phases of atmospheric temperatures exceeding approximately 32 degrees C, second, rapid onset of phases with high maximal temperature after preceding phases of relatively cool weather. The influence of age expressed itself in a peak of heat disorder incidences among older children and adolescents and in an elevated plateau at ages higher than approximately 60 yr. Up to that age, exertional heat disorders prevailed. At higher ages classical, non-exertional heat disorders constituted an increasing fraction. Lethal outcomes among patients suffering from heat disorder was low. Patients older than 70 yr clearly prevailed among the lethal cases. The frequent occurrence of heat disorders among persons of old age puts emphasis on the importance of maintaining social activity to improve well-being in general and physiological resistance against heat in particular, including adequate fluid supply. Because physiological heat defense is limited in an aging population, adequate air conditioning will gain increasing importance in view of the observed tendency for the increasing occurrence of phases with excessively high atmospheric summer temperatures.

Role of afferent pathways of heat and cold in body temperature regulation.

The detection of surface and internal temperatures is achieved by axons terminating at lamina I of the spinal dorsal horn, otherwise approached only by nociceptive afferents. Recent advances in thermal physiology research have disclosed that temperature-sensitive ion channels belonging to the "transient receptor potential" family exist in the peripheral sensory neurons and in the brain. Thermosensory, nociceptive and polymodal afferents project to different thalamic nuclei, and specific pathways to the insular cortex evoke the conscious experience of thermal sensation. The posterior insular region represents discriminative thermal sensation, while the largest correlation with subjective ratings of temperature is located in the orbitofrontal and anterior insular cortex. The insular cortex forms an integrative part of the limbic system and is closely tied with the hypothalamus, the amygdala, the anterior cingulate cortex and the orbitofrontal cortex and emerges as the main coordinator of behavioral, autonomic and endocrine responses to both non-noxious and noxious thermal stimuli. The firing rate of warm and cold receptors is not altered by pyrogens. A strong correlation between the onset of fever and production of superoxide by macrophages following the injection of pyrogens implicates reactive oxygen species as elicitors of fever, a hypothesis strengthened by the observation that oxygen radical scavengers or thiol reductants act as antipyretics. Oxidative stress appears to be sensed by the brain and a likely structure for its detection may be the redox-sensitive site of the N-methyl-D-aspartate (NMDA) receptor for glutamate, in that oxidation of this site causes fever while its reduction lowers body temperature, effects which are abrogated by specific NMDA receptor blockers.