362 B. ANDERSSON AND OTHERS 



vasoconstriction. A raised body temperature, on the other hand, may cause 

 the mobiUzation of various heat loss mechanisms (sweating, peripheral 

 vasodilatation and polypneic panting) by the mediation of central " warm 

 detectors " in the preoptic region and the rostral hypothalamus (the " heat 

 loss centre ") (Magoun, Harrison, Brobeck and Ranson, 1938 ; C. von 

 Euler, 1950). 



The question still remains open whether parallel to central " warm 

 detectors " there may exist central " cold ", or, rather, " hypothermia 

 detectors " which serve to activate various cold defence mechanisms. 

 Local coohng of the rostral hypothalamus in the dog generally induces 

 shivering in spite of little or no inflow from peripheral cold receptors 

 (Hammel, Hardy and Fusco, 1960). For this reason Hammel et al. (1960) 

 have suggested that the rostral hypothalamus and preoptic region may be 

 the site not only of central " warm detectors " with the task to prevent 

 hyperthermia, but also the site of central " hypothermia detectors " having 

 the opposite thermoregulatory function. The preoptic region and the 

 rostral hypothalamus would then be regarded not simply as a " heat loss 

 centre " but rather as a thermoregulatory " centre " of a more general 

 character. Microelectrode studies of neuronal activity in this part of the 

 brain do not support this view. The activity of numerous neurons are 

 found to increase in response to a raised hypothalamic temperature, but no 

 neurons seem to react specifically to a lowering of the temperature (Naka- 

 yama and Hardy, 1962). Further, it does not seem necessary to postulate 

 the existence of central " hypothermia detectors " to explain the thermo- 

 regulatory effects of local cooling of the preoptic region and rostral hypo- 

 thalamus of the goat (Andersen, Andersson and Gale, 1962). In this species 

 even profound cooling of the rostral hypothalamus elicits shivering only if 

 there is an inflow from peripheral cold receptors of if the shivering mechan- 

 ism is facilitated by certain humoral or emotional factors. Thus in the calm 

 goat, fully accustomed to the experimental conditions, local cooling of the 

 preoptic region and the rostral hypothalamus induces shivering in a cold 

 environment only (Fig. 1), or during local stimulation of peripheral cold 

 receptors. But if these goats are subjected to mild stress or to an infusion 

 of adrenal catnecholamines, cooling of the " heat loss centre " may elicit 

 shivering in a thermally neutral or even warm environment (Fig. 2). It 

 thus seems as if the central " warm detectors " of the " heat loss centre " 

 serve not only to activate various heat loss mechanisms but also as a brake 

 on the shivering mechanism. However, only when there is a simultaneous, 

 stimulatory " drive " on the shivering mechanism from peripheral cold 

 receptors or from other sources (emotional excitation, sympathico-adrenal 

 activation, pyrogens etc.) does an inactivation of these " warm detectors " 

 by cooling elicit shivering. (For tentative scheme see Fig. 3). 



It is well known that not only nervous, but also endocrine factors are of 



