CENTRAL NERVOUS REGULATION OF BODY TEMPERATURE 



I 193 



cutaneous blood flow and water evaporation. The 

 two types of thermosensitive elements therefore have 

 significantly different roles in regulation and supple- 

 ment each other. 



When cutaneous cold receptors are activated by 

 external cooling, cutaneous vasoconstriction ensues 

 as a regulatory measure. Skin temperature is thereby 

 further lowered, and the cold receptors are still more 

 intensely activated. This is a positive feed-back system 

 which would lead to large oscillations of cutaneous 

 blood flow in response to small stimuli unless modu- 

 lated by an independent type of receptive mechanism 

 working as a negative feed-back system. It is interest- 

 ing to note that in some animals the .skin areas of 

 dominating receptive importance are not identical 

 with those of dominating effector responses. This 

 would diminish the positive feed-back tendency of the 

 peripheral system (209). In man, on the other hand, 

 these areas seem to coincide (69). 



Another aspect of the interplay between surface 

 thermoreceptors and central thermodetectors is 

 illustrated by an old observation, which anyone can 

 repeat, that heat-lo.ss mechanisms which have been 

 activated by a definite rise of body temperature can 

 be suppressed by cold stimulation of a small skin 

 area : the sweating evoked by a hot bath of long dura- 

 tion stops quickly when one hand is held in cold water 

 (64). The generalized cutaneous vasoconstriction 

 which rapidly follows local cooling of the hands leads 

 to decreased heat loss and a definite increase of rectal 

 temperature (6). If then the blood flow through the 

 cooled extremity is suddenly increased, body tem- 

 perature falls (because of redistriiaution ot lieat be- 

 tween deep and surface body layers) and shivering 

 may start (82). In evaluating such obser\'ations it 

 should be kept in mind that local skin cooling besides 

 activating cold receptors also may evoke pain. 



If the central body temperature is kept sufficiently 

 high, cutaneous blood vessels remain dilated even at 

 very low ambient temperatures (173). Different skin 

 areas have different influences on central temperature 

 regulation, the skin of the face being relatively most 

 important (7). It also shows the quickest vasomotor 

 responses in thermal stress. Localized warming of the 

 skin, especially in the face, can evoke cutaneous 

 vasodilatation (fig. 22) which by increasing the rate 

 of heat loss lowers the body temperature until a new 

 balance is reached (79). This explains the initial 

 sensation of warmth when the face is exposed to the 

 sunshine on a cold spring day, and the sudden chill 

 and slight shivering which follow some quarter of an 

 hour later. 



Marked changes of thermoregulatory effector 

 activities may occur with almost constant rectal tem- 

 perature. As shown in figure 23, oral temperature is 

 better correlated with such changes (55), a fact which 

 reflects the dominating importance of cranial thermo- 

 ceptive structures for the conditioning of central 

 nervous temperature regulation. 



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