CENTRAL NERVOUS REGULATION OF BODY TEMPERATURE 



II77 



of Willis also seems to be of importance in this re- 

 spect. 



The suggestion that the medulla oblongata contains 

 thermosensitive structures influencing cutaneous 

 blood flow (13) and sweating (95) has not been 

 substantiated. 



Mechanism of Activalion of Thetmodetedors 



The question has arisen whether the hypothalamic 

 thermodetectors are sensitive both to warming and 

 cooling, as suggested by early investigators (cf. figs. 

 4, 14). It should be remembered that the single unit 

 response of the detectors has not yet been recorded. It 

 is therefore not known whether a temperature rise 

 which activates the heat-loss meclianisms produces an 

 increase or a decrease (or perhaps both) in firing 

 frequency of the first order neurons into which the 

 detectors discharge (or which may be identical with the 

 detectors). The surface thermoreceptors, the function 

 and unitary responses of which are well known, may 

 be considered analogous to the hypothalamic ther- 

 modetectors (103). Each surface receptor unit shows 

 maximum activity (afferent firing frequency of the 

 first order neuron) at an individually characteristic 

 temperature. This temperature varies considerably 

 within the family of receptor units. A rise of tempera- 

 ture is therefore accompanied by successive deactiva- 

 tion of some receptors and activation of others; 

 within the normal range of body temperature the 

 former may be classed as cold receptors and the latter 

 as warm receptors. The only well-known property of 

 the hypothalamic thermodetectors is that the range 

 of temperatures within which the effector systems 

 show prominent reactions, is relati\ely small, as 

 suggested in figure 4, extending from perhaps i 

 degree C below to a few degrees above the normal 

 brain temperature (187, 190). At brain temperatures 

 above 41 °C or 42 °C the effector systems may show 

 reversal of reaction. It is possible therefore that the 

 hypothalamic thermodetectors resemljle surface warm 

 receptors in having a temperature level for maximum 

 activity slightly above normal brain temperature. 



The activation mechanism (mode of transducing 

 thermal to electrical signals) of the thermodetectors is 

 incompletely known but important information on 

 this prol)lem has been reported by von Euler (203, 

 204). When the brain is warmed via the carotid blood 

 stream, a steady potential field is de\'eloped between 

 the supraoptic region and the rest of the brain (fig. 

 5). The potential change, which may be as steep as 

 0.5 to i.o mv per o. i°C, is not produced by impulse 

 firing; it may be analogous to the steady potential 



HYPOTHALAMIC 



TEMP, 



FIG. 4 Effect of diathermic heating and conductive cooling 

 of the anterior hypothalamu.'; on cutaneous blood flow. Cat is 

 under urethane anesthesia with artificial respiration. Two 

 parallel silver wire electrodes with bare tips, 4 mm apart, are 

 inserted by the Horsley-Clarke technique into the hypothala- 

 mus. Lower record: Hypothalamic temperature measured by a 

 thermojunction placed midway between electrode tips. Upper 

 record: Blood flow from the skin of the forelimb pad measured 

 by a drop interval recorder, ordinate height being inversely 

 proportional to flow. Diathermic heating continuously from 

 I to 3; conductive cooling at 2 and 4. [From Strom (187).] 



fields that can be registered in receptor structures 

 with systematic spatial orientation, such as the retina, 

 and may represent a 'generator potential' (the steady 

 depolarization of receptors or of the finest terminals 

 of first order afferent neurons which generate im- 

 pulses in those neurons). It is interesting to note that 

 a similar local steady potential field develops in the 

 medulla oblongata when the arterial pCOs is changed. 



