CENTRAL NERVOUS REGULATION OF BODY TEMPERATURE 



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Fio. 16. Suppression of spontaneous shivering by electrical stimulation of the hypothalamus 

 (electrodes inserted by the Horsley-Clarke technique). Cat is under light pentobarbital anesthesia. 

 Shivering (SH) was recorded electromyographically by skin electrodes {A and C, thigh muscle; B 

 and D, tail muscle). Hypothalamic suppression of shivering does not influence either cortically 

 evoked muscular contractions {upper part of the figure), or the spinal reflex evoked by muscle stretch 

 {lower pari, stretch reflexes responsible for repetitive high voltage spikes). [From Hemingway el al. 

 (100).] 



becomes obvious in man when during; cooling the 

 central body temperature approaches 36.5° to 36°C 

 (184), then increases at lower temperatures to a 

 maximum at 33°C under light anesthesia (185). 

 .Shivering in man stops at a central body temperature 

 just about or just below 30°C, in the rat at i6°C, and 

 in hibernating animals at even lower temperatures 

 (6°C). Hypoxia inhibits shivering (132). Shivering is 

 obviously influenced by surface thermoreceptors (56) 

 and by thermoreceptors in the tracheal mucosa (54) 

 which explains the intensification of shivering during 

 inspiration. It is also clear that local hypothalainic 

 warming (loi) and electrical stimulation (4, 100) 

 which set a coordinated heat-loss mechanism into 

 action can inhibit shivering (figs. 6, 15, 16) in an un- 

 anesthetized animal which has first been subjected to 

 light cold stress to establish shivering. On the other 

 hand, hypothalamic electrical stimulation can also 

 produce shivering (2). 



It is more doubtful wliether local hypothalamic 

 cooling, to levels well below normal brain tempera- 

 tures, alone can evoke shivering in the unanesthetized 

 aniiTial which is in thermal balance (190) (fig. 14). 

 Such experiments should preferably be made on un- 

 anesthetized animals (49, 132a, 190) as anesthesia 

 suppresses shivering. If a dog with chronic high spinal 

 transection (Ce level) is cooled, the muscles inner- 

 vated from above the transection exhibit shivering 

 (fig. 8) in most cases (181) but not all (14). This 

 response might be evoked either from brain thermo- 



detectors or from those surface thermoreceptors (in 

 the head skin, oral and nasal cavities) still in afferent 

 connection with the cerebrum; the experiment there- 

 fore does not give definite evidence for either possi- 

 bility. Stronger evidence for central elicitation of 

 shivering is obtained if in similar experiments the 

 local skin and brain temperatures are registered 

 during the cooling (49). It has even been concluded 

 that two types of shivering ('reflex' and 'central') 

 can be separately elicited (49) and also distinguished 

 by their patterns of appearance (139). On the other 

 hand, local cooling of the anterior or posterior hypo- 

 thalamus (fig. 14) has not ijeen observed to evoke 

 shivering in a thermally balanced animal (190), a 

 fact suggesting that the hypothalamic thermorecep- 

 tive structures are relatively insensitive in the tein- 

 perature range below normal brain temperature or, 

 at least, have little effect in promoting shivering. As a 

 negative result, however, this might be explained as 

 due to recruitment of an insufiiciently large number of 

 central thermodetectors by the local cooling. In a 

 careful series of experiments on dogs under light 

 chloralose anesthesia, the brain was cooled via the 

 carotid blood stream while the body temperature 

 was kept constant (35a). The oxygen uptake, re- 

 flecting i.a. skeletal muscular activity, was found to 

 decrease continuously with decreasing brain tempera- 

 ture under these circuinstances. As the anesthesia 

 caused only a partial and not a complete depression 

 of the shivering response to whole body cooling, this 



