EFFECTS ON NERVOUS SYSTEM— McQUEEN 245 



They indeed provided the first direct evidence that nervous tissue can better tolerate 

 hypoxia w^hen cooled. Chatfield et alJ demonstrated a species difference in compar- 

 ing the responses from the cooled tibial nerves of the rat and hamster. The critical 

 temperature for the former was 9° C. and for the hibernator 3.4° C. This latter 

 level is just below the body temperature maintained by the animal in deep hiberna- 

 tion. They further showed that the nerves from hibernating and cooled non-hiber- 

 nating hamsters reacted identically. 



Kahana, Rosenblith, and Galambos-- noted the neural component of the round- 

 window response of the hamster to be absent below a temperature of 18° C. They 

 thus established a specificity amongst nerves of the same animal. 



Gasser^^ found the amplitude of the action potential of frog peripheral nerve to 

 be decreased on cooling. The temperature coefficients for conduction time, spike 

 duration and refractory phases were quite distinct, but all were greatly augmented 

 in the lower temperature range. Chatfield and his group^ confirmed these results for 

 the golden hamster. The action potential height, conduction velocity and excitability 

 all decreased in a linear fashion although at a slower rate in the hibernator. 



Denny-Brown et al. in 1945^ studied the results of direct freezing of mammalian 

 nerve, as others had done before. In addition, they enclosed nerves within small 

 metal cuffs, through which cooling fluids were circulated. The nerve temperature 

 was then brought to a level somewhat above freezing. They were so able to estab- 

 lish definite damage to both the myeline sheaths and axis cylinders of the largest 

 fibers at levels of approximately 8° C. and for periods as short as 10-15 minutes. 

 They found motor deficit to precede sensory loss, and the sensation of touch to be 

 affected before that of pain. 



From this and other work (Lundberg-^), it is known that C fibers in general 

 are much more resistant to cooling than A fibers. With minimal lesions, the largest 

 myelinated nerve fibers will be selectively involved. The effects of cold at the range 

 of 8°- 10° C. and perhaps higher are quite comparable to the effects of ischemia. -"^^ 

 The minimal injury induced by cold may well explain in part at least those periph- 

 eral neuropathies now being observed during the course of general hypothermia 

 in the human. It is noteworthy that sympathetic fibers are numbered amongst those 

 most resistant to the action of cold. 



Central nervous system. Electroencephalography has perhaps best monitored 

 the narcotic action of cold on the brain. Records are available for dog,-^ cat,-^ rat,'*° 

 various hibernators,^ monkey*^ and man.^^ The patterns are generally uniform for 

 the group, although a species dift'erence is recognized within hibernators.^" Pre- 

 cooling potentials varying between 50 and 100 //v are registered with the animals 

 under light barbiturate anaesthesia. A decrease in amplitude is usually not noted 

 until a range of temperatures between 32° and 36° C. is reached. The potentials 

 from occipital and parietal regions fall at the upper limit of this range, those from 

 the frontal areas at the lower limit. The voltages then drop evenly to disappear at 

 the level of electrical silence w^hich is at a rectal temperature of 18°- 20° C. One 

 noted exception to this general decline is the introduction of delta waves of large 

 amplitude (100 ,«v) found at the 30° level and predominantly in the frontal region. 

 Changes in wave form and frequency also became first apparent in the 32°- 36° 

 range. An intensification of the delta and theta activity with a loss of intricacy of 



