KAWAMURA, Y. 
cedes shivering. This may mean that at this stage of anesthesia in 
which an increase in muscle tone but no shivering is evident, the 
animal is too deeply anesthetized to permit respiratory facilitation 
of shivering. However, this lack of relation between shivering and 
muscle tonus may be due to the latter phenomenon having spinal, 
rather than central origin. Classical decerebration studies have 
illustrated the predominantly inhibitory action the rostral nervous 
system exerts on the spinal cord. Just as decerebration releases 
the spinal cord from inhibitory influences so it may well be that in 
the light anesthetic state, the supra-spinal inhibitory regions are 
more depressed than the spinal cord. Hence, the initial increase in 
muscle tone is not directly related to temperature regulation but 
represents a stage of anesthesia in which proprioceptive hyper- 
activity predominates just as decerebrate rigidity is due to a hyper- 
activity of myotactic origin. 
In addition to a central activating region being necessary to in- 
itiate shivering, there is evidence that shivering is facilitated by 
afferent impulses from proprioceptive nerve endings that may exert 
these excitatory effects centrally and/or peripherally. 
The evidence is as follows: 
First. During deep anesthesia the somatic muscles are com- 
pletely relaxed and no electrical activity as revealed by unit 
potentials is noted. As the animal recovers from the anesthesia and 
the muscles regain tonus, unit spike potentials begin to appear and 
these are followed by shivering. Present results in these obser- 
vations confirm the earlier work of Burton and Bronk (1937). The 
work here reported suggests that as the muscle tone increases, the 
resulting activation in integrative afferent proprioceptive input from 
annulo- spiral, flower spray, Golgi tendon organ, and deep joint pro- 
prioceptors is favorabletotheproductionof shivering. For example, 
it is well known that increased annulo-spiral discharge from any 
muscle fiber lowers the alpha motor neuron thresholds of that 
muscle. Kawamura et al (1958) have shown that there is an increase 
in the discharge of frequency of the trigeminal mesencephalic 
nucleus (which is considered to receive proprioceptive information 
from the jaw muscles) immediately prior to the onset of jaw muscle 
shivering. 
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