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HANDBOOK OF PHVSIOLOGV 



NEUROPHYSIOLOGY II 



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FIG. 17. Effect of hypothalamic warming (diathermic heating 

 through two pairs of parallel electrodes with bare tips, inserted 

 by the Horsley-Clarke technique into the anterior hypothala- 

 mus) on cortical electrical activity. L, left; R, right; F frontal 

 area; P, parietal area; occipital area. This is an unanesthe- 

 tized fw//)Aa/f !io/f of a cat; the trigeminal ganglia are blocked 

 by xylocain injections. Upper pari: Before warming, asynchronous 

 activity. Loiuer pari: During warming to approximate tempera- 

 ture at hypothalamic heating electrode of 42 °C, variable 

 synchronized activity with 'spindles," [From von Euler & 

 Soderberg (208).] 



result can be interpreted as e\idence that "central 

 shivering' is of little importance. The explanation 

 for these apparently discrepant experiments becomes 

 more clear if the problem is regarded from a quanti- 

 tative rather than qualitative point of view in the 

 following way. 



An intact animal which is subjected to cold stress 

 shows voluntary inuscular activity and signs of alert- 

 ness, even if shivering is not apparent. When it is 

 slowly warmed to rectal temperatures between 37° 

 and 39°C, the animal first becomes more less active, 

 shows drowsiness and perhaps goes to sleep; but later 

 when the rectal teinperature increases even more and 

 the heat-loss mechanisms have long since become in- 

 tensely active, the animal again becomes restless. 

 Moderate local hypothalamic warming can also 

 produce drowsiness. This change in behavior has its 

 counterpart in changes in activity of the cerebral 

 somatic facilitatory or activating systein, localized 

 in the reticular formation in the brain stem, which 

 has been described by Magoun and co-workers (150). 

 This activity can be measured from cortical EEG 

 changes (114), or from changes in the small-fiber 

 motor system to skeletal muscle (gamma-fiber system) 

 in the way suggested by Granit and co-workers (83), 

 using aflferent muscle spindle discharge as an index 

 of gamma-fiber activity. Such experiments in cats 



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FIG. 18. Effect of hypothalamic warming (as in fig. 17) on 

 the acti\'ity of the gamma-fiber motor system, as indicated by 

 the impulse frequency in an isolated afferent nerve fiber from a 

 gastrocnemius muscle spindle {upper tracing). Rabbit is under 

 chloralose-urethane anesthesia. Hypothalamic temperature is 

 recorded by a thermojunction placed at one heating electrode. 

 Gastrocnemius muscle does not contract (mechanogram in 

 lower Iracing). [From von Euler & Soderberg (208).] 



and rabbits under anesthesia, and also on unanes- 

 thetized encephale isole preparations, have been per- 

 formed by von Euler & Soderberg (207, 208). Local 

 hypothalamic warming of moderate degree syn- 

 chronizes the EEG (fig. 17) and simultaneously in- 

 hibits gamnia-filjer activity (fig. 18); intense warming 

 desynchronizes (arouses) the EEG (fig. 19) and simul- 

 taneouslv facilitates gamma-fiber activity (fig. 18), 

 and it may also produce obvious restlessness in a 

 superficially anesthetized raijbit. This result demon- 

 strates the iinportance of the gamma-fiber system and 

 the peripheral servoloop of skeletal muscle for the 

 production of shivering, and explains why deafTerenta- 

 tion suppresses shivering (157). A reasonable con- 

 clusion is that the hypothalamic thermodetectors pro- 

 ject upon and to some extent modulate the activating 

 system in the brain stem, thereby influencing wake- 

 fulness and skeletal mu.scle tone. An interesting 

 further suggestion from these experiments is that the 

 brain-stem activating system, which is inainly con- 

 trolled by nonthermoceptive projections, may in- 

 fluence the activities of the thermoregulatory efifector 

 svstems, including the skeletal muscles, as an in- 

 dependent reference mechanism (209). If such were 

 the case, a certain change in the intensity of lunction 

 of the activating system might balance the coordina- 

 tion of the diff'erent heat-loss and heat-production 

 mechanisms at a new le\el of bodv (or brain) tcm- 



