i3i8 



HANDBOOK OF PHYSIOLOd' 



NEUROPHYSIOLOGY II 



Recruiting responses obtained from stimulation of 

 the intralaminar portions of the thalamus can also be 

 blocked completely by simultaneous more rapid stimu- 

 lation of the midbrain reticular system (59). Also re- 

 cruiting responses are not readily obtained in an 

 animal which shows the activation pattern in the cor- 

 tex due to arousal by sensory stimulation when in the 

 unanesthetized state. These observations demonstrate 

 a close functional relationship between the brain-stem 

 and thalamic reticular systems. 



It has also been shown that large bilateral lesions 

 produced by coagulation of the anterior portion of the 

 thalamic reticular system result in unresponsiveness 

 of animals quite similar to that produced by lesions of 

 the midbrain reticular system (28, 48, 49). There may 

 be some qualitative difference here however in that 

 the profound depths of coma produced by midbrain 

 lesions do not seem to be reproduced as completely in 

 all cases by lesions within the anterior portion of the 

 thalamic reticular system. 



Differences between the cortical activation pro- 

 duced at thalamic or midbrain levels of activation 

 should be pointed out however. In the first place, the 

 recruiting response is not readily obtained by stimu- 

 lation of the midljrain reticular system and usually 

 requires stimulation within the thalamus.' The pro- 

 longed activation of tiie animal with the typical 

 change in cortical electrical activity obtained by 

 stimulation of the midbrain reticular system, or of the 

 basal diencephalon, is not readily produced by rapid 

 stimulation in the anterior portion of the thalamic 

 reticular system. 



The blocking of spindle bursts in the electrical ac- 

 tivity of the cortex occurs only for a brief period of 

 time, sometimes not outlasting the stimulus itself, 

 when the thalamic system is being stimulated. Pro- 

 longed blocking of rhythmic cortical activity however, 

 is readily produced, lasting for 30 sec. to several min- 

 utes, foUowina; stimulation of the midbrain reticular 

 system in the unanesthetized animal. 



It has been shown that cortical activation is of two 

 forms, one of rapid onset and brief duration, and an- 

 other of slow onset and prolonged duration. The for- 

 mer has been called the phasic activation system and 

 the latter the tonic system. It now seems clear that the 

 tonic activation system lies in the basal diencephalon 

 and midbrain and has many properties distinct from 



^ Evarts & Magoun ha\e recently shown that a recruiting 

 response can be obtained under certain conditions with im- 

 planted electrodes in the brain stem of the unanesthetized 

 cat (26a). 



the thalamic system. The phasic type of rapid but 

 short duration arousal seems to be characteristic of the 

 thalamic system. The tonic activation .system of the 

 brain stem is particularly sensitive to epinephrine, and 

 it may well be that the tonic phase of activation from 

 this area is largely due to the humoral aspect of acti- 

 vation of the brain with cells sensitive to this type of 

 activation only in the diencephalon or hypothalamus 

 and in the upper midlirain, but not in the thalamic 

 portion of the reticular system (74). 



There may be also differences in the effects of stim- 

 ulating the thalamic reticular system upon behavior 

 in the unanesthetized preparation. There is some evi- 

 dence that diminished responsiveness or even sleep- 

 like states may be produced from stimulation of the 

 thalamic reticular system, as in the so-called "arrest 

 reaction' of Hunter & Jasper (35) or the sleep reaction 

 of Hess (32, 33, 34). The intense excitement of an ani- 

 mal with attack or fear responses is not obtained from 

 stimulation of the thalamic reticular system but re- 

 quires stimulation either of the posterior hypothal- 

 amus or upper midbrain. It has recently been shown 

 by Akimoto et al. (3) that repetitive stimulation at 

 5 to 10 per sec, producing a high degree of synchron- 

 ous activity throughout the brain, results in sleep, 

 while stimulation at more rapid frequencies causing 

 desynchronization of cortical activity may result in 

 an awakening reaction in unanesthetized animals with 

 electrodes implanted in the intralaminar portion of 

 the thalamus. 



Another important difference between the brain- 

 stem and the thalamic reticular system is that of topo- 

 graphical organization. Very little evidence for any 

 topographical organization can be shown at the level 

 of the hypothalamus or midbrain. There is some ten- 

 dency for activating influences to be most effective on 

 rostral cortical areas in frontal and motor regions, but 

 in general the effects are quite generalized. In the tha- 

 lamic system, on the other hand, different points with- 

 in the system seem to have a different pattern of corti- 

 cal projection. This is not a rigid topographical organ- 

 ization and there are many close interrelationships 

 present, but it seeins likeK that there are portions of a 

 system which can regulate the electrical activity of 

 limited areas of the brain without affecting the cortex 

 as a whole. 



This capacity for some degree of localized activa- 

 tion would suggest functional properties of the tha- 

 lamic system of a inore highly organized character re- 

 lated to specific or different cortical functions rather 

 ihnn to a;eneralized arousal or activation of the brain 



