I316 HANDBOOK OF PHYSIOLOGY — NEUROPHYSIOLOGY II 



MYOGRAM 



CORTICAL STIMULATION 

 / 



ON 



INTRALAMINAR STIMULATION 



» 6 SECONDS ' 



OFF 



FIG. 8. Facilitation of cortically-induced movements in the cat (lightly anesthetized with pento- 

 barbital) during intralaminar stimulation. [From Jasper (38).] 



In the motor system it was first shown that corti- 

 cally-induced movements could be facilitated mark- 

 edly by rapid stimulation of the intralaminar system 

 of the thalamus (38). The facilitation outlasted the 

 period of stimulation by 20 to 30 sec, as shown in 

 figure 8. Some of this facilitating effect remained after 

 removal of the motor cortex, as shown by stimulation 

 of the white matter beneath, so that this may repre- 

 sent largely a descending action upon spinal motor 

 centers. 



Brookhart & Zanchetti (12) have failed to show any 

 effect of recruiting waves upon the activation of pyram- 

 idal cells, as recorded by means of electrodes di- 

 rectly in the tract at the level of the decussation (see 

 fig. 9). Recruiting waves are often of large amplitude 

 in the motor cortex, so that it would be surprising if 

 they did not have some effect upon motor cortical 

 function. Some relationship has been reported by 

 Arduini & Whitlock (6). A definite relationship to 

 spontaneous 'spindle' bursts was shown by Whitlock 

 et al. (86) as well as by Brookhart & Zanchetti who 

 also showed a clear relation between pyramidal dis- 

 charge and augmenting waves in the motor cortex in 

 response to stimulation of the n. ventralis lateralis of 

 the thalamus. It is apparent that there are multiple 

 rhythmic systems in the cortex and thalamus which 



may be dissociated and may ha\e different functional 

 significance (11). 



Repetitive stimulation of specific thalamic nuclei at 

 frequencies between 6 and 12 per sec. gives rise to 

 local incremental responses simulating a recruiting re- 

 sponse. These were called "augmenting responses' by 

 Morison & Dempsey (24, 58). They are distinguished 

 by their location in the area of known projection of the 

 specific svstem and by the short-latency surface-posi- 

 tive evoked potential complex which initiates each 

 successive response. It is the surface-negative compo- 

 nent which shows the greatest augmentation, and 

 there may be waxing and waning in amplitude in a 

 manner very similar to that observed for recruiting re- 

 sponses. It would seem that similar cortical elements 

 and mechanisms of summation must be involved in 

 both recruiting and augmenting responses, but this is 

 not always true. It may e\en indicate that some un- 

 specific cortical afferents, predominantly axodendritic, 

 may also originate in specific thalamic nuclei, as sug- 

 gested by the anatomical studies of Nauta & Whit- 

 lock (62). The longer latency of the true recruiting 

 response mav be due to multisynaptic circuits in the 

 thalamic reticular system (5), while summation and 

 delay in the augmenting response may be largely of 

 cortical origin. Considerable delay may be accounted 



