i3o8 



HANDBOOK OF PHYSIOLOGY ^ NEUROPHYSIOLOGY II 



FIG. I. Effect of unspecific 

 thalamic stimulation in n. cen- 

 tralis medialis of the cat upon 

 electrical activity from the an- 

 terior suprasylvian gyrus. The 

 animal was under light pento- 

 barbital anesthesia. A: Spon- 

 taneous spindle burst. B: A 

 single I msec, shock (s) 'tripping' 

 a spindle burst. C: Repetitive 

 stimulation at 5 per sec. showing 

 waxing and waning of recruiting 

 response. D: Spike and wave 

 response to stimulation at 2.5 per 

 sec. (Unpublished records taken 

 with J. Droogleever-Fortuyn. ) 



A 



B 



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produce a regular wave-and-spike complex as seen 

 in petit mal epilepsy (fig. iD). 



A topographical organization with respect to 

 different areas of cortex was also demonstrated by 

 carefully controlled local stimulation within different 

 parts of the intralaminar system. Numerous electro- 

 physiological and anatomical studies then followed 

 to give a clearer picture of the structure and functional 

 characteristics of the unspecific as compared to the 

 specific thalamocortical projection system. 



RECRUITING RESPONSE 



A single electrical stimulus of brief duration 

 administered to the central portion of the intra- 

 laminar system may result in little evidence of immedi- 

 ate response from the cortical surface. There occurs, 

 however, a burst of rhythmic waves resemijling the 

 spontaneous (spindle bursts) of the resting cortex 

 (fig. 1.4 and B). These bursts may begin in frontal or 

 motor cortex after only a few milliseconds delay, but 

 with delays up to several hundred milliseconds in 

 more posterior areas of the cortex (fig. 2). 



Repetitive stimulation at a frequency approxi- 

 mating that of the dominant spontaneous electrical 

 rhythm (6 to 12 per sec.) results in the recruiting 

 response (fig. iC). Under opdmal conditions this 

 response is characterized by a successively increasing 

 surface negative wave reaching a maximum after 

 two to five successive stimuli. ^Vith continued repet- 



itive stimulation the response may then progressively 

 decline in amplitude and then recur again in a 

 spindle formation, waxing and waning in a manner 

 similar to spontaneous spindle bursts. During this 

 time the recruiting response may completely dominate 

 the cortical electrical activity, no spontaneous rhythms 

 appearing. With strong stimulation, in a favorable 

 preparation, responses may continue with little 

 waning as long as the frequency of repetitive stimu- 

 lation is kept within rather narrow limits. 



Slower frequencies of stimulation may result in 

 doubling of responses while frequencies above 12 per 

 sec. may cause alternation of response. With higher 

 frequencies, above 20 per sec, no responses may ap- 

 pear and the spontaneous rhythmic acti\it\- ot the 

 cortex may be completely arrested. 



The latency of onset of the surface negative wave 

 of the recruiting response is usualK' 20 to 40 msec, 

 following the thalamic stimulus. However stimulation 

 of the rostral portion of the system, in n. ventralis 

 anterior and reticularis, may result in responses in 

 frontal and motor cortex of shorter latency (5 to 10 

 msec.) and rapid recruitment, with a near maximum 

 response even to the first of a series of stimuli. A 

 recruiting response in the motor cortex with a latency 

 of 10 msec, from stimulation of the rostral portion of 

 the thalamus in n. ventralis anterior is compared 

 with a response from the same cortical area conducted 

 from a more caudal portion of the system in the 

 centrum medianum is shown in figure 3. There is a 

 shorter latencv 'tripping' of spindle bursts and rapid 



