CHAPTER L 1 1 1 



Unspecific thalamocortical relations 



H E R B R R T H . J A S P E R | The Montreal Neurological Institute, McGill University, Montreal, Canada 



CHAPTER CONTENTS 



Introduction 



Recruiting Response 



Microelcctrodc Studies of Recruiting Response 



Thalamic Distribution of Unspecific Projection System 



Interrelations Between Specific and Unspecific Projection 

 Systems 



Relation Between Thalamic and Brain-Stem Ascending Re- 

 ticular Activating Systems 



Suinmary and Conclusions 



INTRODUCTION 



THE UNSPECIFIC THALAMOCORTICAL projection svstein 

 constitutes the rostral portion of tiie ascending retic- 

 ular activating system of the brain stem (54, 55). It 

 is sometimes called the thalamic reticular svstem 

 (39). It partakes of some of the properties of the more 

 caudal portions of the reticular system in the basal 

 diencephalon and midbrain with which it is closely 

 connected. It serves to mediate and to distribute to 

 almost all areas of cortex, some (although not all) 

 of the ascending activation originating in the more 

 caudal portions of the brain-stem reticular system. 

 In addition, the thalamic reticular system maintains 

 a more direct control over the rhythmic electrical 

 activity of the cortex and may serve also as an intra- 

 thalamic integrating system. 



The anatomy and distinct physiological properties 

 of the thalamic reticular system were first described 

 by Morison & Dcmpsey in 1942 (22, 23, 57). By 

 exploring the thalamus with a stimulating electrode 

 while recording the electrical activity from local 

 areas of the cortex in the cat, they were able to dis- 

 cover a separate projection system originating in the 

 intralaminar svstem of thalamic nuclei. The lonsrer 



latency, widespread 'diffuse' distribution and 're- 

 cruiting' character of the cortical response to repet- 

 itive stimulation of intralaminar nuclei served to 

 distinguish this projection system from the local 

 short latency respon.ses to stimulation of specific 

 thalamic nuclei with known local cortical connections. 

 These obser\ations led Mori.son & Deinpsev to the 

 important conclusion that "there exists between 

 thalamus and cortex at least two systems, with very 

 different physiological properties: a, the well known 

 specific projection .system with a more or less point-to- 

 point arrangement; b, a .secondary non-specific 

 system with diffuse connections" (57, p. 292). It was 

 postulated that recruiting responses were mediated 

 by the unspecific cortical afferent fibers described by 

 Lorente de No (50). 



The importance of the ascending reticular acti- 

 vating system, brought to light by Magoun and co- 

 workers, was not known at this time. However, Berger 

 (7, 8), Adrian (i) and Rhcinberger & Jasper (71) 

 had observed that there was a general mechanism for 

 the regulation of the electrical activity of the cerebral 

 cortex as a whole. It was related to processes of 

 attention, alerting, general excitement or sleep, 

 interpreted as the maintenance of a general level of 

 cortical excitatory state (37). 



The unspecific thalamocortical projection system 

 was next encountered by Jasper & Droogleever- 

 Fortuyn (42) in their search for the mechanism con- 

 trolling the bilaterally synchronous generalized wave 

 and spike discharge of petit mal epilepsy. It was found 

 that the rhythmical electrical activity of the cortex of 

 both hemispheres could be controlled into a synchro- 

 nized electrical beat by repetitive stimulation of only 

 a few square millimeters of grey matter in the center 

 of the thalamic reticular system. Under certain rather 

 ill-defined conditions a 3 per sec. stimulus would 



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