UNSPECIFIC THALAMOCORTICAL RELATIONS 



I3II 



They ha\'e been considered 10 represent an intra- 

 thalamic association system, interconnecting various 

 specific nuclei within the thalamus, but without 

 direct connections with the cortex. 



The nucleus centrum medianum, which is a rela- 

 tively large structure in higher mammals, has been 

 shown to have direct projections to the striatum 

 (putamen) but not to the cerebral cortex, according 

 to anatomical studies (21, 25, 26, 51, 52, 65, 66). 

 Nevertheless the rostral pole of this nucleus regularly 

 produces widespread cortical recruiting responses 

 when stimulated repetitively. This has suggested that 

 recruiting responses may be conducted to the cortex 

 via the striatum or by a striatothalamic system (25, 

 ■^6, 51, 52, 73,82). 



Both anatomical and physiological studies have 

 suggested to some authors (52, 80) that recruiting 

 responses reach the cortex by means of intrathalamic 

 connections with specific nuclei and then are con- 

 ducted with synaptic delay over specific projection 

 fibers to the cortex. Such connections would be princi- 

 pally with the association nuclei of the thalamus, 

 projecting to frontal and temporoparietal cortical 

 association areas and not to sensory receiving areas, 

 according to Starzl & Magoun (76) and others 

 (52, 80). 



Physiological studies have consistently shown that 

 recruiting responses are less prominent or even absent 

 from sensory receiving areas of the cortex and are 

 especially hard to demonstrate in visual and audi- 

 tory receiving areas. This has been shown, however, 

 when stimulating the centrum medianum or mesio- 

 ventral portions of the intralaminar system. If there 

 exists a topographical organization within the unspe- 

 cific system, there may be other portions of it more 

 directly related to sensory areas. The close inter- 

 connections within the system have given the im- 

 pression to some investigators that it responds in an 

 all-or-none manner with a fixed pattern of cortical 

 projection to only 'association' and motor areas of 

 the cortex (80). 



More recent anatomical and physiological studies 

 have served to clarify some of these problems, although 

 much has still to be learned about the details of the 

 manner in which the unspecific projections reach all 

 areas of cortex. 



The independence of the unspecific projection 

 system from specific thalamic nuclei has been proved 

 by recording recruiting responses from each cortical 

 area, including the sen.sory areas, following complete 

 destruction of the thalamic nucleus known to have 

 specific connections with a particular cortical area 



(31). For example, after finding a portion of the tha- 

 lamic reticular system which produced good re- 

 cruiting responses in the visual area {I'A in fig. 4), the 

 lateral geniculate body was completely destroyed by 

 coagulation and recruiting responses demonstrated 

 to be unaffected or increa.sed from the same point of 

 the visual cortex (fig. 5). 



Siinilar results were obtained for auditory and so- 

 matic sen.sory areas following destruction of the me- 

 dial geniculate and the n. \entralis posterior, respec- 

 tively. It was remarkable that recruiting responses 

 appeared of larger amplitude in all sen.sory areas af- 

 ter destruction of their respective thalamic relay nu- 

 clei. This suggests a competitive interaction lietween 

 the specific and unspecific projection systems for the 

 control of cortical electrical activity in sensory areas. 

 Such an interaction was clearly demonstrated in later 

 studies by Jasper el al. {43) who established again the 

 existence of independent unspecific projections to 

 sensory receiving areas in the cat. 



It is now quite clear that the pathways of the un- 

 .specific projection system to all cortical areas need not 

 pass via specific projection nuclei. There may still be 

 additional connections with specific thalamic nuclei, 

 but these are not essential for the mediation of the re- 

 cruiting response. The efifects of stimulating the tha- 

 lamic reticular system are more difficult to demon- 

 strate in sensory receiving areas of the cortex, most 

 difficult in auditory and visual areas. 



The extent of the thalamic reticular system, as de- 

 termined by stimulation points yielding recruiting 

 responses, includes structures other than those com- 

 monly included in the intralaminar system of nuclei. 

 These areas are shown stippled in diagrammatic cross 

 sections of the thalamus of the cat in figure 6. It will 

 be noted that only the rostral pole of n. centrum me- 

 dian yields recruiting responses while an additional 

 area in the vicinity of the n. suprageniculatus has con- 

 sistently given recruiting responses, especially in ecto- 

 sylvian regions. 



It has been shown by both anatomical and physi- 

 ological methods that the conduction pathway from 

 the centrum medianum extends forward in the inesio- 

 ventral thalamus, penetrating the n. ventralis medi- 

 alis and into the n. ventralis anterior, then into the 

 rostral pole of n. reticularis. This appears to be a 

 mixed system of short multisynaptic connections and 

 fibers. Unspecific fibers are particularly dense in the 

 ventral third of the n. ventralis anterior, as shown by 

 the double cross hatching of this region in figure 6. 



Beginning in the n. centralis unspecific fibers and 

 multisynaptic pathways extend dorsally into the 



