UNSPECIFIC THALAMOCORTICAL RELATIONS I313 



duction of recruiting; responses readily from one part 

 to another. 



The extent to which the nucleus reticularis of the 

 thalamus participates in the unspecific projection sys- 

 tem has been the subject of some controversy. Rose 

 (72, 73) proposed that this nucleus, with its caudal ex- 

 tension as a shell around the thalamus, might well con- 

 tain many of the cells of origin of the final common 

 pathways of the unspecific projection system. A simi- 

 lar conclusion was reached by Hanbery & Jasper (31 ), 

 and by Hanbery et al. (30) based upon physiological 

 and anatomical studies. In these studies it was shown 

 that recruiting responses can be obtained from the 

 thin posterior extension of n. reticularis, but that they 

 are then more restricted to a limited cortical distribu- 

 tion, as might be expected if the final distribution of 

 unspecific projections were \ ia cells and fibers in this 

 nucleus. 



Anatomical and physiological studies (2, 60, 62) have 

 also shown a more important rostrally-projecting 

 pathway passing through the anterior limb of the in- 

 ternal capsule, just beneath and partially within the 

 border of the head of the caudate nucleus. Some of 

 these fibers terminate in the caudate nucleus, a fact 

 which has led some authors to conclude that all un- 

 specific fibers were relayed to the cortex via tlie cau- 

 date. Such a conclusion is not supported in physi- 

 ological or anatomical evidence when care is taken to 

 distinguish between the caudate nucleus proper and 

 fibers of pas.sage in the adjacent internal capsule (30, 

 60). 



Further anatomical studies are necessary, combined 

 with electrophysiological ob.servations, before the 

 exact nature and distril;)ution of the final common 

 pathway of the unspecific thalamic projection system 

 is thoroughly understood. It would seem from present 

 evidence that, although recruiting responses are ob- 

 tained in the caudate nucleus and they can be ob- 

 tained in the cortex upon stimulation of the caudate, 

 the caudate nucleus is not an essential relay station in 

 the thalamocortical conduction pathway of the un- 

 specific projection system. Cortical recruiting re- 

 sponses may be of shorter latency than those simul- 

 taneously recorded in the caudate, and destruction of 

 large portions of the caudate, sparing adjacent fibers 

 of the internal capsule, has little efTect upon thalamo- 

 cortical recruiting responses. However, there is an 

 intimate relationship between the striatum and the 

 unspecific system as well as with other subcortical 

 structures. 



More recent anatomical studies have largely con- 

 firmed this conception of the intrathalamic organiza- 



A 



' Hff?" 



;iin 



B 



FIG. 5. Oscilloscope tracings of: .4, specific visual evoked po- 

 tential in response to a single shock to the lateral geniculate body 

 (time line intervals, 2 msec); B, recruiting potential from 

 the same recording electrodes in the visual cortex response 

 to stimulating nucleus ventralis anterior as shown in fig. 4; and 

 C, recruiting response fi-om same electrode site in the visual cor- 

 tex after destruction of the lateral geniculate body a.s shown in 

 fig. 4. Time line intervals, 10 msec. [^From Hanbery & Jasper 



tion of the unspecific projection system (44, 64). In 

 addition important corticofugal projections into the 

 thalamic as well as the brain-stem reticular system 

 have been demonstrated (27, 41, 63), as well as projec- 

 tions from the cerebellum (Sprague, J. M., personal 

 communication). 



There appears to be a close relationship between 

 the rhinencephalon and the tiiaiamic recruiting or un- 

 specific system. Clertain of the nuclei known to be re- 

 lated to portions of the rhinencephalon (e.g. n. reun- 

 iens and antcromedialis) regularly produce wide- 

 spread cortical recruiting responses. It has also been 

 shown by Nauta that the hippocampus has many pro- 

 jections into the intralaminar nuclei of the thalamus 

 (61). Unspecific thalamic projections have also been 

 shown to the olfactory bulb (4). 



Finally it may he necessary to point out that the so- 

 called 'unspecific' thalamic projection system is actu- 



