UNSPECIFIC THALAMOCORTICAL RELATIONS 



I319 



as a whole. Due to this property of the thalamic sys- 

 tem, it has been proposed by Jasper that the local 

 activation processes of attention in the waking animal 

 could be mediated, in part at least, by this system, al- 

 though it is now known that processes of attention 

 may also affect the specific projection systems, even at 

 peripheral levels of sensory influx. 



The participation of the thalamic reticular system 

 in epileptic seizures of the type characterized by in- 

 itial brief loss of consciousness ("petit mal') is still un- 

 certain since the wave-and-spike pattern, which char- 

 acterizes the EEG in patients with these seizures, is 

 obtained by stimulating the intralaminar system only 

 under very special conditions which are poorly under- 

 stood (36). Seizure discharges are generally more read- 

 ily transmitted o\er specific projection systems (77), 

 although bilateral synchronization in cortical rhythms 

 may be enhanced by epileptic lesions in the unspecific 

 thalamic nuclei (69). It appears that both thalamic 

 and mesencephalic portions of the reticular system 

 may be involved in the central pacemaking circuits of 

 the petit mal attack, and a certain general suscepti- 

 bility of the cortex to this type of response may also 

 be involved. This matter is considered further in Chap- 

 ter XI\' by Gastaut & Fischer-Williams in this work. 



SUMM.^RY .'^ND CONCLUSIONS 



The unspecific thalamocortical projection system is 

 composed of a closely interconnected multisynaptic 

 network of neurons extending largely through the in- 

 tralaminar portion of the thalamus with rostral direc- 

 tion of conduction toward the anterior portion of the 

 thalamus where connections are formed with the re- 

 ticular nucleus and the n. ventralis anterior for the 

 origin of the major projection systems to widespread 

 areas of the corte.x. Important relationships to the 

 striatum are present, although the final common path- 

 ways for cortical projection are as yet uncertain. Some 

 may originate in the n. reticularis of the thalamus. 



This projection system bears a very close relation- 

 ship to certain components of the spontaneous elec- 

 trical activity of the cortex and can regulate this ac- 

 tivity either by timing it into rhythmical sequence or 

 by arresting spontaneous rhythmical activity. The re- 

 cruiting response, which is characteristic of this sytem, 

 participates in timing rhythmic activitv which is in- 

 herent in both the cortex and the thalamus. Such ef- 

 fects can be obtained on cortical activity independent 

 of the specific projection pathways. 



It seems likely that the synaptic termination of the 

 unspecific system in the cortex is chiefly of the axo- 

 dendritic type widely distributed throughout all layers 

 of the cortex ijut under optimal conditions having its 

 major effect on superficial layers. 



This system forms a part of the ascending reticular 

 activating .system of the brain stem, although it pos- 

 sesses properties distinct from the portions of the re- 

 ticular system in the hypothalamus and upper mid- 

 brain. 



The functional significance of the unspecific tha- 

 lamic system is not entirely clear. It seems to be the 

 most important regulator of the spontaneous electri- 

 cal rhythms of the entire cortex and he capable of 

 some local efi"ects in a loose topographical organiza- 

 tion which may give it a more highly integrated func- 

 tion than might be attributed to lower portions of the 

 reticular system. It seems to mediate a more rapid but 

 shorter lasting cortical actixation than do the more in- 

 ferior portions of the reticular system. This is called 

 the phasic type of activation as opposed to the tonic 

 activation which is deri\ed from hypothalamus and 

 upper midbrain. 



Studies of the interaction between the specific and 

 nonspecific system in the thalamus indicate four types 

 of interconnection : a) by means of collaterals from the 

 principal afferent pathways directed medially into the 

 intralaminar .system, h) by collaterals from the intra- 

 laminar system terminating in specific thalamic nu- 

 clei, c) by convergence upon common neuronal sys- 

 tems in the cortex and d) by corticofugal projections 

 reaching various parts of the thalamic reticular system. 



REFERENCES 



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2. Akimoto, H., K.. Negishi .and K. Yamad.a. Foltu psychiat. 

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3. Akimoto, H., N. Yamaguchi, K. Ok.\be, T. Nakagawa, 

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4. .^RDUTNX, .\. and G. Moruzzi. Etectroencephatog. & Clin. 

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