TOUCH AND KINESTHESIS 



4>7 



or only sometimes lost (273) could perhaps be inter- 

 preted to mean that at least some degree of separation 

 exists between the fibers concerned with touch and 

 those relevant for pain. This could be expected if the 

 different observers differed in their routine sectioning 

 of the anterolateral column in regard to the extent 

 and the depth of the cut. However, more exact work 

 is needed before one can conclude that the classical 

 notions in respect to touch hold true for the spino- 

 thalamic system. 



Origin of Spinothalamic System 



The knowledge that large cutaneous fibers are 

 activated by mechanoreceptors and that painful and 

 thermal stimuli usually activate small fibers was ob- 

 viously largely responsible for the deductions regard- 

 the origin of the ventral spinothalamic tract. This 

 tract is often conceived as originating solely from those 

 posterior horn cells which themselves are assumed to 

 be activated by the collaterals of the large myelinated 

 fibers of the medial division of the posterior root. The 

 lateral spinothalamic tract, on the other hand, is 

 pictured as originating from cells which are dis- 

 charged by the small fibers of the lateral division of 

 this root. The evidence at hand is clearly discordant 

 with such concepts since it is certain that tactile stim- 

 uli can activate A fibers of different sizes and since it 

 is probable that even some C fibers can be so activated 

 as well (see p. 394). It follows, therefore, that no cell 

 in the posterior horn which emits an axon into the 

 spinothalamic tract can be excluded at present as 

 potentially responsive to tactile stimuli. The evidence 

 secured with the retrograde degeneration method after 

 cutting the anterolateral column (71, 146, 147, 178) 

 implies that only the large apical, pericornual and 

 basal cells of the posterior horn give rise to the spino- 

 thalamic tract. The findings of Kuru (147) suggest 

 that the large cells just below the substantia gelatinosa 

 give rise to the ventral spinothalamic tract and hence 

 to the tactile component of the spinothalamic system 

 if, indeed, it is true that the fibers carrying touch are 

 running in a separate sector and if this sector lies 

 ventromedially to the other fibers of the system. 

 Curiously enough, no retrograde degenerations 

 (after cutting of the anterolateral column) were ob- 

 served in the suljstantia gelatinosa cells, which is one 

 of the reasons for suggesting that these cells may 

 actually represent a system intercalated lietween the 

 axons of the posterior roots and the cells of origin of 



the spinothalamic system (194). However, more 

 evidence is needed to support this concept convinc- 

 ingly. 



Termination of Spinothalamic System 



Ever since the description of Edinger (61) there has 

 been general agreement (44, 47, 53, 82, 96, 100, 180, 

 204, 263) that the spinothalamic tract is easily 

 demonstrable in man and other primates. However, 

 despite early descriptions of both the spinothalamic 

 and the bulbothalamic components of this system in 

 the rabbit (139, 259, 260) douljts have Ijeen frequently 

 expressed as to whether the system actually reaches 

 the thalamus in forms other than primates. The system 

 is known to be composed mainly of small fibers (1 14) 

 and it seems right to assume that only a fraction of 

 them is actuall)' traceable with the Marchi technique 

 usually employed. It would appear that the poor 

 m\elination of the fibers accounts reasonably for the 

 failure of some workers to trace the system to the 

 thalamus although recently a claim has been made on 

 the basis of studies employing silver technique (173) 

 that, in comparison with the primates, actually 

 fewer fibers of the system reach the thalamus in sub- 

 primate forms. In any case it seems clear that the 

 spinothalamic system reaches the thalamus in all 

 mammals studied and specifically so in the cat (95, 

 I 78) which is used so frequently in modern research. 

 Although some fibers to the centrum medianum, to 

 the parafascicular nucleus and to the intralaminar 

 nuclei are often described, most workers are agreed 

 that the system ends mainly in the \entrobasal com- 

 plex of the thalamus. The spinal component is stated 

 to end in the external element; the Ijulbothalamic 

 component, in the arcuate element of this complex. 

 As far as the tactile system is concerned, the classical 

 concept assumes that the lemniscal and the spino- 

 thalamic systems converge upon the ventrobasal 

 complex and that from here on corticopetal pathways 

 are common to both. 



It is possible that this concept may need a revision 

 since it seems appropriate to suggest that besides the 

 ventrobasal complex a region intercalated between 

 this complex and the medial geniculate i:)ody may be 

 a major terminal station for some spinothalamic 

 fibers. The reasons for this suggestion are as follows. 



Many ol:)servers in the past have been greatly dis- 

 turbed by the scarcity of detectable terminations of 

 the spinothalamic fibers in the ventrol)asal complex. 

 It is clear from descriptions by most of the writers 



