TOUCH AND KINESTHESIS 



395 



clear that the failure did not follow an orderly se- 

 quence according to fiber size. The important finding 

 was that even after the entire spectrum of A fibers 

 failed to conduct, the C elevation was only little im- 

 paired. It can thus be concluded that C fibers are 

 more resistant to ischemia than is the A group and 

 since manifestations of painful sensations are still 

 evokable when only C fibers conduct, one can infer 

 that painful stimuli must activate at least some C 

 fibers. While this finding again agrees with what has 

 been more recently shown by other methods, one can 

 conclude in addition that activity in C fibers alone is 

 apparently sufficient to arouse painful sensations. It 

 could have been expected perhaps that some tactile 

 sensations should be present as long as C fibers are 

 conducting if it be true that mechanical stimuli excite 

 such fibers. The negative findings may mean, of 

 course, that there are no C fibers activated by touch. 

 It may mean as well that activity in C fibers aroused 

 by tactile stimuli under the experimental conditions 

 tested are not interpreted as touch, or finally even that 

 some perhaps obscure qualities of tactile sensations 

 which actually were present were ignored by the ex- 

 perimenters and the experimental subjects alike. 



SUMMARY. It appears that the availaljle neurophysio- 

 logical evidence in respect to the peripheral aspects 

 of the tactile system does not support fully any of the 

 current ideas regarding tactile sensations. 



Despite the arguments advanced by the Oxford 

 workers the evidence seems conclusive that there 

 e.xist in fact specific tactile (as well as thermal) recep- 

 tors, The evidence is also good that the fiber size may 

 be indicative of connections with some specific recep- 

 tors. Thus, the known thermoreceptors seem con- 

 nected with small or medium sized fibers only, while 

 the largest fibers in the cutaneous nerve are con- 

 nected to mechanoreceptors. Bishop (21} points out 

 further that the largest afferent fillers known in the 

 peripheral nerves do not occur at all in the cutaneous 

 branches, and it seems clear that these fibers are con- 

 nected to the muscle stretch receptors. To this extent 

 then von Frey's concepts appear valid. The fact that 

 tactile stimuli can activate A fibers of different sizes 

 may or may not he compatible with the classic ideas. 

 What seems difticult to reconcile with von Frey's con- 

 cepts is the suggestive evidence that C fibers (which 

 presumably ramify in free endings only) are also acti- 

 vated by tactile stimuli. If this should be so a major 

 question to be answered would be whether individual 

 somatic C fibers are modality specific or whether an 

 individual fiber is e.xcited by tactile as well as by 



thermal and nociceptixe stimuli. If the latter should 

 be the case, the classical concepts would clearly 

 need a major revision obviouslv in the direction of the 

 ideas expressed by Head. 



CENTR.\L TACTILE AND KINESTHETIC SYSTEMS 



General Remarks 



It is well known that the dorsal root fibers ramify 

 upon entry into the central nervous system and, by 

 means of their main ijranches and collaterals, estab- 

 lish synaptic contacts with .several nuclear regions. It 

 is convenient to divide into two classes those regions 

 to which discharges aroused by tactile stimuli can be 

 relayed. The first is formed by regions which are, or 

 which can be reasonably assumed to be, instrumental 

 for generation of tactile sensations. To the second class 

 belong those regions which are either not at all .sen- 

 sory, as in the case of the anterior horn cells, or those 

 which receive afferent information but for which 

 there is no reason to believe that their function has an 

 essential bearing upon tactile or kinesthetic experi- 

 ence. It is thus clear enough that the appearance of 

 evoked neural activity in a given synaptic region fol- 

 lowing tactile stimulation may indeed mean that the 

 region in question is relevant for tactile sensations. 

 .Such responses, however, may equally well merely 

 indicate that some other activ'ity, not necessarily even 

 sensory in nature, is modulated by the activity of 

 tactile receptors. Considerable confusion exists in the 

 literature in respect to this problem, since many 

 workers seem to believe that a response evoked any- 

 where in the central nervous system by tactile stimuli 

 is prima facie evidence that the locus in question is 

 linked somehow to tactile sensations. If one considers 

 that most morphological groupings in the central 

 nervous system establish synaptic contacts with 

 more than one other morphological entity, the 

 numijer of potentially activated synaptic regions may 

 be expected to increase in geometrical progression 

 with each synaptic relay. It is likely, therefore, that 

 within a short time a signal in an afferent filler could 

 be relayed, at least in principle, to almost any group- 

 ing within the central nervous system. 



Hence it is not unduly surprising if under certain 

 experimental conditions a response to a tactile stimu- 

 lus occurs in a region which anatomically appears to 

 be an altogether unlikely locus. It is fortunate indeed 

 for an analysis by electrophysiological methods that 

 all potentialities for synaptic transfer are for a num- 



