394 



HANDBOOK OF I'HYSIOLOrn- 



NEUROPHYSIOLOGY I 



excited by pressure are 3 to 5 /i in the cat and 4 to 5 m 

 in the frog. In addition, unmyelinated fibers of the C 

 group were seen by them to be activated by mechan- 

 ical stimuli both in the cat and frog. 



The findings of the Japanese observers confirm the 

 older observations of Zotterman (282) who, recording 

 from strands of the saphenous nerve of the cat, noted 

 that, apart from the usual discharges evoked by 

 tactile stimuli in the large fibers, discharges were 

 evoked in smaller fibers also by very light stroking of 

 the skin. These potentials contribute to the delta ele- 

 vation of the electroneurogram of the saphenous 

 nerve and Zotterman judged the appropriate fibers to 

 be in the range of 5 to g n. He also noticed that with 

 stroking of the skin the delta potentials are followed 

 by a ca.scade of very small spikes which he felt must 

 be conducted by the C group of fillers. 



REL.'^TION OF CUT.'>iNEOUS STIMULI TO ACTIVITY IN 



FIBERS OF DIFFERENT SIZE. Observations on single units 

 indicate then that tactile stimuli can activate at least 

 several groups of afi'erent fibers and, if the data of the 

 Japanese workers are taken as a basis, the conclusion 

 seems inescapable that no fiber whatever within 

 either the A or C group can be eliminated by virtue 

 of its size alone as potentially responding to tactile 

 stimuli. Nevertheless, the data imply that a relation 

 may exist between the size of a fiber and the exact 

 quality of the mechanical stimulus which activates it. 

 The conclusion that fibers of all sizes may be activated 

 by mechanical stimuli must not imply that all fibers 

 in the cutaneous nerve can Ije activated by them. If 

 this were so, each fiber activated by other than me- 

 chanical stimuli would also be responsive to tactile 

 stimulation. This is apparently not the case since at 

 least some fibers which are selectively activated by 

 thermal stimuli have been shown to exist (120-123, 

 281). 



The problem of C fibers is of special interest. There 

 is evidence available (22, 46, 149, 168, 282) that 

 nociceptive stimuli can activate such fibers. From the 

 experiments in which C fiber activity could be identi- 

 fied with certainty it has been inferred (46) that at 

 least some thermal stimuli (warmth) can also cause C 

 fiber activity. All workers who used single unit 

 preparations and were concerned with this question 

 (127, 168, 282) have concluded that not only nocicep- 

 tive but cold, warmth and mechanical stimuli acti- 

 vate C fibers as well. It should lie noted that with the 

 single fiber technique it may not always be possible 

 to decide that a C fiber and not a small A fiber has 

 been activated. Nevertheless, the evidence suggests 



indeed that C fibers can be acti\ated by all modes of 

 cutaneous stimulation. 



In contrast to the findings about A fibers there is 

 no conclusive evidence, either for or against, concern- 

 ing selective sensitivity of single C fibers to various 

 stimuli. It thus remains an open question to what 

 extent the C fibers resemble the A system. 



RELATION OF ELEV.\TIONS OF ELECTRONEUROGRAM TO 



MODALITIES OF SENSATION. The findings derived from 

 observations of single units seem to agree with the 

 studies which relate the different elevations of the 

 compound action potential to the results of psycho- 

 physical and animal experiments in which the periph- 

 eral nerve is blocked by infiltration with local anes- 

 thetic, made ischemic or excited by electrical 

 stimuli. 



It has long been known for man that perineural 

 injection of cocaine (or a similarly acting agent) 

 blocks sensations in a preferential order in such a 

 way that cold, warmth, pain and touch disappear in 

 the order stated. There is some discrepancy among 

 various observers whether it is cold or pain which 

 disappears first, but there is an almost unanimous 

 agreement that it is touch which disappears last. [For 

 some discordant oljservations and a review of the 

 literature .see Sinclair & Hinshaw (221, 222).] 



Since Gasser & Erlanger (89) demonstrated that 

 cocainization Ijlocks conduction in an orderly se- 

 quence, the smallest fibers being blocked first, it can 

 be inferred that the largest fibers in the nerve are 

 activated by tactile stimuli. This conclusion, of 

 course, is but a confirmation of the firmly established 

 findings discussed earlier. It should be stressed that 

 cocaine block does not permit any conclusions as to 

 whether smaller fibers which can be activated by 

 touch exist, but it does imply that activity in a group 

 of the largest fibers alone may be quite sufficient for 

 the arousal of tactile sensations. Direct stimulation of 

 an exposed nerve in man (119) leads to an identical 

 conclusion since it is possible to excite only the largest 

 fibers with an appropriate stimulus and since such 

 stimuli lead only to an arousal of tactile sensations. 



In contrast to a block produced by a local anes- 

 thetic, application of pressure over a limb of man 

 leads to disappearance of sensations usually in the 

 following order: touch, cold, warmth, pain. In experi- 

 ments of Clark, Hughes and Gasser as reported by 

 Gasser (87), compression of a limb of a cat led first to 

 a conduction failure of the delta fibers and of the 

 largest fibers in the ner\e. The exact prosjre.ss of the 

 conduction failure was difficult to establish, but it was 



