470 HANDBOOK OF PHVSIOLOG\' ^ NEUROPHYSIOLOGY I 



0.5 msec. 1.0 

 16-6.5 At 



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4.5-2.5/x 



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6.5-4.5/0. 



FIG. 3. Action potential form in a human sensory nerve. This curve was calculated from the fiber 

 distributions. Inset graph, upper right, gives fiber distribution for the medial cutaneous nerve as 

 fiber diameters. Beneath the curve is indicated the position of the axon potentials according to 

 diameters of fibers, not axons. [From Gasser (8g).] 



man, since no pain had been caused by weaker 

 stimuli activating fibers conducting more rapidly and 

 at lower threshold than the delta-epsilon group, the 

 authors thought these fibers were specific in their 

 pain-producing power. Almost as direct evidence to 

 nearly the same effect has been secured by Brookhart 

 et al. C34) from the tooth pulp in cats and man, a 

 structure chosen because of the assumption that pain 

 is the only sensation experienced when it is stimulated. 

 These workers saw in the cats the unmyelinated 

 terminal arborizations join to form a.xons 1.5 to 6 /n 

 in diameter with a median at 3 yu; the axons were 

 nearly all invested with a myelin sheath, a fact already 

 observed in human tooth pulp by Brashear (33). 

 Brookhart et al. found that strength-duration curves of 

 responses upon tooth pulp stimulation in both cat and 

 man were similar to tho.se obtained for 'A' gamma- 

 delta fibers in the cat's saphenous nerve and markedly 

 different from the 'A' alpha and 'C fibers in the same 



nerve. The index of excitation in the cat was the 

 action potential recorded from the saphenous or 

 trigeminal nerve and in man was the minimal sensa- 

 tion of pain. The conduction velocity of the responses 

 in the cat's mandibular ner\e ranged from 30 to 45 m 

 per sec, putting them well into the 'A' gamma group. 

 [The delta component conducts at 15 to 20 m per 

 sec, according to Gasser (89).] 



The strength of stimulus required to produce 'C 

 fiber activation in vivo has not been attained in critical 

 human study, but Clark et al. (42) have shown that 

 activity in these fibers also is correlated with nocicep- 

 tive reflexes. Thus in deeply anesthetized animals a 

 stimulus exciting 'C plus 'A' fibers was followed by 

 much larger reflexes than one exciting only 'A' fibers. 

 Moreover the 'A' fiber conduction was not necessary 

 for the production of reflexes which could still be 

 evoked after block of all W fibers by a pneumatic 

 cuff surroimding the ner\e. Zotterman (315) recorded 



