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HANDBOOK OF PHYSIOLOGY ^^ NEUROPHYSIOLOGY I 



Hardy et al. (i i8,p. 133)011 the other hand think there 

 are two different quahties of pain independent of 

 the duration of stimulation, that 'pricking' pain is 

 predominantly "fast' and primarily conveyed by 

 myelinated fibers, whereas 'burning' pain is pre- 

 dominantly 'slow' and conveyed by unmyelinated 

 fibers. Sinclair & Hinshaw (248, 249) had made a 

 similar statement, but had added that "in the experi- 

 ence of pin prick, the factor which determines whether 

 the subject reports a feeling of pain or not is probably 

 not the initial prick conveyed by fast fibers, but what 

 we may term 'unpleasantness' which arrives subse- 

 quently by the slower fibers." They also state that 

 "after the slow unpleasantness is removed in pressure 

 blocks there is a period when it is exceedingly difficult 

 to determine whether a pin prick should be reported 

 as 'sharp' or as 'sharp and painful'." 



Landau & Bishop (150) have extended the analysis 

 of pain sensation by the techniques of differential 

 block by cuff pressure and procaine to subcutaneous 

 tissue, periosteum, muscle and fascia. They elicited 

 pain by both electrical stimulation and chemical in- 

 flammation. They concluded that the presumably 'C 

 fiber pain is "of slower onset, but of severer and more 

 penetrating character and of longer after-effect." 

 Periosteum, muscle and veins were found to be sup- 

 plied by fibers of both types, whereas inflammatory 

 pain from the subcutaneous injection of turpentine 

 and from bee stings, as well as the pain following in- 

 jection of 5 per cent sodium chloride solution, was 

 assigned almost entirely to activity in 'C fibers. De- 

 tailed consideration of their results reveals some in- 

 consistencies, however; thus with procaine block to 

 these deep endings which should block 'C before 

 delta fibers "only a partial loss of deep pain results 

 before prick is blocked." The pricking pain should 

 have been the last to go if the authors' hypothesis were 

 to be fully confirmed. 



Further evidence that two different qualities of pain 

 may result from different types of responses of the 

 same peripheral nerve was obtained by Pattle & 

 Weddell (206) in an experiment which included 

 direct exposure of one of VV'eddell's own digital nerves. 

 The threshold sensation following single graded con- 

 denser shocks to the ner\-e was a "pain of unpleasant 

 quality like a wasp sting." This response occurred at 

 all strengths of shock from o. i to 6 ^f capacity of con- 

 denser. But the discharge of a condenser of 7 nf 

 capacity "produced a long-lasting, severe, aching 

 pain, which was completely different in quality from 

 the wasp sting reaction." The reaction time from 

 stimulus to closure of an electrical contact b\' the sub- 



ject was exactly 1.27 .sec. for each type of pain, how- 

 ever. The study was carried out on a nerve in which 

 only a few fibers appeared to be responding to stimuli 

 following injection of the local anesthetic procaine 

 which produced complete insensitivity of the whole 

 distal phalanx of the finger. The two types of pain 

 here are however manifestly different from the experi- 

 ences recorded in experimental studies on double pain 

 from the intact skin. 



An astute maneuver to measure the conduction 

 time for human pain sen.sation which eliminates the 

 time from cerebrum to motor response has been de- 

 scribed by Gordon & Whitteridge (106). The alpha 

 rhythm of the human EEG can be disturbed by un- 

 expected stimulus to the skin and in individuals in 

 whom this response was clear-cut these workers 

 found that the time between painful stimulus and 

 alpha interruption averaged about 0.25 sec. at both 

 normal fingers and toes. The delay was much greater 

 when the iiase of finger or toe was compressed for an 

 hour or more by a cuff occluding its blood supply. 

 It averaged 1.04 sec. from the asphyxiated finger 

 and 1.40 sec. from the asphyxiated toe. In the latter 

 state they measured the fiber conduction time for 

 the 'second' pain at about i m per sec. 



Sinclair & Hinshaw (248, 250) and Sinclair (247) 

 have put forward some sharp and cogent criticisms 

 of much of this work on double pain and 'second' 

 pain, pointing out that a delay between stimulus 

 and perception of pain also occurs in procaine blocks, 

 a situation in which the slowly conducting fibers 

 are supposed to fail first. Critical scrutiny of the data 

 obtained during the asphyxia caused by cuff com- 

 pression also leads to doubts regarding the original 

 interpretation. 



Thus, Lewis & Pochin's (174) average figures in 

 two subjects for appearance of second pain upon pin 

 prick of a normal finger and toe and for appearance 

 of delayed pain after a cuff block were about the same 

 at 1.2 sec. But when each subject is considered 

 separately, there is a statistically significant differ- 

 ence in both of them between the control and as- 

 phyxial readings. Moreover, Lewis & Pochin (174) 

 found after cuff asphyxia that the latency of the pain 

 response, the reaction time, rises abruptly from 0.3 

 sec. to be constant at 1.5 sec. Sinclair & Hinshaw 

 (250) have recorded delays much in excess of this 

 figure up to 5 sec. — longer than would be required 

 for conduction from finger to brain of any normal 

 'C fiber. Ashby (11) has likewise pointed out many 

 recorded instances of much longer delays in tabes, 

 and I have seen one such striking patient who showed 



