PAIN 



473 



a delay greater than 3 sec. upon stimulation of the 

 forearm. If the delayed pain under abnormal con- 

 ditions such as asphyxia is indeed pure 'C fiber pain, 

 then the abrupt rise in latency from 0.3 to 1.5 sec. 

 described by Lewis & Pochin is consonant with the 

 final failure of conduction in 'A' delta fibers. How- 

 ever, Wortis el al. (312) did not confirm this abrupt 

 change; they found delays in the pain response at 

 intervals during asphy.xial compression studies in 

 man to vary upon stimulus to the foot from o.g to 

 1.7 sec. Another major criticism rests upon the fact 

 that reaction time to pain is influenced greatly by 

 the intensity of the stimulus, there being a hyperbolic 

 decrease in time with increasing intensity according 

 to Pieron (212) and Eichler (73). In general the in- 

 tensity of stimulus has not been maintained constant 

 in the studies tending to identify delated pain under 

 abnormal conditions with second pain under normal 

 conditions. Even the less complex sensation, touch, 

 exhibits a reaction time which \aries in\ersely with 

 the intensity of the stimulus. It also varies with 

 the cross sectional area of the stimulus and changes 

 from day to day, from subject to subject and from 

 testing site to testing site (162). Likewise thermal stim- 

 uli even when ineasured from the threshold intensity 

 rather than from an absolute zero show the same 

 type of variation as shown by Lele & Sinclair (161). 

 Since the reaction time represents the sum total for 

 initiation of conduction, for actual conduction over 

 both afferent and efferent paths, for perception and 

 judgement and for synaptic transmission, the assump- 

 tion that changes in the reaction tiine are due to 

 changes only in afferent conduction rate would 

 seem unwarranted. 



It will be noted that much of the work on second 

 pain and double pain in nerve fibers has been car- 

 ried out in abnormal situations of ischemia, pharma- 

 cologic insult or disease. Weddell et al. (289) have 

 suggested that the delay in these abnormal conditions 

 need not depend on the existence of two discrete 

 groups of fibers conducting at different rates and that 

 the delay could occur in the central rather than the 

 peripheral nervous system as a consequence of sim- 

 plification of the impulse pattern reaching the brain. 

 Such an explanation fits more satisfactorily with the 

 observation of gross variability in the delay and its 

 occurrence with all maneuvers depressing conduction. 



However, such an explanation does not account 

 for the occurrence of second pain under normal con- 

 ditions. I have never personally been able to con- 

 vince myself that I could perceive two separate pains 

 in response to a single noxious stimulus even after 



following Gasser's (89) prescription of flipping the 

 back of my finger against a hot incandescent light 

 bulb or metal hot water faucet. Weddell (personal 

 communication) has had the .same trouble. And 

 it has been a mystery to us how Thunberg (264) 

 and Lewis & Pochin (174) could measure such pre- 

 cise reaction times for a .sensation we could not con- 

 sistently discern. 



It was with some relief that I read of Jones' (137) 

 experiments. When she applied a rigidly mounted 

 needle algesimeter calibrated in 0.25 gm steps to 

 three different spots on the dorsal forearm of each 

 of eight subjects, not one of them reported a double 

 pain after any stimulus. (The point of the needle had 

 been sharpened under a microscope to minimize the 

 stimulation of pressure sensation.) The needles were 

 not held by hand, as one infers was done b\- previous 

 investigators, because the pain stimulus might vary. 

 In another effort to elicit double pain she permitted 

 the needle to remain at the site evoking a response. 

 These 'adaptation trials' were carried out at the 

 threshold for pain, i gm above threshold, and in 

 four highly practiced subjects at 2 gm abo\e threshold. 

 The pain did not vary in a smooth way; instead "the 

 course of adaptation showed fluctuations; in about 

 one-fifth of the trials there were only two peaks 

 which naive observers might po.ssibly have inter- 

 preted as double pain." The four experienced 

 subjects looked carefully for possible double pain, 

 and with suprathreshold stimuli they reported it 

 twice out of 20 trials; in two other trials there were 

 other types of double sensation, one of cold and 

 pain and another of pressure and pain. The possi- 

 bility exists that a suprathreshold stimulus may 

 excite two discrete receptors sequentially and Jones 

 suggests this interpretation of the results. Woollard 

 et al. (310) had already correlated 'first pain' with 

 penetration of the needle point into the epidermis 

 and "second pain' with attainment by the point of 

 dermal levels. This they did by measuring on the 

 needle the depth at which each sensation was pro- 

 voked and then correlating this with an actual histo- 

 logic study of the skin in that area. 



Thresholds to the pain upon electrical stimulus 

 with a square-wa\e pulse from a Grass stimulator 

 were also studied by Jones in 120 trials on each of 

 the four experienced subjects. No double pain and no 

 single delayed pains were felt. Jones regards this 

 form of stimulus as well suited to analysis of the double 

 pain hypothesis because if more than one receptor is 

 stimulated they are all stimulated simultaneously. 

 She points out that no experimenter has reported 



