474 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY 



double pain with a single electric stimulus. Heat, 

 on the contrary, one of the more effective stimuli 

 in eliciting double pain, does continue to penetrate 

 deeper into the tissues and to stimulate more remote 

 receptors even after the stimulus is removed. Landau 

 & Bishop (150) also found in eight normal, unpreju- 

 diced subjects "only three who could recognize a 

 second pain response when the skin was tested with 

 heat or brief mechanical stimuli." They ascribe the 

 failure of brief stimuli to evoke delayed pain to a 

 masking effect of the pricking pain. In any event it 

 would seem to me that, since 'C fiber activation in 

 animals requires a much more powerful stimulus, 

 Jones' studies at threshold levels and up to 4 gm 

 above threshold may never have activated these 

 fibers. 



It would thus appear that the whole suljject of 

 pain conduction by nerve fibers of specific size is 

 worthy of careful review. The technique of Dawson 

 & Scott (58) of recording nerve action potentials 

 through the intact skin in man may increase the feasi- 

 bility of securing the crucial information. The method 

 has already yielded valual)ie data on this score in 

 the hands of Magladery et al. (183) who have found 

 that the ischemia of cuff occlusion has a generalized 

 depressant effect on the conduction in both afferent 

 and efferent "A' fibers in peripheral nerve trunks 

 in man. They studied oscilloscopic records of the 

 action potentials and correlated these with serial 

 sensorimotor examinations as the cuff was inflated 

 and deflated. Thus 17 min. after the onset of ischemia 

 voluntary power was still relatively normal. But 

 recognition "of all forms of .sensory stimuli except 

 those producing deep pain was diminished." With 

 this one exception, no subject found one sensory 

 modality impaired disproportionately to another. 

 Figure 4 taken from their work shows the steady re- 

 duction in the potential from the rapidly conducting 

 fibers as the ischemia continued. No "C fiber poten- 

 tials appear in this record, obtained after a "maximal 

 single shock' to the ulnar nerve in the low forearm 

 with recording over the ulnar nerve above the elbow. 



In an important study in man by Collins, Randt & 

 Nulsen (unpublished observations), the exposed sural 

 nerve is being stimulated distally while action poten- 

 tials are recorded oscilloscopically from a more 

 proximal position. .Such studies immediately precede 

 and follow therapeutic incisions into the pain path- 

 ways of the spinal cord. Reporting on the five pa- 

 tients thus far studied, they say tentatively that the 

 sensation corresponding to an 'A' gamma-delta 

 elevation on the oscilloscope has been equivocal — 



not clear-cut pain. But at the first intimation of "C 

 fiber activation the patients have had severe pain; 

 this has been the case also even if 'A' fiber conduction 

 was profoundly depressed by local cooling of the 

 nerve. 



In relation to double pain an effort has been made 

 to generalize even more widely regarding sense 

 organs supplied by nerve fibers of significantly 

 varying diameters by Katsuki fl al. (139). They say 

 that the thin fibers carry impulses from receptor 

 elements of lower threshold to physiologic stimuli 

 with a lower rate of adaptation, a lower maximal 

 frequency of discharge and a greater tendency to 

 continuous or spontaneous firing. Bullock (36) has 

 drawn attention to the applicability of this principle 

 to nine different sense organs, the thin fibers supply- 

 ing the more sensitive and tonic, the thick fibers the 

 more discriminating and phasic receptors. However 

 his attempt to bring pain fibers into this concept 

 stands up only in the roughest way under close 

 scrutiny. He cites Maruhashi ft al. in his support, 

 i)ut these workers actualh' describe distinct tonic 

 or phasic behavior mainly in two groups of small 

 myelinated nociceptive fibers in the toad all within 



FIG. 4. Nerve action poten- 

 tials in man following maximal 

 single shocks to ulnar nerve in low 

 forearm. Surface recording over 

 ulnar nerve above elbow. Pres- 

 sure cuff on upper forearm in- 

 Hated to 200 mm Hg. Top record, 

 before ischemia; lower records, the 

 stated number of minutes after 

 onset of ischemia. Time: i and 

 5 msec. [From Magladery et al 

 (183).] 



