477 



FIG. 5. Hyperpathia in relation to anesthesia following section of a cutaneous nerve. Examina 

 tion 34 days after division of middle cutaneous ner\e of thigh. VVttkin Ike continuous line: anesthesia 

 to camel's hair brush. Within the dotted lines: two areas abnormally sensitive to pin. Broken lines show 

 course of superficial \-eins; spots marked X show maximal sensitisity to pin prick. [From Trotter & 

 Davies (270).] 



exaggerated explosive type of response upon effectual 

 stimulation. Accompanying this concept was his 

 notion that many types of fiber in their poorly in- 

 sulated regenerating phase may conduct impulses 

 giving rise to pain, since he presumed that pain was 

 the only sensation evoked from the uninsulated, i.e. 

 naked, unmyelinated end organs. He goes on to say, 

 "With the advance of regeneration the fibers serving 

 touch, heat and cold, become once more connected 

 with end-organs, and then their insulation, by the 

 junction of the neurilemma with the capsule of the 

 end-organ, can be completed. The completely insu- 

 lated fiber, having lost its teinporary resemblance to 

 the pain fiber, becomes once more sensitive to the 

 finer stimuli and ceases to yield exaggerated re- 

 sponses." Later work already discus.sed which demon- 

 strates the capacity of naked unmyelinated endings 

 to transmit impulses concerned with touch cuts the 

 ground from under the latter part of this reasoning, 

 but the concept of lack of insulation has become more 



appealing since the work of Granit et a!. (109). 

 These investigators showed that a generalized break- 

 down in 'insulation' occurs at the site of injury to the 

 sciatic nerve in cats. This is so striking when the nerve 

 is cut across that impulses set up in spinal ventral 

 (motor) rootlets are transmitted to the sensory fibers 

 at the cut and can be picked up via an oscilloscope 

 from the dorsal (sensory) rootlets of the same seg- 

 ment. Such an 'artificial synapse' or 'fiber interac- 

 tion' also develops at the crush of a knot tightened 

 around a nerve and even appears from time to time 

 after moderate pressures of 50 to iio gm against a 

 nerve insufficient to stop conduction beyond the 

 point of compression. Granit et al. have assumed 

 that pain fibers would be among the most easily 

 excited by fiber interaction. These observations 

 clearly present a mechanism whereby impulses trav- 

 ersing one fiber may abnormally pass to many at a 

 site of injury, thereby pennitting abnormal central 

 excitation. 



