47' 



only 'A' delta and 'C fiber activity from the saphenous 

 nerve of a cat when the corresponding skin was burned 

 by a special stimulus applied without mechanical de- 

 formation of the surface. He obtained similar records 

 upon etching the skin with acids. Maruhashi et al. 

 (185) have recently studied preparations of single 

 afferent fibers in the cat so that their conclusions as to 

 fiber size are derived from direct measurement. They 

 found one group of large and another of small 

 'nociceptive' fibers in the range 3 to 1 1 m- Activity in 

 such fibers was evoked by a pin prick or strong pull on 

 a hair. The receptive field of a fiber in the toe pad was 

 2 X 2 to 3 X 3 mm; it was about 10 times larger in a 

 hairy area. The extent of the field was clearly defined 

 and within it the receptive spots were densely dis- 

 tributed. Following the stimuli used in the.se studies 

 the impulse discharges were phasic and ended in about 

 0.2 sec. ; but if a scalded area was stimulated mechani- 

 cally a protracted after-discharge was present in both 

 small myelinated and unmyelinated fibers. 



Double Pain Responses or Second Pa/n 



From the foregoing type of ob.servation it has been 

 concluded that pain is conducted in meduUated 'A' 

 fibers at 15 to 45 m per sec. and in unmyelinated 'C 

 fibers at less than 2 m per sec. The gap between the 

 two groups of impulses is conceivably sufficient to 

 permit a perceptible differentiation between the slow 

 and the fast group and, indeed, long before the speeds 

 of conduction in sensory nerves were known a double 

 pain response to a single stimulus was described by 

 many observers. Thus Rosenbach (225) in 1884 and 

 Gad & Goldscheider in 1892 (86) thought the re- 

 sponse to a pin was an immediate sensation of prick 

 followed after an interval without sensation by a 

 second prick. Thunberg (264) investigated what he 

 considered to be two separate prick sensations with a 

 great difference in reaction time between the two. 

 Zotterman in 1933 (3 14) first associated the 'second' 

 pain with 'C fiber conduction. He confirmed the 

 observation by Lewis el al. (173) that a compression 

 cuff around the arm blocking the circulation causes 

 early paralysis of the sense of touch, alters the pain 

 sense, but does not cause analgesia even after arrest 

 for 40 min. These two groups of workers fell in line 

 with Ga.sser & Erlanger's (91) conclu.sion that arrest 

 of the blood flow to the nerves causes a progressive 

 loss of their function in accordance with the character 

 of the fiber. The first fibers blocked are those in the 

 'A' delta elevation; with progression of asphyxia the 

 larger medullated 'A' fibers are next affected; finally 



after even the largest fibers are no longer conducting 

 the 'C fibers are blocked. Zotterman, using his com- 

 pression cuff to switch off all the 'A' fibers, found that 

 the pain which persists is felt only after a delay, and 

 his measurement of the time of this delay agreed well 

 with the reaction time for 'second' pain recorded by 

 Thunberg. The conduction velocity in the sensory 

 fibers (calculated from the reaction time) was not lower 

 than 0.5 m per sec. which is only slightly below the 

 conduction rate of the slowest 'C fibers in mammalian 

 nerves observed by Erlanger & Ga.sser. Upon checking 

 the differences in time of appearance of the second 

 pain in relation to the site stimulated, Lewis & 

 Pochin (174) found the expected shorter time when 

 thigh rather than toe was the area pricked. Thus 

 calculated, the rate of conduction in the limb of the 

 second response was again at the 'C fiber speed of 

 0.5 to I m per sec. Confirmatory evidence of this con- 

 cept has arisen from studies upon cocainization of 

 nerve fibers. Gasser (89) found this drug blocked the 

 'C fibers in his animals early, and then blocked the 

 medullated A' fibers in the same way as asphyxia, 

 i.e. beginning first with the smallest. He points out 

 that "it is misleading to state that asphyxia blocks the 

 large fibers first, while cocaine blocks the small fibers 

 first." But cocaine does block the 'C' group before the 

 'A' group, and corresponding with this Lewis & 

 Pochin (174) found that in man "cocaine reduces and 

 ultimately abolishes the second pain response, before 

 it similarly affects the first pain response." They are 

 both agreed that there are great difficulties with any 

 further attempt to correlate in a clear-cut way sensory 

 function and fiber size, that the fibers belonging to 

 different modalities must be widely distributed 

 throughout the various fiber sizes and that there seems 

 to be little possibility of associating any one sensation 

 with an elevation in the electroneurogram. Sinclair & 

 Hinshaw (248, 249), after an extensive study of com- 

 pression and pressure block of peripheral nerves in 

 man, subscribe wholeheartedly to the notion that 

 such association is impossible. Even after a large 

 number of experiments with procaine they found it 

 impossible to generalize as to the order of loss of the 

 various modalities of touch, pain, warmth and cold 

 since by suitable adjustment of the experimental con- 

 ditions "almost any desired order of sensory loss may 

 be recorded." 



Lewis (171) and Gasser (89) are agreed that both 

 the fast and slow impulses evoke the same quality of 

 sensation. Lewis adds that brief noxious stimulation 

 produces the sensation of 'pricking' and that a pro- 

 longed no.xious stimulation elicits a sense of 'burning.' 



