469 



Tower (267) has shown that stimuli to various por- 

 tions of the field of ramification of the same fiber 

 produce different responses in that one fiber; this intro- 

 duces another variable in the data presented to the 

 brain increasing the likelihood of precise spatial 

 discrimination peripherally. She worked in the corneo- 

 conjunctival region of the cat using a preparation con- 

 taining but one to three fibers. The stimuli with hairs, 

 needles or glass rods were nearly all well above 

 threshold and would presumably have caused pain at 

 the human cornea. She made oscilloscopic recordings 

 of the action potentials from the preparation, noting 

 that one isolated nerve fiber yielding fairly large im- 

 pulses fanned out over roughly one fourth of the 

 cornea and some of the adjacent sclera. "Low thresh- 

 old and slow adaptation characterized the central 

 region of the terminal fields of individual fibers, and 

 rapid adaptation more than high threshold, the 

 peripheral parts." A strong stimulus near the center 

 of the field of a fiber might push the frequency of the 

 response nearly to the limit permitted by the re- 

 fractory properties of the fiber, namely about 500 per 

 sec. In general the frequency, duration and rate of 

 adaptation of impulses within the field of one fiber 

 were determined by site as \vell as by intensity of 

 stimulus. When many fibers remained active, the 

 normal situation of course, their fields overlapped in a 

 fashion inextricable to the experimenter. But, pre- 

 sumably, the brain of the subject uses all this informa- 

 tion, analyzing signals from fibers excited minimally 

 which encircle fibers excited more vigorously to 

 achieve better localization. The frequency of dis- 

 charge in the most excited fiber would still re\eal the 

 intensity of the stimulus. 



Fiber Dicinietcrs and Pain Conduction 



Gasser (90) and his collaborators also have amassed 

 evidence correlating physiological with anatomical 

 properties of nerve fibers (see his Nobel Lecture, 

 1946). Their classification is based on the duration 

 and form of the three components of the action po- 

 tentials in the fibers — the initial negative spike, then 

 the negative and finally the positive after-potentials. 

 Their 'A' fibers embrace all of the medullated fibers 

 in somatic nerves and some in the visceral nerves as 

 well. The 'A' fibers are divisible into five subgroups 

 designated in order of diminishing diameter by the 

 letters alpha through epsilon. The velocity of con- 

 duction in these fibers varies directly with the di- 

 ameter of the axon, ranging between go to 115 m 

 per sec. for the largest fibers 16 to 20 ju in diameter 



and around 10 m per sec. in the smallest myelinated 

 fibers 2 to 4 M in diameter (fig. 3). They have called 

 the unmeduUated fibers in sensory nerves 'C fibers; 

 these have a diameter of 2 /x or less and conduct at 

 from 0.6 to 2 m per sec. Each component of a 'C 

 fiber's action potential lasts much longer than the 

 corresponding part of the action potential of an A' 

 fiber. The action potentials in most of the medullated 

 fibers of visceral nerves difier so much from either of 

 these that they have been placed in a separate cate- 

 gory and called 'B' fibers. L'sually a single elevation is 

 present with no \isible negative after potential. 

 Gasser & Erlanger found no such fibers in the dorsal 

 roots. 



The more recently introduced designations of 

 Lloyd (i 78) are also in current use. His Group I fibers 

 from 20 to I 2 M are seen only in muscular branches 

 of nerves; Group II fibers from 12 to 6 ju are seen infre- 

 quently in muscular branches but present a larae 

 peak in cutaneous ner\'es; Group III fibers mainly 

 from 4 to 3 /i correspond to 'A' delta and occur in 

 nerves to both mu.scle and skin; and Group IV' are 

 unmyelinated or G fibers. 



Of the numerous efforts in animals to correlate 

 pain with certain nerve fibers, the early experiment of 

 Ranson & Billingsley (219) is still one of the more 

 widely cited. As the posterior rootlets enter the spinal 

 cord they divide into a lateral bundle of fine, mostly 

 unmyelinated, fibers and a medial bundle of large 

 fibers. These authors found that after .section of the 

 small (lateral) fibers stimulation of the remainder no 

 longer evoked the pain reflexes' of struggling, altered 

 breathing and arterial pressure, whereas after section 

 of the large (medial) fibers these reflexes persisted. 

 More specifically pain impulses have been associated 

 both with the delta-epsilon 'A' fibers and with the 'C' 

 fibers. The most direct evidence of as.sociation of pain 

 with impulses in delta-epsilon fibers was obtained by 

 Heinbecker et al. (i 25) from the cutaneous nerves of a 

 man's leg which was later amputated. No sensation 

 was evoked at operative exposure of the nerves until 

 stimuli at a frequency of 1 2 per 5 sec. caused grimacing 

 and a verbal report of unequivocal pain (as if he were 

 being whipped). At the efl"ecti\e stimulus parameters 

 there was a clear-cut delta elevation in the action 

 potential from a companion nerve in the leg, but no 

 'C' fiber activity. The threshold for the 'C' fiber eleva- 

 tion in this nerve was five times as high. Earlier 

 Bishop & Heinbecker (28) had established in animals 

 that the thresholds of response to electrical stimula- 

 tion of fibers in peripheral nerve trunks varied in- 

 versely with the fiber diameter. So in their study in 



