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HANDBOOK OF PHVSIOLOGV 



NEUROPHYSIOLOGY I 



on examination with calibrated thorns. Strughold 

 (256) found up to 200 per cm^ and Woollard (307), 

 testing sensation over a small area of his own thigh 

 after removal of each of a succession of thin slices of 

 skin, found pain to be the most superficial as well as 

 the most extensive in depth of the modalities tested. 

 In one region of his own epidermis which was es- 

 pecially sensitive to a needle tip he saw histologically 

 a plexus of finely beaded nerve fibers. Woollard et al. 

 (310) examined a biopsy of human skin taken just 

 distal to an ulcer made 8 days earlier with solid carbon 

 dioxide. Pain was the only sensation elicitable from the 

 tissue and the subsequent microscopic examination 

 revealed only fine naked nerve terminals. However, 

 when Foerster & Boeke (cited in 77, p. 16) examined 

 sections of skin in man following division of cutaneous 

 nerves and beginning regeneration, they found in 

 areas from which pain was the only type of sensation 

 no free intraepithelial nerve endings. The positive 

 findings of the previous authors are probably more 

 significant. Despite the density of the pain points there 

 are, however, spots analgesic to pain. Tindall & 

 Kunkle (265) find that these are still demonstrable 

 both during induced erythema (with lowered thresh- 

 old) and during experimental ischemia (with raised 

 threshold). They conclude that the analgesic spot 

 represents a gap in the fiber network rather than a 

 pain fiber with an unusually high threshold. The 

 great ease with which pain is elicited from the cornea, 

 the tympanic membrane and the dental pulp — all 

 structures probably supplied only with delicate free 

 filamentous nerve terminals and no specialized nerve 

 endings — has convinced most workers that such 

 endings can initiate nervous impulses giving rise to 

 pain. [For the neurohistology of the tympanic mem- 

 brane see Wilson (300).] 



There has been less agreement as to whether or not 

 other modalities of sensation might be evoked via 

 these delicate nerve endings. Waterston (283) actu- 

 ally believed that the nerves of the normal epidermis 

 mediate only touch, since he could slice this tissue 

 painlessly from himself with a razor. In the skin 

 shavings he saw nerve fibers ending in loops and fine 

 arborescent figures. In Woollard's (307) detailed de- 

 scription of his similar studies he says that, when the 

 first 5x22 mm slice of his skin was cut, he felt pain 

 only at four previousy mapped 'pain spots'. But he 

 must have felt touch during the rest of the slicing 

 process and this is likely to have been mediated, at 

 least in part, by the other fine nerve terminals in the 

 epidermis. Personal repetition of studies with cali- 



brated hairs and needle points along with a review of 

 much of the voluminous literature on cutaneous sensa- 

 tion led us in 1955 (296, p. 10) to record agreement 

 with Goldscheider's (loi) original observation — that 

 the threshold sensation to minimal stimulation with a 

 minute pointed needle is one of touch at the great 

 majority of all spots on the skin of the body. This I 

 noted even in areas specifically recommended by von 

 Frey (274) for eliciting pain in preference to touch, 

 such as the skin over the eyelids, the biceps brachii 

 and the clavicle. Consequently, the cutaneous sense of 

 touch being even more widespread than that of pain, 

 it .seems likely that proper stimulus to many or even 

 any of the most widespread endings, the fine un- 

 myelinated type, elicits normally a sense of touch. 



Cornea 



Because the great majority of anatomists find only 

 such fine endings in the cornea its sensation has been 

 much tested. General teaching since von Frey (273) 

 and in agreement with Lewis (171) has been that one 

 may evoke only pain from the cornea. Since even a 

 speck of grit on the normal eyeball is so intensely 

 painful, repeated reports in the literature that a sense 

 of touch may be elicited from the cornea found little 

 general acceptance. However, many normal people 

 do in fact descrilje only a sense of touch without pain 

 or annoyance when a wisp of cotton rests on the 

 cornea. 



Lele & Weddell (163) have recently summarized 40 

 publications on corneal sensation and have carried out 

 a series of critical experiments which may well become 

 a cla.ssic of well-controlled study in the complicated 

 field of sensation. In 25 of the 40 earlier publications 

 the various authors record a feeling of touch upon 

 corneal stimulation, and Lele & Weddell obtained 

 this response invariably from each of 10 subjects when 

 a fine nylon suture was brought into contact with the 

 cornea. Reports of each subject never "included even 

 a suggestion of pain". Contrariwise, a heavier nylon 

 thread touching the cornea caused invariably a blink 

 and a report of sharp pain. In their further studies a 

 jet of air at warm, cold or neutral temperature, a warm 

 or cool copper cylinder, or an infrared beam of radi- 

 ation was applied to and restricted to the cornea. The 

 stimulus excited an appropriate sensation of tempera- 

 ture in the overwhelming majority of instances. 



From such findings it seems likely that the fine 

 naked nerve endings in the surface layers of the body 

 are capable of setting up impulses which will enaijle a 



