467 



man to distinguish not only a potentially noxious from 

 a harmless stimulus — pain from touch — but warmth 

 and cold as well. 



Ahn 



il Skin 



There are, however, areas of abnormally innervated 

 skin in man from which only pain can be aroused. 

 Weddell (285) studied biopsies of such skin from a 

 patient with a lesion of the sciatic nerve and from a 

 patient with a plastic tube pedicle of the abdominal 

 wall. On histologic examination he saw in each speci- 

 men only fine nerve fibers giving rise to superficial 

 nerve nets with beaded endings. 



Special Cutaneous Sensory Endings 



Discussion of the functions of the elaborate cu- 

 taneous sensory nerve endings of Meissner, Ruflini and 

 Krause and of the deeper such endings, the Vater- 

 Pacinian corpuscles and neurotendinous endings of 

 Golgi is germane to our theme only to point out that 

 many of them have a long slender 'accessory fiber' 

 (Reniak fiber or Timofevew fiber) with a fine un- 

 myelinated naked nerve ending similar to those in 

 the cornea. Assuming that the most elaborate forms 

 of sensory nerve terminal subserve some specialized 

 function such as touch,, warmth or coolness, does ex- 

 cessive stimulation of such a receptor cause pain as 

 well, and if so i's that pain mediated via impulses in the 

 accessory fiber? VVoollard (308), in support of this 

 hypothesis, has illustrated an 'accessory' fiber derived 

 from what he calls the 'subepidermal pain plexus' 

 terminating at a Krause's end bulb. Lavrenko (158} 

 and K0I0S.SOV have shown that the.se accessory 

 fibers are not connected with the sympathetic system; 

 their specific association with pain remains at the 

 moment a speculation. Trotter & Davies (270) re- 

 garded the sensation of 'hot' as a combination of 

 warmth and pain, the sensation of 'cold' likewise would 

 combine coolness and pain. With increasing thermal 

 difference the sense of temperature disappears and 

 pain alone is perceived. (See above for comments of 

 Nafe on sensations arising from smooth muscle.) 

 Elucidation of all the mechanisms of the combined 

 forms of sensation is a task for the future; but one can 

 say that the 'intensive' theory of pain is right to this 

 extent, that sufficiently pronounced mechanical and 

 thermal stimulation of fine unmyelinated nerve end- 

 ings will cau.se pain. 



Deeper Somatic and Visceral Receptors 



Correlation of deeper somatic and visceral re- 

 ceptors with particular types of pain or other sensa- 

 tion is likewise in an elementary stage. Free unmye- 

 linated nerve endings occur in serous membranes, the 

 subserous coat of gut, intermuscular connective 

 tissue, tendon surface and substance, deep fa.scia and 

 periosteum — from all of which the suitable stimulus 

 evokes pain. [For specific references see White & 

 Sweet (296, p. 15).] The plexus of nerve fibers is 

 much better developed in the adventitia and mu.scu- 

 laris of arteries than of veins, according to Dogiel (66). 

 This finding correlates well with the severe pain com- 

 monly felt on arterial puncture in man in contrast to 

 the absence of or minor pain on venepuncture (283). 



Terminating also in clo.se relation to capillary walls 

 are fine unmyelinated endings derived from sheathed 

 stem fibers of dorsal root origin described by Weddell 

 et al. (288). Their afferent function is further .sug- 

 gested by Landis' (151) observation in man that pain 

 occurred when his micropipette penetrated these tiny 

 channels. 



TERMINAL SENSORY PLE.XUSES 



The nerve fibers ramifying in the subcutaneous 

 tissue and skin are so interwoven as to give the im- 

 pression of a continuous net or syncytium, but even in 

 densely innervated areas such as the cornea Zander & 

 Weddell (313) have never seen fusion between 

 daughter axons originating from neighboring nerve 

 fibers, although they have occasionally seen nets 

 formed by fusion of daughter axons arising from the 

 same parent fiber. Even though the stem nerve fiber 

 from one dorsal root ganglion cell supplies a large 

 area of skin, the capacity to perceive and localize pain 

 correctly to a single spot is well known. It appears to 

 be mediated by the multiple innervation of each 'spot' 

 by branches from different stem fibers. Thereby a tiny 

 area of skin gives ri.se to a pattern of excitation differ- 

 ing enough from its neighbor to permit localization 

 and two-point discrimination. This disposition of stem 

 nerve fibers was first seen by Bethe (23) at the sensory 

 end organs of frog tongue. Boring's (30) penetrating 

 analysis of sensation in his own forearm after deliber- 

 ate division of a cutaneous nerve led him to the same 

 concept. Weddell (285, 286) was the first to demon- 

 strate histologically in human skin biopsies that a spot 

 of skin especially sensitive to one modality of sensation 

 was in fact supplied by two or more nerve fibers 



