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HANDBOOK OF PHYSIOLOGY ^ NEUROPHYSIOLOGY I 



Identification of Thermal Receptors 



Since the recording of spike activity of single 

 thermal nerve fibers by Zotterman and co-workers 

 (22, 52, 96), there can be no doubt about the speci- 

 ficity of cold and warm nerve endings in the mammals 

 as these thermal receptors discharging into func- 

 tionally isolated nerve fibers responded to cooling and 

 warming, respectively, but not to mechanical stimu- 

 lation. The abundance of information about the func- 

 tion of the thermal receptors obtained in recent years 

 from electrophysiological investigations has, howev-er, 

 not been followed by any corresponding widening of 

 our knowledge of the morphological structure of the 

 receptors. The old attempts to identify the receptors 

 histologically h\ e.xcision of human skin beneath the 

 cold and warm spots, respectively, failed almost en- 

 tirely [see von Skramlik (92)]. The statement in most 

 textbooks that the Krause end bulbs are believed to be 

 the receptor for cold and the Ruffini end organ that 

 for warmth were based on histological studies by von 

 Frey (91) and Strughold & Karbe (84) on sensory 

 end organs within the cold sensitive periphery of the 

 cornea. After mapping the cold spots on the conjunc- 

 tiva bulbi, Strughold & Karbe dropped methylene 

 Ijlue into the eye and found a very good topographical 

 correlation between the cold spots and the blue 

 stained end bulbs observed in the corneal microscope. 

 Similarly Bazett et al. (5) in their attempt to identify 

 the end organs for temperature and touch in the pre- 

 puce injected methylene blue intra-arterially. They 

 described seven different types of end organs, among 

 which end bulbs of the Krause type were distributed 

 in good agreement with the cold spots. Their average 

 number was about 15 per cm- compared with 6 to 12 

 for the cold spots, but some of the end bulbs were so 

 close together that their number obviously must 

 exceed that found by mapping the cold spots. Further, 

 there are reasons to believe that some nerve fibers 

 branch and supply more than one end organ. Spots 

 sensitive to warmth in the prepuce are few in number 

 (one or rather less per cm-). The distribution of the 

 Ruffini end organs agreed fairly well with that of the 

 warm spots. 



Whether the Krause end bulbs are the receptors 

 for cold in other parts of the skin is still uncertain 

 since conventional histological methods have failed to 

 reveal any end bulbs of the Krause type in the skin 

 underlying the cold spots. Recently Lele et al. (65) 

 maintained that, in limited areas of the skin (as opposed 

 to mucous membranes) in which encapsulated nerve 

 endings are abundant (the palm of the hand, sole of 

 the foot and parts of the dorsum of the digits), the 



diversity in size and configuration of their cellular 

 and neural elements is such that any classification of 

 encapsulated endings in the skin becomes purely 

 arbitrary. On the other hand they draw attention to 

 the fact that in both glabrous and hairy skin en- 

 sheathed nerve fillers arising from the cutaneous nerve 

 ple.xus give rise at all levels in the skin (from the 

 stratum granulosum of the epidermis to the junction 

 of the dermis and the subcutaneous tissues) to a wide- 

 spread series of fine naked axoplasmic filaments 

 which interweave but do not fuse with one another. 

 These unencapsulated nerve endings cannot be dis- 

 tinguished from one another on morphological 

 grounds; they can be distinguished only by the fact 

 that they are situated in a different stratum of the 

 skin and thus lie among different tissue elements. 

 Thus the morphologically nonspecific nerve endings 

 found beneath the epithelium should be reduced in 

 temperature and therefore be stimulated by cooling 

 of the skin. The deeper endings situated close to the 

 lilood vessels are generally heated up by the blood so 

 that a positive temperature gradient between the 

 ending and the axons should be the usual mode of 

 stimulation. According to Lele et al. the temperature 

 modes are related not to the stimulation of morpho- 

 logically specific endings, but to the manner in which 

 nonspecific nerve endings of fibers in the skin are 

 stimulated. These unencapsulated endings should 

 thus, according to these authors, be looked upon "as 

 universal receptors which give rise to bursts of action 

 potentials, the pattern of which is related to the way 

 in which the stimulus affects the skin." Consequently 

 they also maintain that Johannes Miiller's 'law of 

 specific energies' and the thesis that there are specific 

 nerve terminals which subserve specific sensory 

 modalities is unsupported. 



The absence of encapsulated nerve endings does, 

 however, not exclude the possibility of functional 

 specificity. The tongue of the frog does not contain 

 any such endings although it contains afferent fibers 

 responding specifically to touch, salt and water (98). 

 In recent years it has often been suggested that the 

 capsule of an end organ, for example in a Pacinian 

 corpuscle, protects the nerve ending from being 

 damaged when the organ is subjected to strong and 

 lasting mechanical stimulation as in the beak of a 

 wood pecker. The capsule should thus have nothing 

 specifically to do with the energy transformation. 

 This would imply that the specific process of transfor- 

 mation should be inherent in the morphologically non- 

 specific naked nerve endings or in the structures 

 where these are situated. The sensation experienced 



