THE SIGNIFICANCE OF THE TERMINAL STRUCTURE 

 OF AFFERENT NERVE FIBRES 



A. Iggo 



Department of Veterinary Physiology, University of Edinburgh 



Specialized structures, 100 to 300// in diameter in the superficial layers of 

 hairy skin in the cat, contain in their modified epithelium a layer of large 

 cells, numbering 20 or more, each of which encloses a disc of nervous 

 tissue, approximately 10// wide and 1// thick, densely packed with mito- 

 chondria, which is the terminal expansion of a branch of a myelinated 

 afferent nerve fibre (Iggo and Muir, 1962). A discharge of afferent impulses 

 in the myelinated fibre is most easily aroused by mechanical stimulation of the 

 epidermis, threshold 3 to 5 mg, adapts slowly to steady pressure and can 

 also be provoked by a fall in cutaneous temperature (Iggo, 1962). These 

 properties, particularly the rate of adaptation, distinguish the axons from 

 other similar myelinated axons innervating hair follicles in the same skin, 

 and are always associated with the specialized region of dermis and epid- 

 ermis, for which the name " touch corpuscle " is suggested. 



if the skin was denervated by cutting the saphenous nerve the distinctive 

 structure of the epidermis was disorganized, the layer of special cells dis- 

 appeared and the epidermis became similar to the adjacent unspecialized 

 parts, but the capillaries and Schwann cells remained in the dermis, at 

 least for 30 days. When, after crushing the saphenous nerve, the afferent 

 fibres were allowed to regenerate, the growing axonal tips did not reveal 

 distinctive responses to mechanical and thermal stimuli, even when the 

 growing axons had reached the dermis (Brown and Iggo, 1962). There was 

 a brief discharge in all axons, with a relatively high threshold, in response 

 to mechanical stimuli and no discharge in response to temperature changes. 

 Two spatial patterns of mechanical sensitivity emerged among the large 

 myelinated axons ; in one type mechanical stimuli were effective over 

 areas of more than a square centimetre while in the other type the res- 

 ponsive areas were restricted to a few square millimetres, probably corres- 

 ponding to hair follicle and " touch corpuscle '' axons respectively. 



There was a period after the nerves had reached " touch corpuscles " 

 and were dividing in the dermal papilla when the response to mechanical 

 stimuli was still undifferentiated. At this stage the epidermis, which earlier 

 had shown signs of disorganization, had once more thickened but the 



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