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HANDBOOK Ol I'insKlLOGY 



m 1 Roi'HVsioi.iKJV hi 



von Frey, there has been a continuous search for 

 special receptors subserving touch, deep pressure, 

 cold, warmth and pain. (These arc considered in 

 detail in Chapters XVII through XIX of ihis Hand- 

 book.) The early discovery of "spots' of special sensi- 

 tivity mi the skin encouraged the belief that each 

 of these different qualities" has its own kind of re- 

 ceptor. In fact, .1 little evidence and considerable 

 imagination led to the assignment of a particular type 

 of receptor for each quality: Mcissner's corpuscles 

 for touch, Pacinian corpuscles for deep pressure, 

 Krause end bulbs for cold, Rullini cylinders for 

 warmth and free nerve endings for pain. The results 

 of experiments in which 'dissociation' of these separate 

 miim' qualities was produced by peripheral nerve 

 section and regeneration (24, 29, 43, 58, 89, 1 1 7, 154, 

 177, 190, 209), or by procedures designed to produce 

 temporary alteration of skin sensitivity, and the dis- 

 covery that the qualities were selectively disturbed by 

 spinal cord lesions (57, 86, 105, 197, 214, 221, 222), 

 all added up to a quite convincing argument in favor 

 of a place (or specific nerve energy) explanation of 

 somesthetic qualities. Despite the Imlk of evidence ac- 

 cumulated by researchers holding this point of view 

 and despite many thousands of words written in 

 support of the evidence, there were a few dissenters 

 w ho cited phenomena difficult to account for by a place 

 theory and who produced alternative theories and 

 some evidence to support these theories (for details, 

 sec Chaptei XVII). Particular credit must be given 

 to Nafe (i", 1 153); to Bishop (23); to Jenkins (10 ;>, 

 and to Wcddell, Sinclair, Lelc and their collaborators 

 (K)i) for questioning the adequacy of the evidence 

 produced to support the explanation of discrimination 

 of pain, touch, warmth and cold, simply in terms of 

 a specific receptor and specific neural unit theory. 

 It now seems clear that an adequate theory of dis- 

 crimination of somesthetic qualities must recognize 

 thai the same receptor endings are stimulated l>\ 

 differenl kinds of stimuli and that discrimination of 

 quality cannot be based upon events in separate 

 sensory-neural units but only upon differences in 

 temporal and spatial pa 11 cms of events in the same oi 

 in similar units (85, 91, 121, 192,217 219,229 231). 

 The possibility remains that al the spinal cord level 

 there is some differentiation of the paths subserving 

 the qualities ol somesthesis At the thalamii and corti- 



11 1 )i modalities Since tic same principle "I specific nerve 



energies has been used to account foi the difference in quality 



and the difference in modality, it has made little difference to 



research whether touch, temperature and pain were classified 



parate modalities "i as separate qualities "I 9 esthesis 



cal levels there is almost no evidence to suggest that 

 the separate qualities arc represented in spatially 

 separate areas (187). 



Studies of the effects ol ablation of the cortical 

 projection areas of the somesthetic sy stem or of lesions 

 in subcortical pathways and centers are, in com- 

 parison to similar studies for vision and hearing, rela- 

 tive!) scarce and the quality of the evidence is less 

 satisfactory because of the difliculty in adequately 

 controlling the parameters of stimulation. In experi- 

 mental investigations only the tactual and kinesthetic 

 senses have been examined with any care. The kinds 

 of deficits which occur arc described in other sections 

 of this chapter. 



Early psychophysical studies of taste indicated that 

 different spots or regions of the tongue were par- 

 ticularly sensitive to stimuli which produced the 

 sensations of salty, sweet, bitter and sour. Whether 

 these are the basic taste qualities, and the only ones, 

 may be questioned. (See the discussion of taste by 

 Pfaffmann in Chapter XX of this Handbook, 1 At least 

 they are the ones most readily differentiated and 

 named by the human subject not only under experi- 

 mental conditions but in common experience. At- 

 tempts to identify receptors having structural charac- 

 teristics which might indicate differences in function 

 have met with no success. Nor has it been possible to 

 explain satisfactorily on the basis of the chemical 

 composition of a given substance the taste that it will 

 produce. 



When substances known to arouse the sensations 

 of salty, sweet, sour and bitter are applied to the 

 tongue of experimental animals, records of activity 

 in single fibers of the nerves supplying taste receptors 

 show that some fibers are activated l>v acid alone, 

 others hv acid or salt, and still others bv acid or 

 quinine (ibfi, 167). Experiments bv Kimura & 

 Beidler (11 1) provide evidence thai the same taste 

 bud may be excited by chemical substances which 

 in man arc known to produce more- than one quality 

 of sensation. This kind of experimental analysis in 

 peripheral units leads to the inference that in response 

 to substances which produce separate taste qualities, 

 patterns of neural impulses, which differ both spatially 

 (different nerve fibers) and temporally (frequency in 



individual fibers) arc transmitted to the central ner- 

 vous system 



Electrical stimulation of the glossopharyngeal and 

 chorda tympani have made possible the mapping ol 



cortical areas for taste in the rat and cat (21, 159) 



although the possibility thai these a teas may be tactual 

 areas for the tongue cannot be completely ruled oui 



