390 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



offers a solution of the problem for the neuromuscu- 

 lar blocking agents themselves have powerful effects 

 upon central synaptic transmission. For this reason 

 the method of recording from electrodes chronically 

 implanted in the brain or upon its surface is now fre- 

 quently employed. Recovery of the animals from 

 the anesthetic allows the study of evoked electrical 

 activity for periods of weeks or months under condi- 

 tions closely approaching the normal. The developing 

 technique of recording from single units by means of 

 implanted microelectrodes promises to be fruitful. 



METHODS OF RECORDING. An analysis of the electrical 

 signs of neural activity evoked in the central nervous 

 system by sensory stimuli is treated extensively in 

 Chapters X and XII of this volume. Here we merely 

 wish to point out that it is the initially positive slow 

 wave which has proved of great value, particularly in 

 the experiments designed to determine the locations 

 of the responsive regions and the pattern of the 

 sensory projection. The usefulness of this response is 

 hardly minimized by the fact that its nature still re- 

 mains obscure. 



The method of single unit analysis introduced by 

 Adrian precipitated a rapid advance in neurophysi- 

 ology. So useful has it become that an appreciable 

 share of present day research is based upon it. Both 

 intra- and extracellular microelectrodes are commonly 

 used. Successful application of the extracellular 

 method requires that the unit observed be held under 

 study for considerable periods of time in a relatively 

 undamaged state. The method permits determination 

 of response properties and topographic and modality 

 attributes of a sample of cells at a given location which 

 allows a reconstruction of the behavior of the popula- 

 tion. A full reconstruction, however, would be possible 

 only if the sample were sufhciently large and unbiased. 

 This last requirement is proljably not met, for it seems 

 most likely that the smaller elements of the neural 

 population are in fact rejected by the recording instru- 

 ments presently available. 



CURRENT THEORIES OF CUT.^NEOUS SENS.^TIONS 



Classic Concept 



According to the concepts developed by von Frey 

 (244-247) pain, cold, warmth and touch represent the 

 four basic modalities of cutaneous sensation and spe- 

 cific receptors can be assigned to each modality. The 

 older anatomical and psychophysical findings were 



generally interpreted in the light of these notions and 

 unqualified support of the orthodox view was given 

 by the work of VVoollard et al. (275), and earlier work 

 by Weddell (264, 265) and VVeddell & Harpman 

 (266). The views of von Frey, however, did not remain 

 altogether unchallenged and two formulations differ- 

 ing greatly from the classical concepts will be briefly 

 considered. 



Pattern Thcnry 



Recently the Oxford group of workers (113, 154, 

 155, 220, 223, 267-269) seems to deny altogether the 

 existence of modality specific receptors. The conclu- 

 sion of the group is that different cutaneous sensations 

 arise not as a result of selective activation of specific 

 receptors but because different stimuli affect the same 

 sets of fibers in a different manner. Thus, in the first 

 order neurons different discharge patterns in the 

 same fiber bundle, and not a selective activation of 

 some fibers in the bundle, are thought to determine 

 the different cutaneous sensations. The reasons for 

 this deduction are that these workers were unable to 

 relate specified endings to specific modalities in sev- 

 eral skin areas (113, 155, 223) and that all modalities 

 of sensations were obtained by stimulating the cornea 

 (154) which is known to have free endings only. 

 Moreover, histological evidence led them to believe 

 that all endings in the skin are essentially alike since 

 all arborize into fine, naked, axoplasmic filaments. 

 They further conclude that a classification of en- 

 capsulated endings into various types is untenable 

 since a large number of morphologically intermediate 

 variants exists between the usually recognized types. 



We believe that the Oxford workers did produce 

 suggestive evidence that stimulation of free endings 

 may cause sensations which can be classified in the 

 broad spectrum of touch. Such findings however do 

 not at all establish that specific endings do not exist, 

 and the conclusions drawn from histological observa- 

 tions do not appear convincing. If a crisis exists in 

 respect to evaluating the morphology of the endings, 

 it is a crisis of abundance and not of scarcity. One 

 hesitates to accept as a solution to the vexing problem 

 of the morphology of the encapsulated endings a 

 declaration that virtually all morphological differ- 

 ences between them are either insignificant or are due 

 to artifacts of the technique. 



In any case, a further elaboration of the idea of the 

 discharge patterns led at least some workers of the 

 Oxford group to opinions about touch which are 

 closer to classical notions than one would expect. 



