392 



HANDBOOK OF PHYSIOLOGY' 



NEUROPHYSIOLOGY I 



in some (12, 105-108), the receptors themsehes were 

 identified. 



It was observed early and since confirmed by all 

 workers in the field that the largest fibers in the cu- 

 taneous nerves are activated by certain types of 

 tactile stimuli. It may be stressed immediately that 

 the converse statement does not hold and that ac- 

 ti\ity aroused by tactile stimuli i> nut limited to large 

 fibers only. Since for technical reasons discharges 

 occurring in large fibers are particularily convenient 

 to study, the information given below regarding 

 tactile receptors pertains to those which are con- 

 nected to such fibers. 



The specificity of tactile receptors manifests itself 

 both in their exquisite sensitivity to mechanical 

 stimuli and in their lack of sensitivity or high threshold 

 for other than such stimuli. The threshold to mechan- 

 ical stimuli is low, apparentlv commensurate with the 

 capacitv of the animal to recognize them, and the 

 details of mechanical application of the stimulus are 

 as a rule critical for evoking discharges. Direct meas- 

 urements made on the mesenteric Pacinian cor- 

 puscles (105) indicate that the minimal movement of 

 the stimulus probe necessary to excite the receptor is 

 of the order of 0.5 m in too /xsec; the Pacinian cor- 

 puscle in the toe (107) showed a similar sensitivity. 

 There are no figures available for any other ending. 

 From qualitative observations, though, there is no 

 reason to doubt that at least a number of them is 

 equally sensitive. 



The problem of sensitivity of mechanoreceptors to 

 other than tactile stimuli has attracted but casual 

 interest of most observers, chiefly because it is known 

 that thermal and painful stimuli characteristically 

 cause discharges in the smaller spectrum of fibers. 

 Ne\ertheless, it was observed by .\drian & Umrath (5) 

 that thermal stimuli did not excite the mechanorecep- 

 tors they studied, and Hogg (127) stated that thermal 

 and chemical stimuli are less effectise in the frog in ac- 

 tivating large fibers than small ones and that the re- 

 verse is true for tactile stimuli. Hensel & Zotterman 

 (124) recently presented interesting data on the re- 

 sponse of some mechanoreceptors to cold. In the 

 tongue of the cat they found mechanoreceptors not 

 sensitive to strong thermal stimuli as well as thermo- 

 receptors unresponsive to tactile excitation. In addi- 

 tion to these receptors they also found a group of fibers 

 which responded both to pressure and to cooling. The 

 response to cold differed in important aspects from the 

 response of a typical thermoreceptor for the response 

 occurred only to ver\ low temperatures and rapid 

 cooling (while thermoreceptors respond with a sensi- 



ti\ity to about a tenth of a degree below 40°C:) and it 

 adapted to extinction within a few seconds (while a 

 typical response to cooling persists for as long as the 

 stimulus is applied). How to interpret such a response 

 to cooling is an open question. It is conceivable that it 

 represents a secondary effect due, for example, to a 

 displacement of a mechanoreceptor through vasocon- 

 striction, although this interpretation is considered as 

 rather unlikely by the authors. 



TYPES OF DiscH.\RGES. The mechanorcceptors in the 

 skin can be divided into fast and slowly adapting 

 types. Most receptors activated by displacement of 

 hairs are fast adapting. Those responding to pressure 

 adapt either fast or slowly. It is of interest that recep- 

 tors activated by kinesthetic stimuli appear to have 

 the same properties as the mechanoreceptors in the 

 skin although the slowly adapting ones seem to pre- 

 dominate greatly in numbers (29, 225). 



It is unfortunately not known how the morpho- 

 logical structure of the ending relates to the dis- 

 charge pattern since, except for the Pacinian cor- 

 puscle, none of the other receptors has ever been 

 studied in isolation. For that reason the significance 

 of the capsule and of the accessory fibers is entirely 

 obscure. Recently Boyd (28) and Skoglund (225) 

 identified Ruffini's endings in the joint capsule as 

 slowly adapting receptors and the modified Pacinian 

 corpuscles as the fast adapting ones. These identifica- 

 tions, howe\er, must be considered tentative since 

 they are indirect. The Pacinian corpuscle, the best 

 known receptor at present, is fast adapting (107). It 

 is known for this receptor (106) that its adaptation to 

 mechanical stimuli is a property of the receptor itself 

 rather than of its fiber. Loewenstein C161), working on 

 frog's skin, presented some data to suggest that a 

 fast adapting receptor may be made to discharge for 

 a long time if the tension in the receptor region is 

 greatly increased, thus implying that whether an 

 ending adapts quickly or slowly may be determined 

 by the mechanical arrangement of the ending. If this 

 be so, the degree of coiling of the terminals could be a 

 determining factor in whether a receptor is fast or 

 slowlv adapting. The merit of this suggestion 'is at 

 present difficult to evaluate. 



RECEPTOR POTENTi.\L. As already mentioned, the 

 only receptor the functional properties of which 

 have thus far been studied is the Pacinian corpuscle. 

 It has been recently established by Alvarez-Buylla & 

 de Arellano (12) that mechanical stimuli produce a 

 local response which Gray & Sato (108) propose to 



