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56 The Physiology of Sense Organs 



of connective tissue in which the ending itself is imbedded. 

 Earlier work has shown that mechanical deformations applied to 

 the capsule are transmitted to the nerve-ending, where they produce 

 a graded receptor potentia l which varies in amplitude as a function 

 of stimulus intensity. However, the duration of the receptor 

 potential in the intact corpuscle is invariably brief, even with 

 prolonged applications of pressure, and rarely is more than a single 

 impulse generated in the adjacent regions of the axon following 

 the sudden application of an adequate stimulus. This brief 

 response is due partly to accommodation of the impulse-generating 

 membrane of the sensory neuron, since externally applied currents 

 — even of great intensity — are ineffective in producing more than 

 a very few impulses. Indeed, in six preparations so tested, only a 

 single neuron produced as many as three consecutive impulses 

 when stimulated with prolonged currents of an intensity which 

 was twelve times greater than the threshold for a single impulse ! n 

 This is an especially striking demonstration of the importance of ^ 

 accommodative factors in sensory adaptation. In addition, the 

 presence of the connective tissue capsule in the Pacinian corpuscle 

 is apparently a construction which ensures that the receptor 

 potential generated even by very intense mechanical deformations 

 will not last longer than 6-10 milliseconds, a period which is 1 

 usually only long enough for the generation of a single propagated / 

 action potential. 



Different results were obtained when the capsule of a Pacinian 

 corpuscle was dissected away from the sensory nerve-ending 

 itself.'^ Since this operation exposes the nerve-ending to 

 experimental manipulation, it enables observations to be made 

 upon the effects of the direct application of mechanical energy to 

 the ending. As shown in figure 24, the results were quite con- 

 clusive. When the mechanical filtering properties of the capsule 

 were by-passed, the time-course of the receptor potential re- 

 produced that of the period of stimulus application much more 

 faithfully. The potential still underwent a continuous decay 

 following the sudden application of pressure to the naked ending, 

 but the slope of this decay was greatly reduced and a discern- 

 ible depolarization was still present 60 milliseconds afterwards. 

 Furthermore, if an artificial capsule (composed of several layers 

 of thin mesothelium) was re-established between the nerve-ending 



