6: Absorption of Stimulus Energy 



While the molecular architecture of the receptor membrane and 

 the alterations which occur during application of a stimulus 

 remain very largely unknown, some inferences can be made 

 concerning structural organization from the nature of electrical 

 changes evoked by increments in stimulus energy. Thus, it has 

 become convenient to regard sensory membranes as containing 

 a multiplicity of independent sites. Each of these sites is supposed 

 to be capable of absorbing a fixed amount of stimulus energy, and 

 all of them must be activated before the maximal response of a 

 sensory cell (or sensory neuron) can be realized. According to 

 this hypothesis, each site converts part of the stimulus into a 

 receptor current, larger receptor potentials being obtained by the 

 recruitment of additional current generators. It follows from this 

 idea that the progressive removal of small amounts of the receptor 

 membrane should have little effect upon the receptor potential, 

 other than a reduction in its amplitude in proportion to the 

 percentage of sites lost. An experiment designed to examine this 

 premise was successfully carried out with the Pacinian corpuscle.'^ 

 The receptor membrane of this sensory neuron consists of an 

 encapsulated (but non-myelinated) nerve-ending. If the capsule 

 is carefully removed, the nerve-ending may be stimulated directly 

 with a fine probe (as illustrated by the records in figure 39). 

 Single, identical mechanical stimuli applied to the nerve-ending 

 resulted in progressively smaller receptor potentials as increasing 

 fractions of the total membrane were removed. However, neither 

 the response latency nor the waveform was interfered with. The 

 implications of these results were clearly stated by the authors: 

 * No matter where the cut or compression was done, or how many 



