I 



96 The Physiology of Sense Organs 



frequency by membrane potential, will eventually yield to electro- 

 physiological analysis. But it is becoming increasingly clear that 

 these techniques, at least at their present level of sophistication, 

 will not be adequate for meaningful analyses of the primary 

 structural or configurational changes which the stimulus imparts 

 to the sensory cell. Models, using convenient living preparations. 



Fig. 44. Conductance change in the membrane of a lobster 

 axon due to a mechanical stimulus. A brief current pulse 

 (top, left) is reduced in amplitude when it is superimposed 

 upon a * receptor potential ' evoked from the axon membrane 

 by a direct mechanical stimulus. Effects of two different 

 stimulus-strengths are shown. (From Goldman.'*) 



can be useful. Goldman^^ has found that isolated giant axons 

 of the lobster respond to mechanical forces which tend to stretch 

 the axonal membrane. A large increase in membrane conductance 

 occurs which can depolarize the axon sufficiently to fire an 

 impulse (fig. 44). Goldman suggests that this occurs as a specific 

 result of stretching, which would be expected to increase the 

 distance between molecular elements of the membrane. However, 

 this suggestion remains in the realm of conjecture, for we have 

 little knowledge about the mechanics of membrane changes. 

 Nothing has been said so far of the work being carried out on the 

 sensory responses of higher plants, such as those found in the 

 Venus fly-trap. ^^ Receptor potentials can be recorded from these 

 organisms, and they may prove to be more favorable experimental 

 material than animal preparations for an analysis of the primary 



