68 The Physiology of Sense Organs 



of part of the eccentric cell membrane, allowing inward ionic 

 current to flow, which moves outwards in adjacent regions and 

 critically depolarizes spike-generating membrane parts. The 

 exact proportion of eccentric cell membrane involved with the 

 light-induced resistance changes has so far been impossible to 

 calculate, but it is thought to include most of the distal process 

 of the neuron (rhabdom) which is surrounded by the primary 

 sensory cells. It is important to note in figure 29 that, since the 

 current-voltage curves are linear over the ranges tested, current 

 per se has little direct influence upon membrane resistance. The 

 membrane is electrically inexcitable, and some other agent, 

 possibly chemical, which is released as a result of the absorption 

 of light, appears to be involved. The equivalent circuit figured in 

 the inset of figure 29 also implies that there is a functional differenti- 

 ation of the eccentric cell membrane. In this circuit the membrane 

 batteries, Eg and E^, are assumed to be identical in the unstimulated 

 state, and the internal resistance coupling the two membrane types 

 is ignored. The resistance of the sensory membrane, Rg, is 

 variable and is related to light intensity, so that a decrease from its 

 resting value will cause current to flow in the circuit in such a 

 direction as to reduce the e.m.f. of the resting membrane. 



Results similar to those described above were obtained by 

 FuORTES^^ several years later from primary sensory cells in the eye 

 of the dragonfly. Thus, it seems probable that changes in mem- 

 brane conductance may be a universal consequence of photo- 

 chemical reactions in light-sensitive structures. 



WoLBARSHT^®^ was the first to provide evidence that increases 

 in ponductance follows stimulus-application in primary mechano- 

 sensory neurons. Using the tactile sensilla of various insects, he 

 showed that there is a linear relationship between impulse 

 frequency and the amplitude of the receptor potential produced 

 by translational displacement of the sensory structures. In 

 addition, however, he found a linear relationship between the 

 receptor potential and the amplitude of the resulting nerve 

 impulses: At maximum stimulus intensities, impulse height also 

 attained its greatest values (fig. 30). To understand this, it is 

 necessary to examine the rather unique method used to record the 

 electrical activity of neurons associated with these insect sensilla. 

 Each of these hair-like structures contains two inner canals, and 



